Stone (to use the more anthropological and less geological term, in the context of human usage) seems to be forever. That is what we imagine when we’re choosing our gravestones. The North American settler intention for burials is that when the grave plot is purchased, it belongs to that family in perpetuity. The grave it eternal, so that means so should be the marker, right?
Stone has another idea, though. Stone, like everything else on this planet has a birth, a lifespan, and a death. Yes, even the gravestone so lovingly chosen to be the last earthly reminder of an individual, shall too eventually give in to the forces of nature and crumble, crack, and fall (to paint a dramatic picture). While we, as monument conservators, are working to conserve the historic gravestones which have sunken, broken, or fallen out of their keys, we also recognize that sometimes the gravestone is too far gone. Restoration of these gravestones could do more harm to it than good, repairing missing sections, attempting to patch together crumbling pieces, obscuring text, and potentially cracking the remaining pieces in an effort to restore them.
Today we will be looking at the major players in the downfall of these gravestones in the form of different types of weathering. Weathering comes in many different shapes and sizes as natural processes, and can impact the lifespan of the gravestone directly.
The following terminology is based on the list developed by Dr. Inkpen (ND), and is summarized below with examples, and additional examples.
Sugaring: Mineral grains on the surface of the stone are coming loose. It will feel a bit like sand or sugar to the touch, and grains will fall. Letting takes on a more rounded appearance. Also called ‘granular disintegration’.
Flaking: Fragments of stone are detaching from the surface of the stone. This is particularly noticeable on sandstones. This is also known as ‘spalling’.
Blistering: The surface of the gravestone will appear raised or domed, and will sound hollow when tapped gently.
Contour Weathering: The entire surface of the gravestone breaks away in one sheet. It’s extremely dramatic!
Pitting: Depressions in the surface of the stone, caused by any form of weathering. These can be measured and compared in the future to track the rate of degradation.
Black Crust: One of the most distinctive forms of weathering is the infamous ‘black crust’ that forms on all types of stone. It often appears in more sheltered areas, such as below trees, the crust is comprised of calcium sulphate, a ‘crust formed by dry deposition’ (Inkpen ND). This crust forming usually indicates an accumulation of soot or other debris on the surface of the stone.
Organic forms / Biological Growth: The growth of lichen or moss on the surface of a grave stone often indicates that the area is moist. You can notice lichen growth on particular portions of a gravestone if the top is shaped in such a way that water pours down the face only in specific areas.
How does it happen?
But what causes a stone to fail, to die? The natural processes of weathering work in every corner of this planet, shaping mountains, pushing trees, changing what might otherwise seem impermeable. Weathering is the break-down of rocks while in situ (Tymon 2012). Weathering is often mixed up with erosion: Erosion There are three types of weathering that all effect the look, feel, and longevity of a gravestone:
– Chemical Weathering
– Physical Weathering
– Biological Weathering
“Processes of weathering are generally associated with particular types of gravestones or particular environments” (UCL 2019). This means that acid rain will eat away at limestone faster than granite, due to its high calcium carbonate content. Water collecting in the base of a softer stone may cause it to break in that area quicker, due to increased weathering in conjunction of the high moisture content. We will explore the major components of weathering below.
Chemical weathering is caused by chemical reactions between substances such as acid rain with the surface of the stone (Tymon 2012).
Hydrolysis, caused by rain which is acidic due to picking up carbon dioxide in the atmosphere. This reacts with minerals, causing them to break down and be washed away.
Oxidation is caused by iron in the stone oxidizing, or rusting, when water and air are present. This causes stains to the gravestone, and can often be seen as the orange/yellow/brown patina on sandstones, due to their mineral content.
Carbonation is caused by rain water reacting with minerals which contain carbonate. This is particularly an issue for calcium carbonate-heavy stones such as marble, limestone, and sandstones. If water gets inside the stone it can cause pieces to fall off the face of the gravestone as the minerals is eaten away.
There are two main types of physical weathering that you might notice while out and about in the cemetery: Expansion-contraction & Freeze-thaw.
Expansion-Contraction Weathering is also known as exfoliation or delimitation of the gravestone, and is caused by the expansion of the stone itself in the heat, and the contraction of the stone in cold. This is particularly an issue in modern cemeteries with sprinkler systems (NCPTT 2018).
Freeze-Thaw Weathering deals with water seeping into cracks in the stone and freezing, which causes the water to expand and can further crack or break the stone. This is especially noticeable in places that have temp extremes like..oh…Ontario?
There is also weathering by wind, which is a force against the stone but often combines with other weathering processes to react with the surface of the stone. Say the stone was weakened by chemical weathering, and then wind blows against the surface, blowing away those loose particles and cause the surface of the stone to alter. This is common, and you can often see more heavily weathered stones in windy, wet areas.
Biological weathering is defined as damage done to the gravestone as the result of plant life impacting the stone. It may be surprising to hear, but lichens, moss, and vines attach themselves to the stone and take nutrients out of the stone, such as calcium. This essentially means they eat into the surface of the stone, taking away physical pieces off and weakening the structure. The attachment points of moss and lichen are called rhizoids, which secrete acid which cause chemical breakdown of the materials, while ivy attaches with small root-like structures or suckers, and while the leaves can provide protection from the rain, they suck nutrients and therefore structural components from the stone itself (Tymon 2012). Have you ever seen a house that had vines removed, and it is now covered in streaks from where they were attached? That is because it was eating into the brick! (there will be a whole post on biological growth and weathering!)
There you have it, the story of weathering and how it causes the eventual breakdown of even the toughest of stones. If we didn’t have weathering and erosion, we wouldn’t have canyons, hoodoos, or the need for monument conservators!
While we try our best to conserve gravestones that can be helped, sometimes the gravestones have been at the mercy of weathering and erosion for far too long, and it would be detrimental to that stone to attempt to put it back together. It is a case by case basic, and when a gravestone has begun to rapidly deteriorate, like the example shown here, we do our best to display the stone and conserve it in its present state. This might involve limestone screening to keep it from sinking. When we restore a gravestone, we want to make sure it is the right action to take for the lifespan of that stone!
Weathering impacts all gravestones, no matter what conditions they are in. You can see the effects of weathering on many of our gravestones throughout Woodland Cemetery, on all types of stones. We are planning a post detailing biological weathering impacts, with an in-depth look at what plant life can do to gravestones, so keep your eyes peeled!
Thank you for following our progress so far this summer! Keep an eye out for more information on our walking tour, which will be held on July 6th, 2019.
Inkpen, Rob. ND. Gravestone Weathering. Gravestone weathering.
NCPTT. 2018. Gravestones Bite The Dust. National Centre for Preservation Technology and Training.
Tymon, Alison. 2012. Weathering Processes on Headstones and Monuments. West Yorkshire Geology Trust. Weathering Processes on Headstones and Monuments.
UCL. 2019. Gravestone Weathering. University College London. Gravestone weathering.