STONE ID – is our section that helps you to identify and understand the stone types you may run across in the cemetery. Many types of stone have been used over the years and centuries for making grave markers and monuments. If it can be carved, it has most likely been used for this purpose. For now, we are going to focus on the most common varieties and families of stone. We are going to do this by first understanding stone by looking at Rock Cycle, Mohs Scale, and Porosity and Permeability. These 3 areas will make better sense of the stone types that follow.
THE ROCK CYCLE:
The Rock Cycle
By Don Hilton Geologist
The rock cycle springs from the idea that the planet always reworks and recycles everything; even that which seems permanent to us.
The grave markers we most often work with represent the three general types of rocks. Knowing how these rocks form helps to understand and consider their relative strengths and weaknesses when working on conservation.
There is no beginning or end to the rock cycle, so let’s pick a starting point and move from there.
There are two types of igneous rocks: Extrusive igneous rocks are fully-melted material of any other kind of rock that is either blown out of volcanos, or placed on, or very, very close to the surface of the earth. These rocks can be so full of gas that they float, like volcanic ash and pumice, or very dense, like heavy lavas. But all cool rapidly, even in human terms, and show little, if any, signs of crystallization. Some, like obsidian, solidify so fast that they take a glass-like form, even though they contain a complex mix of elements. In the U.S. it is rare to see extrusive igneous rocks in grave markers because they tend to be light and easily damaged or heavier but not very attractive.
On the other hand, granite, in its many colors, is an intrusive igneous rock, formed by fully-melted rock material that slowly cooled over thousands of years deep within the earth. It’s difficult to comprehend, but the granite outcrops you can see in quarries were, at one time, miles beneath the surface where temperatures and pressures were severe enough to keep rock material in a liquid state.
Intrusive igneous rocks like granite cool slowly enough to allow different minerals to crystalize from the melt. Generally, the larger the crystals, the slower the solidification. Generally speaking, intrusive igneous rocks are mechanically tough, acid-resistant, and have little in the way of porosity, though they are brittle and can be broken. Their rough-hewn or highly polished surfaces are found in a multitude of grave markers around the world.
The sandstone and limestone grave markers we see are examples of sedimentary rocks, made from bits and pieces of all other types rocks.
Sandstone is mechanically transported, compacted and cemented. Limestone, probably the most complicated of all sedimentary rocks, can be created in this way, but can also be the result of chemical deposition, made up of biological artifacts (like shells or clam poop (honest)), or a combination of all three. Sedimentary rocks can be created from the worn remains of any other kind of rock, so they end up as the garbage pail of rock types. As a result, their strength and weathering characteristics are highly variable. There’s a lot of good-looking but crummy sedimentary rock in markers that peel, break, melt, and fall apart—we’ve all seen them. That’s why “good” sandstones or limestones in large enough amounts to be quarried are so highly prized as building and monument material.
Because of the high variability in their source, deposition, and subsequent hardening you never know what you have with a sedimentary rock. For example: A quartz-rich sandstone with a quartz cement and no porosity can stand up to almost anything except a direct blow with a hammer. The same quartz-rich sandstone with a calcium-rich cement will fall apart with exposure to acid rain. Limestone, in its various forms can be hard and dense, light and easily broken, or anything in-between.
These are all kinds of rock that has been partially melted and squeezed by the heat and pressure of deep earth burial. Like the formation of igneous rock, metamorphism typically happens deep underground, but is not radical enough for a complete melting of the original rock material. This results in a curious hybrid effect.
Gneiss (says “nice”) -pictured- often has the strength of its granitic beginnings. It also holds all of granite’s minerals, but moves them about to arrange them into distinct layers that can be nice and neat, dramatically swirled, or any combination of the two.
Marble, perhaps the most familiar of metamorphic rock used in grave markers, was limestone before being worked over and left to cool. As such, it has a weakness against acids inherited from its limestone parent, but is generally more resistant and less layered because of the partial melting. Remember the variability described above in limestones? All of that shows up in marble, too!
Slate, once a very common marker material, is metamorphosed shale. The process renders it durable and nearly impervious to water but it can retain a tendency to thinly flake.
The formation of quartzite removes the porosity from a silica sandstone and keeps the simple mineral composition while generally removing any bedding or layering from the parent.
The Rock Cycle and Geography:
Early graves are most often marked with local rock types because only the well-to-do could afford to have a heavy marker brought in from a far distance. Look to your earliest common burials to see what kind of stone was used: Igneous, sedimentary, or metamorphic. That will provide you with information on the geology and geography of your area. A general change in rock type over time can often be tied directly to the development of local transportation. These changes may occur slowly, but can happen within a few years, or even months. An abundance of one type of stone in one cemetery and a lack of it in a nearby cemetery often points to social differences. All of this is secondary information that clues you into what was important to the people whose markers you conserve.
The three major rock types: Igneous, Sedimentary, and Metamorphic are well represented in the grave markers we work to save. Knowing their origins, general characteristics, and how they were used over time can help make us better conservators.
Interested in more about rock types? Try:
Rock Cycle from:
Gneiss image from:
MOHS SCALE OF HARDNESS:
Mohs Scale of Hardness
By Don Hilton Geologist
In 1812, a German geologist named Friedrich Mohs created a scale of mineral hardness based on the ability of one natural mineral to scratch another. There is no apostrophe in the name “Mohs Scale.”
The Mohs Scale is popular with earth scientists because it is easily understood and used. There are 10 relatively easily obtained minerals (except, probably, diamond). Each is assigned a relative value depending on which can scratch which. In addition, common objects can be used as “testers” to compare a mineral’s hardness to, say, a fingernail, a piece of glass, or a fine knife’s steel blade.
Not to be overlooked is the fact that the scale has been in worldwide use, virtually unaltered, for a very long time. Any description of a mineral listing a Mohs hardness will be understood by any earth-bound geoscientist who has lived in the past 200+ years. Indications are the scale will continue to be used despite its shortcomings.
The scale is relative. That is, the minerals are placed in order of hardness, with the mineral talc assigned a value of 1 (one) and diamond given the value of 10 (ten). But the differences in hardness are not linear; diamond is much more than 10 times harder than talc.
The Mohs Scale is for pure minerals. Very few rocks are comprised of just one thing. A grain of sand in a stone may be one mineral, but the stuff that holds the grains together may be something entirely different. People can be tempted to say that a rock is harder because the minerals in it are “harder,” but that ignores other, often more important considerations.
The Mohs Scale ignores chemical considerations. On the scale, quartz is harder than talc, but quartz can be dismantled by acids that would barely bother talc. Also ignored are other characteristics such as durability. A piece of diamond may be cleaved with a blow that wouldn’t put much more than a dent in a block of gypsum.
Finally, an individual mineral crystal can show a different Mohs hardness depending on the direction it is scratched. Quartz, for example, is softer when scratched along a crystal’s axis but harder when scratched across the lines of crystal growth.
The Mohs Scale is useful for identifying minerals but lousy at determining how hard a mineral actually is. Rocks are usually composed of many different minerals held together in many different ways. This, along with structural and chemical factors, renders the Mohs Scale inadequate for measuring a rock’s total strength.
The Mohs Scale with Additional Information:
POROSITY AND PERMEABILITY
Porosity and Permeability
By Don Hilton Geologist
Ever spray water onto a grave marker, see the moisture soak into the surface and declare it to be “porous?” If so, you’re making one of the most common mistakes when it comes to describing rocks. There are two separate, but interrelated measures that are used when describing how liquid (or gas) moves through a rock. This article will help you understand the difference between the two and what’s really going on when a liquid is (or isn’t) absorbed into the surface of a stone.
ALL rock has porosity. For ease of understanding, what follows discusses sandstone.
Porosity is the void space within a solid material. Simply put, porosity is the amount of space inside a rock that isn’t rock. A freshly-poured pile of undisturbed dry sand is about 40% empty space, that is, it’s porosity is about 40%. Funny thing is, a freshly-poured pile of fist-sized-and-shaped rocks is about 40% empty space, too, because porosity doesn’t depend as much on the size of the material as it does the shape of the material and what happens to the rock as, and after, it forms.
Grain shape: Imagine a pile of baseballs. Such a pile has about 40% porosity. Meaning about 40% of the pile isn’t baseballs at all, but the space in between them. No matter what you do, as long as you simply toss the baseballs into a pile, there will be plenty of space around them. Now, imagine a pile of bricks. Not so much space, is there? It’s the shape of the bricks that allows them to naturally fill the space. They can interlock in a way that baseballs cannot.
Sorting: Your pile of baseballs is going to have a lot of space in it no matter how you try to stack them. That is the nature of spheres. Bricks, however, can be stacked in such a way as to leave very little space in between; think of a brick wall. Natural processes, like the action of wind and water, are as capable as we are when it comes to modifying and sorting shapes. Ever been to a wave-washed stone beach or a cobble-rich flowing stream and been amazed by the apparent uniformity of the rocks you find there? That’s some of nature’s finest sorting in action.
Compacting: Any potential rock material is subjected to compaction. If the grains within it are hard (like quartz) they won’t squeeze together as much as minerals with give (like mica). The grains of harder material may melt, recrystallize, interlock and reduce the porosity, but they won’t deform all that much. Minerals that do deform can drastically reduce the space within a rock.
Microscopic examination of a sandstone shows the following. This sample is cut and polished thinly enough to allow light to shine through it. Magnification is 200x:
The porosity is the blue, filled with an epoxy to keep the rock from coming apart as it is prepared. The “clear” grains are quartz. The dirtier one are feldspars. The muddy brown ones are limestones. Here we see about 18% porosity, which is high, but not unusual. Grain shape is generally angular with some rounded corners. The grains are poorly sorted for composition and direction but fairly well sorted for size. Look to the green box. See the mineral with the vertical, but wavy lines? This is muscovite, a type of mica, that has been distorted by compaction. Just below it, in the green circle, is a dark, soft clay of some sort that has been squeezed at right angles around harder minerals. If you look around the thin section you can find other examples of compaction.
Infill and Cement: Typically, as a rock hardens (or lithifies), mineral-rich water circulates through it, depositing material as it goes. This cement decreases the porosity of the stone as it fills in the space around the grains. As it fills the pore spaces it also makes the rock heavier. A cubic foot of dry sand weighs about 100 pounds. The same amount of sandstone is usually calculated at 150 pounds. The difference is porosity being filled by compaction and cementation.
Secondary Porosity: This is empty space created within the rock after it forms. This can be chemical, like the dissolution of cement or certain minerals in the rock, or mechanical, like the fracturing of the rock material.
Notice that, so far, there’s been no mention of liquids flowing through a rock. That’s because such flow is measured by a different variable.
Permeability is the ease of flow of air or liquid through a material. Porosity is how much space there is in a rock, permeability is one measure of how interconnected that pore space is. Go back to your pile of baseballs. If you pour a bucket of water in the top of the pile nearly all of it will come out the bottom. So, you have high porosity and high permeability. Take your pile of bricks and stack them in a solid cube. If you pour a bucket of water on top of it nearly all of it will flow off the top and down the sides. So you have low porosity and low permeability.
Now… Take your pile of bricks and rearrange them so there is big room inside, but the walls and floor are solid. If you pour a bucket of water in it, the room holds the liquid but very little of it flows out. In this case, you have high porosity and low permeability.
One more: Take your bricks and arrange them so there is a single hole that goes from the top to the bottom of the pile. Pour your bucket of water into the hole and it flows straight out the bottom. Now, it’s low porosity and high permeability.
What does all this imagining prove? That you have to have some porosity to have any permeability, but the two things, while related, are completely different to each other.
What makes fluids flow within rocks:
Water flows downhill, right? Nope. Not always. Within a restricted system (like a grave marker) Water flows for two major reasons:
Capillary action: The best example of this is a sponge which is very porous and has high permeability. Water is pulled up into the sponge’s structure by the water’s need to stick together. Anything that’s absorbent takes advantage of water’s surface tension to “pull” it in the direction you want it to go. This is what happens when you spray water and it vanishes into the surface of a grave stone that has both porosity and permeability.
Changes in pressure: When constricted, water always flows from areas of greater pressure to those with less. These pressure differences can be great, like a burst pipe, or subtle, like water evaporating from the surface of a stone. This, along with capillary action, is one big reason water rises from the earth into and out to the surface of a porous and permeable stone. The weathering along the bottom of some markers are indicative of this behavior.
Other controls of fluid flow:
Flow within any stone is often directional. In sandstone grave markers, water moves along the path of any obvious bedding lines which might appear as stripes or layers within the stone. Remember that the sandstone has those lines of preferred flow, even if you cannot see them.
In non-sedimentary stones, like granites, water has a tougher time finding its way along. Rule of thumb is that nearly any stone has at least a few percent of porosity. But many are considered to have little to no permeability. Ever hear of “fracking” wells? They’re fracturing the rock to make the fluids within them flow more easily.
Porosity and permeability can vary widely, even within a very small distance. All grave makers, even those that are inside of buildings, are subject to weather that alters the characteristics of the surface. Smoothing and filling in porosity, in some instances, but more often, coarsening and increasing the porosity. Hand in hand, an increase of porosity tends to increase permeability, making it more likely that the stone, at least at its surface, will accept and incorporate liquids more easily. This process then feeds-back on itself. Slow changes lead to faster change. Minor degradation leads to major failure. This is true of all natural stone marker.
This is a big reason that markers must be carefully cleaned. Using the wrong cleaner (like vinegar) “opens the pores” on many different kinds of rocks. Others (like bleach) leave behind salts that will damage the stone’s surface as they repeatedly dissolve and recrystallize. Some (like “fatty” soaps) clog what pores and permeability are already present and while keeping water out sounds like a good thing, it also alters any establish flow of liquids within the marker, setting it up for future failures.
ALL rock has porosity (void space) and most have permeability (interconnected voids). The initial porosity and permeability of raw material are altered by the process of becoming a rock and can also be changed after the rock has formed. While necessary for fluid flow, permeability, like porosity, can be controlled by a marker’s internal structure. They can also be changed by poor cleaning techniques.
Thin section: Squaw Lake Formation, Alaska North Slope, c640-caco3, 1985-07, METC, WV, courtesy of Don Hilton.
November 25, 2018 – CEMETERY CONSERVATORS FOR UNITED STANDARDS
By Don Hilton Geologist
The sedimentary rock type of sandstone often makes up a large number of markers in older cemeteries. Widely available, seemingly resistant to weather, easily carved, and attractive, it’s a dead-cinched choice for a gravestone. But not all sandstones are created equal. The composition and formation of the rock determines it ultimate fate as a marker.
To start, understand that the term “sandstone” is generic. It refers only to the sand-sized grains of the material that make up the rock. Those sand grains could be quartz, broken dinosaur teeth, or even plastic. As long as they are sand-sized and form a stone, they are a “sandstone.”
Those in the know usually add at least a color and size modifier, like “a light-gray, coarse-grained sandstone.” Closer examination may allow the addition of what the grains are made of and what’s sticking the sand together: “a light-gray, coarse-grained, calcite-cemented, quartz sandstone.” Typically, a conservationist wants to know all of the stuff in the long name, and more.
Go to the hardware store and look at the “sandpaper” there. It runs from very coarse to extremely fine. The sandstones we conserve are the same way. Their grains come in many different sizes.
Geoscientists use a variety of size measures and terms, but the most easily understood is the “Wentworth Scale” (1922). Shown her are the size ranges and their names. Yes, a “boulder” has an actual formal size.
When the sand in a sandstone grows fine enough, it becomes a “siltstone” and then, finally a “claystone.” Geologists argue about the point when a claystone becomes a shale. It’s best to just stay the heck out of their way.
For grave markers, starting at very coarse sand, siltstones are about as fine-grained as you’re going to get. Some siltstones make wonderful markers, smooth and able to hold the sharp edge of a carver’s work. Claystone is a poor material for monuments since it tends to break into layers.
Layers v. Bedding:
To a geoscientist, a “layer” of rock is the whole layer of rock, no matter what it is, or what’s happening inside of it. Instead we’re going to use the term “bedding” (an Inappropriate Geologist once pointed out that bedding and layers sure seemed related, but there’s no use going into that here).
“Bedding” is part of the nature of sedimentary rock. By definition, such rocks are made up of material that has been eroded from some other material, transported some distance, deposited, and then re-lithified into stone. The nature of whatever movement and deposition took place leaves traces inside the sedimentary rock as bedding.
Sometimes the bedding is completely obvious. The pictured marker (front and back) is that of Maryann Keeler (died 1827) and is one of several similar in the Brownhelm Cemetery, Brownhelm Township, OH. It is 16.5-inches (42 cm) across and is a tan-to-brown, fine-grained, heavily-bedded, quartz sandstone. The western face is lit by a full autumn sun, the back is in shadow and looks a different color.
The question is, of course, what is causing the remarkable mottling of the stone’s surface? At first glance, it looks like a cleaning job gone haywire in a serious way, when in fact, the answer is in the structure of the rock.
Here’s a view looking down from above, the face of the stone is the top edge of this photo. The 3-inch thick marker shows strong bedding of a peculiar type. In this case, the grains that make up the sandstone were deposited in water shallow enough to form ripple marks created by near-shore wave action. To put that another way, in the past, this sand, before it became stone, was in shallow water with waves in it that moved and sorted the sand into ripple marks which were then preserved in the rock!
Within the ripple marks, each difference in color is variability in grain size, grain composition, cementing material, or any combination of the three. Such differences make for an unusual marker, but they also render the stone inherently weak, as can be seen in the photo, above, of the marker’s face. Spalling of the surface and serious vertical cracks are readily observed. The future of this marker is not good and without some form of stabilization it will surely peel apart and fall to a more stable state, like its neighbor.
As an aside, this stone, and those like it in Brownhelm and other nearby cemeteries all seem to have dates in the late-1820s to the mid-1830s. Nearby rock quarries were not in operation at the time, but it seems likely they are what is known as “Berea Sandstone.” It is unknown if their natural decoration was seen as a flaw or a selling point.
The example presented by the Keeler stone is extreme and unusual. Most of the bedding you normally see will run in stripes parallel to the carved face of the maker, but keep in mind that, whether you can see it or not, ALL sedimentary rock has bedding. For the most part, any bedding you see is a probable location of past, present, or future failure. Water typically moves most readily along bedding and, as we all know, water means trouble.
Sandstone without obvious layers is often referred to as “massively bedded” or, simply, “massive.” Locations capable of producing very large pieces of massive and uniform stone were and are highly prized. But even if you can’t see any bedding, it’s still there. This is sometimes referred to as the “fabric” or “grain” of the rock.
The most apparent characteristic of unseen bedding is uneven, or differentiated wear. For example. You may see a large sandstone column, either square or round. Two opposite faces are holding up well. The other two are weathering at a faster rate. This is because the alignment of the grains within the stone is such that it presents a more readily eroded surface along unseen bedding.
Always keep in mind that stone is not uniform throughout, even if it appears to be so.
With a sandstone, the idea of “maturity” refers to the minerals that make up the sand grains (and, to a lesser extent, their shape). Simply put, the more mature a sandstone is, the harder the minerals.
Imagine you putting a bunch of different kinds of rocks in a clothes dryer and then running the dryer for, oh, a year. At the end of that time, the rocks will be broken into smaller pieces. The softest of them will have turned to powder long ago. What remains is the hardest of the batch. The same is true of the grains in a sandstone. As they are moved from one place to another, the softest of them are worn to nothing and only the strong survive.
Practically speaking, the more quartz grains a sandstone contains, the more mature it is because quartz is the most abundant but relatively hard mineral on the planet. There are many rarer minerals as hard or harder than quartz, like zircon, and finding these in a sandstone boosts its maturity. But, as mentioned above, the sand in sandstone can be anything, even very soft minerals, and these lend themselves to faster rates of wear and weathering.
The other advantage of quartz is that there aren’t too many things in it that lichens and algae wish to metabolize. Other minerals contain elements like aluminum, potassium, magnesium, and iron… All very yummy to our little lichen pals.
Cementation and Permeability:
The permeability of a sandstone, its ability to transmit gas and liquid, depends on how the grains are stuck together. Rocks that are fully cemented with silica are the most resistant and when combined with quartz sand grains form excellent grave marker material, not only because of its strength but because such stones allow very little water inside them.
Calcite (limestone) cement is less desirable because it can be weathered by acidic rains and is prone to lichen and algae growth because of the calcium and carbon it contains. A sandstone marker with calcite cement is prone to cement loss along the surface due acid exposure and bio-activity. This opens the marker’s pores and allows water to penetrate the material. Water in a marker is bad news.
The stone here is that of McQueen family (1825 and 1830), also in the Brownhelm Cemetery, Brownhelm Township, OH. Pictured is the bottom of a tapered square column about 18-inches (46 cm) across. It is a rust-red-brown, very fine-grained, massively-bedded sand-to-siltstone. The sun-lit face looks south.
Here, we readily see the damage caused by the infiltration of water and the subsequent cycles of freeze-thaw, or what’s sometimes called “frost-wedging.” It’s easy to see the cracks moving up the face, but take a moment to look closely at their alignment, all running parallel along bedding that is not obvious at first glance. As bad as the sunlit side is, the northern face is completely gone as are ornaments carved into its square, peaked, finial.
Because these cracks go completely through the monument, side-to-side and along parallel planes, it’s likely that the infiltration of water is due to the original nature of the rock. In real life it is a striking dark-red and was, no doubt, an expensive and impressive monument, but the permeable stone is ill-suited to a life out-of-doors, at least in a climate with repeated freeze-thaw cycles. Inside, out of the weather, or in a warmer spot, it would probably still be a good-looking marker, particularly since it seems to have some sort of natural immunity to bio-growth. It carries almost no moss, algae, or lichens.
Sandstone is one of the most common grave marker material, but its variable nature can make it a very poor choice. The complexities of sandstone’s grain-size, composition and depositional history, along with subsequent cementation and weathering characteristics make it difficult to predict exactly how the stone will behave over time. Understanding this allows a conservator to make better choices on whether to take action on a simple cleaning, move to a more proactive preservation, or chose to let nature take its course.
Grain Size: https://en.wikipedia.org/wiki/Grain_size
Search “Grain Size” at https://www.forestry-suppliers.com/index1.php
Berea Sandstone: http://www.ohiohistorycentral.org/w/Berea_Sandstone
Courtesy of Don Hilton, 2018
By Don Hilton Geologist
The sedimentary rock type of limestone once ranged from the most “classy” to the most common of markers. Limestone is not widely found but may make up a large number of stones when a good and plentiful local/regional source is available. The rock’s chemical make-up renders it very susceptible to damage by acidic pollution and precipitation. Limestone markers fell out of favor when its more attractive and seemingly more durable metamorphic cousin, marble, became widely available. While limestone may be unfamiliar to many, understanding it helps us to conserve not only it, but similar-in-composition marble markers.
Limestones are difficult to categorize because they can form in so many ways: from purely chemical to purely biological to purely mechanical to any mix of the three. One method of categorizing limestones is the textural classification of carbonate sediments developed by Folk (1959). It combines the chemical, biological, and mechanical characteristics of the rock type.
The first thing to notice is that the word “limestone” is completely missing from the chart. That’s because, to many geoscientists, the term encompasses so many things that it is, essentially, meaningless. Who are we to argue?
The “Carbonate Sediments” used by Folk refers to substances that are formed from or with the help of calcium-rich carbonic acid (a chemical mixture of carbon dioxide and water).
Moving from the left to the right of the diagram takes us from calm to vigorous environments of deposition. It also moves us through different types of cement and “allochems,” Folk’s word for “stuff in the rock that isn’t cement.”
So, how does the Folk Classification work? Right off, we’re at a disadvantage because our conservation frame of mind does not allow us to use one of the geologist’s most common tools. Calcium, the main component of carbonate rocks, vigorously bubbles when it comes in contact with dilute hydrochloric acid (10% HCl). It even bubbles, though to a lesser extent, with white vinegar. But, we’re a little shy about putting such acids on markers, so what else is available to us?
Carbonates are relatively soft and can be easily scratched with a knife blade, or even a car key. They can be almost any color, from white to black to red and anything in between. A “traditional limestone” is, in the Folk Classification, a “Micrite” (bottom left of the chart). A freshly cleaned marker is a creamy white to light brown to light gray rock that is often “mottled.” That is, uneven or blotchy in color. When highly polished, limestone can be smooth to the touch but it takes very little exposure to the elements to give it a “grabby” feel. Badly weathered micritic limestones look “melted” but typically retain a surface that appears smooth but feels almost like a very fine sandpaper. A surface that’s melted and looks and feels like sugar on a table-top belongs to marble.
Newish and reasonably weathered micritic limestone often appears almost like a fine-grained concrete, which is not surprising since one is very much like the other. Because it is almost always carved, and not always polished, the initial surface depends on its final treatment. In the image above (Mark Morton, 2018, Clermont County, OH), the left side carries chisel marks, the right side is smoother. Note the mottled color which is one of the clues that it’s limestone! In the hands of a good carver, limestone can be light and airy, heavy and massive, or anything in-between. But, in the end, even if it’s beautifully worked, the material almost always appears work-a-day: solid, massive, and substantial.
Some conservators worry about confusing limestone and its metamorphic form, marble. But, when seen together, as in the picture to the left, there can be little confusion over the commonplace, gray limestone and its finer, elegant, and very nearly glowing cousin (Mark Morton, 2018, Clermont County, OH).
How about less traditional limestones? One example is the famed “White Cliffs of Dover,” made up of more fossilized shells of small, dead sea creatures than you can imagine. Or how about this highly unusual purple-and-gold, volcanic-ash-infused, cliff-forming limestone from the southwest United States that has a surface sharp enough to cut flesh? (Don Hilton, 1982, Parowan, UT). Imagine that as your marker!
Perhaps something that, on first glance, looks a little less exotic?
Pictured just below is the newly-cleaned marker of Anson and Phoebe Cooper (Don Hilton, 2018, Brownhelm Twp. Cemetery, Lorain County, OH). Excluding the fine-grained, brown, quartz sandstone base, this mottled pink marker is 39 inches high, 21 inches wide, and 4 inches thick (100 cm, 53 cm, 10 cm). One thing you can’t tell from the image is how rough the surface feels… your hand almost sticks to it.
A close look at the face of the marker shows the carving to possess a softened look. It seems as if their might be layers of some kind running diagonally. There is a mottling of color, gray to pink, though this could be the D/2 still at work. For sure there are hundreds (thousands?) of small to somewhat large chunks of oddly-shaped “things” that form a sort-of-maybe pattern. For certain the rock is made up of smaller parts.
An even closer look at the side of the marker tells us what we need to know. Here, the right edge of the rock is the back side of the marker (about 20x magnification).
It’s obvious, now, that the rock holds a multitude of fossils. The thick lines that look like stacked checkers are the remains of crinoid stems. The sort-of-round item with all of the holes in the top of it, left side, middle, is either a crinoid head or, more likely, some sort of fossilized coral (you can find many examples and more information on these with a simple web search). Other sections of the rock show many corals. There are no obvious “shell fossils” to be seen, so this rock is made up of the cemented-together remains of a prehistoric coral reef!
What’s more… You can see why the marker feels so rough. The fossils have weathered to a point below a more resistant, pink and white, sharp, crystalline (spar) calcite cement.
In Folk’s Classification, this is a textbook example of a “Biosparite” –sorted, or unsorted, I’ll leave that up to you. Keep in mind that, if these myriad fossils were broken down into sand-sized particles, you might argue that this marker isn’t a limestone at all but is, instead, a very calcite-pure sandstone!
There are limestones constructed of nothing but calcite in its crystalline form. When broken they have a sugary look, but feel solid and smooth to the touch, almost like a broken quartz. But the calcite’s softness always gives it away—3 on the Mohs Hardness Scale. Quartz is 7. A marker of just crystalline carbonate (“Sparite” in Folk’s classification) would be a very rare find.
Other limestones are comprised nearly completely of shell material. “Coquina” (say “koe-KEEN-ah” or “KOE-ki-na”) is a good example of one. A “Packed Biomicrite” for Folk, it can be a tough and durable carbonate rock, in fact, it was used in building Castillo de San Marcos (Fort Augustine), in Florida (circa 1672). It’s a common base material in some cemeteries along the south-eastern coast of the United States, but its coarse-grained nature makes it uncommon for carved markers.
Weaknesses of Carbonates:
In places where there is little rainfall, limestones last a long, long time. This illustrates the main vulnerability of all carbonate markers: their susceptibility to chemical weathering.
The Cooper stone, above, was likely polished as smooth as silk when it was first placed. Exposure to precipitation, even that of a relatively low acidic nature, has left the surface very rough. This rougher surface retains more precipitation and speeds degradation of the rock. Proof is the top of the Cooper monument. It holds rain and snow and is already far more weathered than the marker’s vertical sides.
A close-up of the N in the word BORN below Phoebe’s name shows the present level of damage in the marker’s face. With the softer fossils eroded and the crystalline cement exposed, the carver’s edge has already suffered a great loss of detail. It’s plain to see that there will come a time when the face is illegible. There’s not much we can do to stop it.
Wet and highly acidified temperate environments, urban or industrial, are even tougher on carbonate markers. Those downwind of such places are subject to accelerated weathering, especially on fresh or freshly-cleaned surfaces. This leads some conservators to leave carbonates in a dirty state, particularly when there are no lichens present. Some think the layer of filth helps protect the rock from further damage. Passing by a readable marker we know we can clean and “make like new” is almost impossible for many of us!
Carbonate markers placed directly in or on the earth with no drainage are subject to attack by the humic acids present in many soils. Slab-type stones are especially weakened under such conditions. Markers set in a base should be above grade. Those on or in-ground (as describe in the “rule of thirds”) should be on, or surrounded by, crushed limestone that, itself, is sacrificed to ground-sourced acids.
Constructed mainly of calcium, carbon, and oxygen, limestone markers are a tasty treat for all kinds of algae and lichens. The acids produced by such life readily dismantles the stone into its base components, making it even easier to digest and the chance for bio-produced damage even greater.
In addition to their chemical and biological weaknesses, limestone markers are physically soft. The same characteristics that make them relatively easy to carve also make them prone to chipping along corners and scratching on their faces. Even moderately-tall monuments that take a tumble are apt to break.
All of this adds up to a marker that must be carefully conserved. Digging must be done with care. Lifting, as well. Scraping and brushing must be done gently, especially on carved surfaces, and even a mildly acidic cleaner can do great harm and an acidic cleaner that is left to dry can be catastrophic. Limestone seems very solid, but it’s face is prone to damage and must be protected from scratches and gouging that accelerates weathering.
Like all sedimentary rocks, carbonates contain evidence of their deposition. For those created in deep, calm water, bedding might take the form of a simple, dark layer running thru the rock. For limestones formed in more active environments, like the Cooper rock, above, the material may be all jumbled together, but show alignment inside of the rock. Take a second look at the close-up of the edge of the Cooper marker. See how the long direction of the crinoid stems align? This is a characteristic of the bedding of the rock.
Bedding represents zones of weakness within the rock, but it’s unusual to see a limestone split apart like sandstones are prone to do. That’s because such material was hardly ever used as markers or, if it was, those examples are long gone. Besides, limestones are usually melted by their cemetery environment well before natural parting occurs. Additionally, small splits in carbonates tend to “self-heal” through the dissolution and re-precipitation of the calcium inside them.
The permeability of a carbonate, that is, its ability to transmit gas and liquid, depends on how the grains are stuck together. For many Micrites, most porosity and permeability will be secondary in nature. That is, created after the rock has formed. For Sparites, it depends on how complete the cementation is. The Cooper example, above, is a very solid marker that allows for little-to-no infiltration of water.
With any carbonate marker, any ability of water to flow through it is very detrimental because the rock is so easily dismantled by even the weakest of acids. Placed in a wet environment, limestone monuments with cracks around old pins are in a very bad situation.
Limestones, or carbonates, were once one of the most popular, regional rocks to be used as markers. Finer examples are beautifully carved. Its susceptibility to chemical weathering by acidic precipitation make older pristine examples in outdoor settings difficult to find, particularly in heavily urbanized areas. All of the weakness of limestone is passed on to marble, its metamorphosed form, so an understanding of carbonates enables a greater appreciation of the much more popular marble grave markers.
Folk’s Carbonate Classification: http://www.sepmstrata.org/page.aspx?pageid=89
Folk’s classification, used in this article, is only one of a number of schemes used to categorize limestones. Dunham created another that has some popularity with geoscientists. Dunham’s chart, and its subsequent modifications by later researchers can be found here: https://en.wikipedia.org/wiki/Dunham_classification
Sometimes, a marker considered to be limestone is actually its close chemical relative, dolomite. Formed in much the same environments, dolomite, or “dolostone” as it’s sometime called, substitutes the element magnesium for some of its calcium atoms making it a calcium-magnesium-carbonate. It still bubbles in weak acid, but to a far less degree. Dolomites are tougher, both chemically and physically, tend to be grayer than limestones and are usually thought to be less attractive and “lower class” than limestones. There is a chance that the limestone shown in the images at the top of this article is, in fact, dolostone.
By Don Hilton Geologist
Make sure to read the article for Limestone, first. Without it, you are missing the origins of Marble!
The metamorphic rock type of marble was once a very popular material for grave markers. Widely found, marble may make up a large number of stones in middling-old sections of cemeteries. Marble’s chemistry makes it very susceptible to damage by acidic pollution and precipitation. Its physical construction renders it vulnerable to extreme damage by bio-sources such as lichens and algae. Outdoor marble is a challenge to conserve.
Marble is the metamorphosed form of limestone, and it is the completeness of this change that defines, in large part, how marble will behave as a grave marker. In all cases, there is a recrystallization of the calcium found within the parent limestone. As the process continues, the calcite crystals grow and interlock with one another. With incomplete metamorphism, the crystals are so small that they’ll show only as fine-grained sparkles on a freshly-broken surface—you’ll need magnification to see them. The longer and more complete the change, the larger and more interlocked the calcite crystals become and the more the rock seems to “glow” (the Greek root of the word, marble, is marmaros, meaning “shining stone”).
Complete metamorphism destroys all of the original structure of the parent. Fossils, cementation, and any layering are wiped out by the process to leave a single mass of crystalline rock.
The forces that encourage the change from limestone to marble can be widely regional, like two tectonic plates crunching together, or more local, like a magma moving through overlying rocks. Some metamorphism is so limited in scale that one side of a quarry can produce limestone and the other, marble!
Like limestone, marble’s main component is calcium which vigorously bubbles when it comes in contact with dilute hydrochloric acid (10% HCl). But since conservators typically do not use acids on markers, we need to use other means.
Marble can be almost any color, from white to gray to black to red and anything in between, but we mostly see white. Many have lines that form a “swirly” pattern (Mark Morton, 2018, Harrison County, OH). That pattern may be of a more-or-less resistant material that will either stand proud or sink into the surface as the marker erodes.
Marbles are relatively soft and can be easily scratched with a knife blade, or even a car key—that’s what makes them easy to carve. Since “dirt shows first on the cleanest cotton,” what we typically see, in grave markers, is a filthy-looking rock with some patches showing the stone, beneath.
Out-of-doors, marble markers are commonly badly weathered and look “melted.” The surface, when examined very closely, will be constructed of small crystals that reflect the light and look something like crystalized candy. If you touch the weathered surface with your fingertips it will feel something like sugar on a table-top. This is the “sugared surface” some conservators refer to when discussing marble markers (weathered marble image, 10x magnification, by Elaine Storer Hinton, 2018, Adams County, OH).
Weaknesses of Marble:
The relative strength of marble depends, in large part, on the completeness of its change from the limestone source material. The best marble you’ll see is that used for U.S. military markers. Fully recrystallized and with a tightly interlocking structure, they are tough stones.
The trouble is that most older markers for civilians were made of inferior marble. Many (most?) of the marble markers we conserve are made of lower-grade material that has nearly fallen apart with time. Typically, horizontal surfaces that retain moisture are damaged the most, but vertical surfaces are also quickly weathered to illegibility. As with limestones, there’s not much we can do to stop it.
Even the best outdoor environments are hard on low-grade marble markers. Urban or industrial settings are much worse. Stones in such places experience extremely rapid weathering.
Anything that can be done to keep marble markers as dry as possible is a good thing. Those placed in-earth should be surrounded by sacrificial limestone gravel that will counteract the effects of ground-generated acids.
Lichens and algae love marble! The poorly-interlocked crystals of low-grade marble allow some lichens to send runners up to nearly an inch (2.5 cm) below the surface. The acids produced by such life readily widens the inter-crystal spaces allowing more life, or water to infiltrate a fair depth into the stone. Freeze-thaw cycles then aggravate the problem.
The surface of a weathered marble will not stand up to any type of scraping or brushing. The removal of lichens and algae , even with a no-harm biocide like D/2, leaves a rough, pockmarked, hummocky, sugary, unattractive stone. A look at the top of a typical white marble marker shows that, often times, there is no good middle ground (5x magnification image, Elaine Storer Hinton, 2018, Adams County, OH).
Chemical and biological weaknesses cause marble to grow increasingly fragile with age. Leaning against a tall marble tablet with just light-to-moderate pressure can cause it to snap wherever it is weakest. Their sugary surfaces are impossible for us to improve using field-based techniques and are sometimes difficult to epoxy.
The best thing for a marble marker is to be properly kept with ongoing maintenance from the very start, something that doesn’t happen very often.
Complete metamorphism obliterates any bedding that was present in the parent limestone. Swirls of colors are one thing, but any parallel bands of markings or crystals in the stone indicates a low quality marble.
Take another look at this close-up of a weathered marble surface. See the very faint bands within the grains running across the stone, slightly lower-left to slightly upper-right? That’s how subtle the traces of bedding can be.
The permeability of a marble, that is, its ability to transmit gas and liquid, depends on how tightly interlocked its crystals are, and to the extent damage has opened any available space. Bio-growth is likely the biggest cause of secondary porosity and permeability. Lower-grade marble comes apart quite easily when exposed to repeated cycles of freeze-thaw.
The metamorphic rock type of marble makes up many of the markers we struggle to conserve. It’s limestone source along with its formation make it a unique material. High-quality marble and ongoing maintenance provide an attractive, long-lasting marker. Low-quality marble lacking care is fraught with weakness against chemical and biological agents of weathering. Many times, we are left with no good choices when it comes to conservation.
By Don Hilton Geologist
The metamorphic rock type of slate was once a popular material for grave markers and is experiencing something of a resurgence in hand-carved monuments (and kitchen counters). In the more distant past, large pieces of slate often broke when shipped, so most historic slate markers are found near to the source rock. This results in heavy regional use. Slate’s chemistry makes it stable and mostly impervious to damage by acidic pollution and precipitation. On the other hand, its physical characteristics render it vulnerable to extreme damage by mechanical means and it can be difficult to conserve. Severe bio-damage by lichens and moss usually occurs only over a long period of time.
The word “slate” has had many meanings over the years, including things that aren’t really slate, at all. Our use of the word describes the metamorphosed form of very-fine-grained siltstones, mudstones, and shales. The degree of metamorphism determines, in large part, how slate behaves as a grave marker.
In all cases the pressure and heat of the metamorphic process changes the clay minerals in the parent rock to micas. These micas “foliate.” That is, they align themselves perpendicular to the direction of the metamorphic “squeezing” experienced by the source rock. The foliation controls the direction in which slate can be easily split (the root of the word slate is the Old French term esclate, meaning “split piece,” or “splinter”).
The metamorphism that creates slate does not always destroy the original structure of the parent, so a slate can have both bedding and foliation. Layering and fossils in the parent rock may still be found in an altered state within the resulting slate. If the metamorphism continues, the slate goes on to a more highly foliated rock called “phyllite” (fill-lite) which is rarely, if ever, used for monuments.
The typical geological forces that produce slate are widely regional, for example, the tectonic activity along almost the entire the eastern region of the United States (also tied to western England, Europe, and Africa). The U.S. also has slate deposits in its Superior region, along its extreme north-central border with Canada.
The historical use of slate is regional. Where available, it may comprise the majority of markers in a cemetery. In many of the oldest eastern U.S. graveyards, it’s used for almost all of the stones with monuments remaining wonderfully crisp and clear even after hundreds of years. The carving on older stones may be primitive or faint but still legible.
Nothing else really looks like slate. When in good shape it is smooth and usually gray but the color depends on the mineralogical makeup of its parent. Slate can be green, red, blue, brown, very light to dark gray to black. Rarely, it may be multiple colors. The rock does not polish to a shine, but the surface is typically “honed” to a silky-smooth finish. Slate’s ability to hold an edge is obvious in the beautifully fine lines of text and carving often seen on its surface (detail from a photo of the 1833 headstone of John Green. St. Edmunds, Warkton, Northamptonshire, England, by Marian Crenshaw Austin, 2018 – note, also, the green algae).
The edges of slate markers may be smooth, or display a ridged, cut, or sawn appearance, all related to the way the marker was created. Edge-finishing is often controlled by the time and/or location of the stone’s manufacture.
Slate may have inclusions of pyrite (pie-rite), an iron- and sulfur-rich mineral. In old markers these weather to rusty or metallic streaks along the face of the rock that may be impossible to clean from the surface.
Slate is quarried in large blocks that are cut to a manageable size and split, or “cleaved” along the plane of foliation. The surfaces exposed by the split become the front and back of the marker. The more pronounced the foliation, the easier it is to split the rock. The thickness to which the rock splits controls its ultimate fate. Rock that separates into thick layers may be used for flagstones or grave markers (or kitchen counters) while thinner-splitting material is more suitable for rooftops. In the extreme, slate can separate too finely to be of use.
This marked characteristic of separation along lines of foliation makes slate unsuitable for large monuments. The maximum height and width of a typical slate marker is measured in a few feet (< 1 m) and, unlike most other material, the thicker it is, the more susceptible to damage it becomes.
In some slate-rich areas it’s not unusual to find all, or just the most important part of old slate markers encased and “preserved” in the carved bed of a much-stronger rock, usually granite. The example shown is the 1730 marker of Hanna Lothrop, wife of Isaac (photo by Kerri Klein, 2018, Burial Hill, Plymouth, MA). It is not known how deep this marker goes, or how thick it is, but the abbreviated artwork at the top indicates that it’s not the entire stone. You also can’t tell if the vertical and horizontal cracks in the rock’s face (unusual in a slate) are the reason for, or the result of, its hard-rock helper.
Several neighboring stones may also be encased. Sometimes there is an obvious relationship, like a family group. Sometimes, maybe, it’s related to just how good the sales-pitch was.
Weaknesses of Slate:
The ability of slate to be cleaved along planes of foliation is what makes the stone attractive for use as grave markers. At the same time, this splitting is slate’s greatest weakness.
Unusually enough, it’s thought that sunlight is one of slate’s greatest enemies. The stone’s dark color absorbs bright sunlight and turns it into heat. Because the mica in slate is a poor transmitter of heat, sunshine causes highly uneven temperatures through the thickness of the rock. The heat-related swelling and subsequent contraction with cooling causes stress within the slate that cracks the rock vertically, parallel to its face, along natural planes of foliation. Water enters these cracks and typical mechanical weathering, along with freeze-thaw, and biological growth begins.
Heat-related damage is not restricted to hot climates. In a cold, snowy winter, the brightly sunlit face of a slate marker can be dozens of degrees hotter than its frigid backside.
Thinner slate markers of a couple inches or less (< 5cm) are the most stable because they heat and cool more evenly. Thicker, more substantial monuments are far more prone to heat-stress and delamination. In many cases it’s the well-to-do, who bought the biggest markers, whose stones are damaged the most. The first split is often near the middle of the thickness of the marker and the process works its way towards the edges. You sometimes see a thick marker that has been, in effect, delaminated into several thinner stones, looking like uneven slices of bread.
Keeping a thick slate marker safe from sunshine can be a tall order. Conserving a delaminating slate is complicated by the fact that any patch has to do four separate things; Stick to the slate, keep the water out, fill a large void, and remain flexible over time.
In some old cemeteries you may see slate markers capped with strips of lead or copper intended, in part, to “hold the stone together.” The ability of such metal caps to keep the slate from delaminating is questionable. Their application sometimes damages the stone and the forceful removal by vandals / scavengers certainly does.
You may see historical attempts to hold broken stones together. These usually involve drilling holes in the marker to attach iron or copper sheets (right) or “cramps” anchored with soft lead (left). Slate tolerates front to back efforts fairly well, as long as the repair isn’t made of a rustable material that swells, like iron. Almost any repair to the edge of the marker can start or aggravate delamination. (Wrap-around 2-inch (5cm) wide copper-sheet patches from a photo by Kerri Klein, 2018, Burial Hill, Plymouth, MA –and– detail of lead-anchored 4-inch (10cm) iron cramp and delamination / off-set from a photo of the 1803 marker of Grace Green, St. Edmunds. Warkton, Northamptonshire, England, by Marian Crenshaw Austin, 2018.)
Slate is much less likely than other rock types to show dirt so most people are less prone to scrape and scrub which is a good thing because the markers are like glass—relatively hard but fragile. Corners and edges are easily broken and its surfaces can be scratched. Besides large-scale delamination along foliation planes, some slate is prone to flaking along both its edges and faces. For this reason any scraping / scrubbing must be done gently and when possible, in a direction toward any cut edge. It might be best to skip such action all together.
The slightest touch with a metal implement, when scraping (NO!) or digging can mar the slate’s fine finish. This damage cannot be repaired in the field by most conservators. Slate is about 5.5 on the Mohs Hardness Scale, so it can be damaged by silica- / quartz-rich gravel.
Very foliated but otherwise sturdy markers can be subject to ground-water wicking. Any salt that find its way between the rock’s fine mica layers causes spalling (the small slate marker of Patience Ellis showing edge damage and surface spalling, photo by Kerri Klein, 2018, Burial Hill, Plymouth, MA).
As a general rule, heat-related damage starts at the center of the marker’s thickness and works its way out. Damage going the other way, from the edges to the center is caused by water or physical strikes.
Quality slate results in a marker that is amazingly thin for its height and width (some say it “rings” when lightly struck with the knuckles). The rock is amazingly strong when pushed or pulled to or from its face but it can be broken in that direction. The difficulty of repair depends not only on how clean the break is, but also on the size of the mica. Rocks with larger mica sheets can be difficult to epoxy because the mica easily shifts. Adding to the difficulty is temperature-related expansion and contraction. The rock is somewhat flexible. The epoxy, not so much.
The good news is that slate shows little in the way of chemical weakness. It stands up to acidic precipitation that would dismantle limestone and marble. It is also reasonably unappetizing to lichens. That’s not to say you don’t see bio-growth on the surface of slate markers, but the damage done by such growth is usually minimal, especially when compared to marble.
Bedding and Foliation:
The bedding that existed in the sedimentary parent may remain in slate. It can run in almost any direction through the marker and usually does not indicate zones of weakness. The more complete the metamorphism, the less obvious any bedding.
The alignment of internal mica plates always runs perpendicular to the squeezing that caused the metamorphism. This means that foliation (the way the rock actually splits) and the bedding (where the rock looks like it may split) have nothing to do with each other. They may match, they may not. It’s all up to the pressures applied during the formation of the stone.
A good, tight slate has little ability to transmit gas and liquid: a good thing because these markers are usually set in-ground. But all bets are off once the process of delamination begins. The infiltration of water and combination of freeze-thaw and bio-growth can completely dismantle a slate marker over a short period of time.
The metamorphic rock type of slate sees heavy regional use. Most such markers are found near to the source. Slate’s chemistry makes it stable and mostly impervious to damage by acidic precipitation. Its physical characteristics makes extreme damage by mechanical means highly likely. Thinner markers tend to do better than those that are thicker. Badly damaged and delaminated slate is difficult to conserve because any patching must fill a variety of roles.
Foliation diagram: http://itc.gsw.edu/faculty/bcarter/physgeol/metrx/agents.htm
Granite – Igneous Rocks Identification
By Don Hilton Geologist
You may wish to review the Rock Cycle article before reading this!
Old geology joke: “This rock sure is tough! See?” – “You don’t have to show me, I’ll take it for granite!” Crystalline igneous rocks are a popular choice for monuments. Attractive, widely available, and able to stand transport, they became widely used for modern grave markers once tooling grew strong enough to work them in a reasonable fashion. They are found on all continents, but Brazil and India dominate the market, particularly for darker stones. Resistant to weathering and mechanically strong, biological activity in the form of algae and lichens causes little concern, initially, but can damage the stone over the long haul if not kept in check.
Non-geoscientists misuse the word “granite” by applying it to almost any obviously strong, crystalline rock they see, usually modifying it with a color, as in pink-granite, white-granite, gray-granite, black-granite, and so on. Monument companies do much the same, but as can be seen from a simplified igneous rock classification chart, granite comprises only one of a number of igneous types that could be used as grave markers. The chart shows sharp divisions between the rock types, but nature doesn’t work that way. There are transitional forms between each type and there are fine-grained, near-surface versions that contain the same minerals, but in crystals too small to be seen.
If we were to speak as geologists, granite would be restricted to pinkish-to-red, but reasonably light-colored, fairly coarsely crystalline igneous rocks comprised mostly of quartz and feldspar with minor amounts of darker minerals. But, most of us don’t want to be mineralogists or geologists—and that’s okay! This article discusses the three most popular igneous rock types. A simple web search on the rock names, listed along the bottom of the chart shown, will give you some idea of the different types of markers you may see.
What all of these rocks have in common is that they were formed from completely liquid material, either original or recycled, that was placed and cooled deep within the earth. It’s thought that the temperature of the melted parent (from 1,300 to 2,200 F (700 to 1200 C)) and the rate of cooling controls both the type and size of the minerals found in any particular sample. These rocks are most often found in the cores of mountains, either present, or ancient and eroded, or in the very oldest “shield rocks” of the planet.
Identification; In General:
Crystalline igneous rocks are exceptionally hard. Elegantly wrought examples are rare in most cemeteries because of the cost and time it takes to create them. Even modern stones tend to be simple shapes that de-emphasize carving for extravagant finishes, engraving, and artwork.
Because of this toughness, older markers sometimes incorporate igneous rocks as a portion of a “hybrid” monument with carving on softer, less exotic stone. The one shown has a base of fine-grained, brown sandstone topped by a combination of dirty white marble and a 5-foot long, 10-inch diameter (1.5-meter, 15.4-cm) pink granite column (Lee monument, around 1872, section N, Westwood Cemetery, Oberlin, Ohio, photo by Don Hilton, 2018). The entire marker is about 9 feet (2.7 m) in height. The granite likely arrived as it was placed. Only the white marble is carved with text. Also, see how filthy the marble has grown over the years while the granite looks unaltered by time. Imagine how this marker would appear were it well-cleaned!
Identification; Granite (“gran-it”):
Granite’s bright pink to brown to red color and obvious interlocking crystals makes it one of the easiest igneous rocks to identify (detail of the photo, above). In the example shown, the lighter crystals are quartz and feldspars while the darker materials are micas and a mineral called amphibole (“AM-fib-bowl”). In large part, igneous rocks contain just a few, simple ingredients. The combination of them determines the rock type and its characteristic colors.
The mineral crystals here are easy to see but not very large. A common variation in granite markers is crystal size: from so small that you can barely see them, to larger crystals called “pegmatites” that are found in showier stones. Large pegmatites can be several inches to dozens of feet in size (30 cm to tens of meters). Crystals that extreme are rare in most cemeteries, but you will sometimes see a marker made of a single, very large quartz crystal.
The finish of an igneous rock changes its appearance. This is especially true for granites. A close-up of a polished and worked 14-inch (36-cm) diameter granite column shows how dramatically surface treatments change the look of the stone. Polished, the crystals are obvious. Worked, they are obscured. It almost seems like two different rocks. Look closely and you can see the lines left by the carver as he (she?) worked along the surface of the stone. As an aside, these marks can sometimes help you determine if the marker was hand-carved (uneven, perhaps curved with obvious strikes) or dressed by a machine (smooth, mechanical, and straight).
The granite to the right contains larger crystals and is much darker than the one above, the result of a higher percentage of biotite (a dark mica) and amphiboles. From a middling distance, the color of the stone is a rich chocolate-brown (Severance monument, around 1916, section H-L, Westwood Cemetery, Oberlin, Ohio, detail of photo by Don Hilton, 2018).
Granite’s natural surface looks very different from one that is either polished or worked. The example, left, with a top-to-bottom field of view of about 6 inches (15 cm), shows a broken and unworked granite. You can clearly see blocky, pegmatitic crystals of feldspar scattered throughout the rock. The color of the surface is somewhere between one that is polished and one that is worked but left rough (Bell monument, around 1988, section I-N, Westwood Cemetery, Oberlin, Ohio, detail of photo by Don Hilton, 2018).
Identification; Diorite (“DIE-or-ite”):
Granite’s more modest cousin, diorite, isn’t well known by its own name, but chances are good that you’ve walked right past it. Sometimes called “white-“ or “gray-granite,” its solid, light-to-dark gray face sometimes makes up entire sections of cemeteries (multiple monuments of diorite, Westwood Cemetery, Oberlin, Ohio, photo by Don Hilton, 2018).
Diorite’s heavy and massive aspect seems to make it a favorite of those creating markers that appear “half-finished”—said to be a representation of the transition of passing.
Like other igneous rocks, the earliest modern monuments use diorite in hybrid markers that incorporate it and a more easily-carved material, like limestone, marble or sandstone.
The marker shown to the right has a typically blocky diorite base in three sections totaling 5 feet (1.5 m) in height. The mourning figure, about as tall, is carved from a white marble. Diorite can be worked to the same degree of fineness, but the time and expense involved would discourage all but the most well-to-do (Platt monument, dates between 1823 and 1916, section A-A, Westwood Cemetery, Oberlin, Ohio, photo by Don Hilton, 2018).
Like granite, diorite is constructed of easily seen, interlocking crystals, it’s just that the pink feldspars are gone, leaving shades of white, gray, and black; like salt and pepper!
Diorite shows some change depending on how the surface is finished, but not to the degree of granite. In the close-up, the right side is polished. The left is worked and rough. A weathered diorite looks drab, gray, and solid, but close up, you can still see the salt and pepper crystals (Weaver monument, dates between 1903 and 1923, section N, Westwood Cemetery, Oberlin, Ohio, detail of photo by Don Hilton, 2018). The earliest diorite markers often have both polished and rough surfaces. Modern markers tend be polished. Those in-between are most often seen with a smooth but not shiny finish.
Identification; Gabbro (“GAB-row”):
With an unappetizing name like “gabbro,” it’s easy to understand why those who sell monuments usually call it “black-” or sometimes “green granite.” There are older grave marker that take advantage of gabbro’s elegantly severe character, but more modern stones display the rock’s dark drama and capacity to take a mirror-like polish to great effect. Gabbro’s popularity received a huge boost in the early 1980s when it was used for the Memorial Wall at the Vietnam Veterans Memorial in Washington, DC.
A close look reveals the familiar interlocking crystals we’ve seen in other igneous rocks, but besides a few lighter feldspars, they are the dark minerals: amphibole and pyroxene (“peh-ROCKS-seen” or “PIE-rocks-seen”).
See how, like granite, gabbro displays a strong contrast with finish types. Highly polished, it’s very dark. Worked smooth, but unpolished, it’s a much lighter shade of gray. This makes any lettering or design “pop” from the surface. These markers are almost always legible. The color contrast that makes the monument look like a gray-trimmed black tuxedo, almost always indicates a gabbro.
Completely natural surfaces are a color between light and dark, but the crystals are not always so obvious and the rock can be much darker, almost to the point of pitch black. (Heusner monument, with a diorite base, 6-foot total height (1.8 m), dates between 1883 and 1937, section E, Westwood Cemetery, Oberlin, Ohio, photo by Don Hilton, 2018).
Gabbro takes on a decidedly different color and texture as we move towards the next rock type.
The Swanson monument displays the dark and light contrast of a gabbro but also has a sort of grainy or fibrous appearance. A closer look shows us interlocking crystals, but the marker isn’t really black. It’s a black-green. And it looks sort of “smeared,” almost like the scales on a fish or serpent.
This is because the rock contains almost none of the lighter feldspars and is made of fairly large crystals of pyroxene and olivine (“olive-een”).
This stone, perhaps sold as a “serpentine granite” sits just about at the far edge of crystalline igneous rocks that can be used to create lasting grave markers (Swanson monument, 4-foot total height (1.2 m), section X, Westwood Cemetery, Oberlin, Ohio, photo by Don Hilton, 2018).
Identification; Peridotite (“per-ID-oh-tite”):
In a way, gravestones are orphans, pulled from their homes and forced to live in conditions they find hostile and foreign. Peridotites are a perfect example of this idea. Formed under extremely high temperatures, they are chemically unstable at surface conditions. Although there are makers sold as “peridotite monuments,” like maybe the Swanson stone, above, it is highly unlikely that you will ever really see one. Large, structurally sound blocks are almost non-existent because the rock’s main ingredient, olivine, reacts with water and transforms into softer minerals that are unsuitable for traditional grave markers. A stone that really is peridotite won’t stand the passage of time.
Weaknesses of Igneous Markers:
Typical crystalline igneous rocks are tough customers. Besides the softer micas, most of their individual minerals rate 5 or higher on the Mohs Hardness Scale. But that doesn’t mean they’re indestructible. They can be chipped by mechanical means (think poorly-steered lawn mowers). They lack the flexibility of other rocks and are fragile under tension or when cross-loaded. Long spans over uneven support may crack. Like all other stones, once water reaches its innards, the rock will yield to repeated cycles of freeze-thaw (Hall monument, dates 1915 and 1935, vertical field of view is about 2 feet (61 cm), entire structure is 11 feet long, 5 feet high, and 6 feet across (3.4 x 1.5 x 1.8 m), all of diorite, section M, Westwood Cemetery, Oberlin, Ohio, photo by Don Hilton, 2018).
Monuments cut from crystalline igneous rocks tend to be thick because thin slices can be brittle, particularly when the crystals are large. If not properly supported, thin slabs of pegmatitic rock are broken with amazing ease. When massively cut, igneous stone possess very high compressive strength which is why they are often used to build some of the largest and tallest monuments in a cemetery.
Rough or natural surfaces can host lichens and algae which do little harm to the stone, at first. With enough time, persistent lichen growth can reach between the rock’s crystals and begin to pull things apart. Bio-attackers enjoy mica-rich stones the best, not only for their relative softness, but for the elements those minerals hold.
Highly polished igneous rock surfaces are resistant to bio-growth. The variability in bio-hosting between the marker’s differently worked surfaces often creates very strange-looking stones: placing nearly pristine areas hard against others with heavy growth.
In this example, the clean granite surface is polished. The area hosting the lichens and algae is smooth, but unpolished. This phenomenon can be seen on gabbro, but is most widely found on granite markers which are more likely to host biological activity. Note also, how the marker above the bio-growth has dried from a recent rain while that below remains wet, thus creating a micro-climate that encourages the growth of more algae. (Martin monument, around 1965, section I-N, Westwood Cemetery, Oberlin, Ohio, detail of photo by Don Hilton, 2018).
While not a weakness in the traditional sense, crystalline igneous rocks are very dense. Even the smallest of such markers are often beyond human strength. The material has a fair tolerance for lifting straps and the like, but surfaces can be scratched and corners and edges chipped. Columns and obelisks can break if they fall. Patching a broken igneous marker is a challenge because it is, commonly, highly polished and has a distinct, multicolor nature.
Within a few human lifespans, crystalline igneous markers appear immune to acidic environments and precipitation. This is an illusion. The surface of the rock slowly alters. Darker stones may grow lighter and colorful markers may fade. Nothing lasts forever. At the same time, carvers say that such stones become harder with time, that a freshly quarried granite is much easier to work than one that has been exposed to the elements for a substantial number of years.
Because igneous gravestones stand the passage of time so much better than we do, it can be difficult to decide just when they were placed. A good conservator can look at the condition of a marker made of, say sandstone or marble, and judge on the basis of weathering and wear if it is the original or a replacement. That’s much harder to do with a marker made of igneous rock. Keep in mind that we had to wait until the mid- to late-1800s before the technology to reasonably work these very tough rocks came to be. It’s not just the dates—let the characteristics of the stone in total: its shape, decoration, and manner of being worked guide you!
Bedding, Crystal Alignment, & Grain:
Crystalline igneous rocks have no bedding. Any that existed is destroyed when the parent material is liquefied. Strictly speaking, there is supposed to be no alignment of crystals within an igneous rock—their placement is defined as being random.
In real life, it’s not unusual to find some orientation of crystals within an igneous marker. A high degree of non-randomness can point to flow of material during, or some metamorphism after, formation. Moderate levels of orientation can be difficult to see with the naked eye in rocks constructed of finer crystals.
Pegmatites make the task much easier. In the highly-polished example to the right, pegmatitic quartz and feldspars clearly show orientation and segregation by mineral type. Here, the pegmatites are 2 inches (5 cm) in length. (Heinzerling monument, around 1990, section I-N, Westwood Cemetery, Oberlin, Ohio, detail of photo by Don Hilton, 2018).
Orientation of crystals can create a “grain” that defines a direction of weakness in thinly-sliced rock. Like wood, it will break much easier parallel to the grain than across it—just ask anyone who installs granite countertops! Careful observation shows that markers are usually cut to accommodate strong crystal orientation. Countertops should be, too.
January 12, 2019 – CEMETERY CONSERVATORS FOR UNITED STANDARDS