Yesterday I went for a social-distancing walk at Woodland Cemetery, visited a few repairs that we did last summer (all are holding up well!), and made note of a few gravestones that have suffered some new damage. One of these stones was one that was repaired a few years earlier by another team, using one of the pinning methods that is accepted as a safe conservation technique for historic gravestones.
Historically, gravestones that had pins in them were attached together using iron, lead, or copper. The issues with the iron pins is, of course, that they can easily rust the moment moisture gets into the space, which in turn will cause the stone to crack from the inside. This damage is difficult to repair, which is why we don’t use these anymore! In place of iron, stainless steel pins have been used in more recent decades to hold broken stones together, and more recently these are being replaced with fibreglass rods. However, these materials, while they don’t rust, they have their own issues.
When a historic ‘simple’ gravestone breaks, one of the only ways to repair the stone is by using pins and a stone epoxy. This is an invasive process, as it involves drilling into the marble, and applying material that is difficult to remove. While the goal of conservation is to preserve the object, and restoration is to return the object to its original state these processes are meant to be reversible, so the object can be returned to its original state. Restoration on gravestones is very difficult to reverse, especially when certain practices can cause more harm than good, the more we learn about them.
The main problem with stainless steel and fibreglass rods is that they are harder materials than the stone they are typically set into, stuff like marble, sandstone, or limestone. The pin is meant to provide support to the stone, provide stability, but not to hold the stone up if it is going to fall again. If the repair fails, what you want is to just have to do it again, but not to have the original repair, plus additional damage caused by said repair…which is exactly what happens if a gravestone with stainless steel or fibreglass pins fails.
When the repair fails, as it might if the epoxy doesn’t work, the wrong sealant is used, a deer hits it, etc, the weight of the gravestone falling puts pressure on the pin. Suddenly, instead of just adding some stability to the stone vertically, pressure is being put on the pin on an angle, pushing the pin into the surrounding stone. Because stainless steel or fibreglass pins are harder than the soft, weathering, cracking stone, this results in the stone itself breaking around the pin as it falls (see above images!). It causes the stone to spall with the pressure!
This is, obviously, not what we want. Extra structural damage makes it harder to stand the stone back up. One of the more recent changes to gravestone conservation has been to use wood pins instead of harder materials. They provide the same support to the repairs (I checked our repairs from last summer, they are all still bright, clean, and standing!), but if the repair fails, the weight of the stone will snap the wood pins, not causing any additional damage to the stone. We used wood dowels at Woodland, and they absolutely do break if you put too much pressure on them, because we broke several putting that gravestone together in the image on the right.
If the repair fails, the wood pins break, and even though they are basically glued in place with stone epoxy, they are easy to drill out. With the holes cleared again, it would be simple to redo the same repair. This is a more sympathetic conservation process than metal or fibreglass, and cheaper as well! It is very easy to get wood dowel at affordable rates, which makes this option more feasible for small sites having restoration undertaken (by a qualified professional, please!).
Basically, wood pins can cause less damage to historic gravestones if the repair fails, and are a more cost effective method of pinning. Conservation is a field which is constantly evolving as we learn more about what practices look like over time, their effects on objects, and how we can improve upon them. Now, the CCI (Canadian Conservation Institute) has some info on stone artifact conservation, does not have material specifically to deal with gravestones, so much of what we as monument conservators work with are adapted processes for stone conservation, specifically stone conservation in heritage structures/architecture (see the Engine Shed, Stirling). At the moment, this method of pinning seems to be the safest, so I’m happy to be able to share these images and information with you today!
(Always remember that repairs to historic gravestones should be undertaken by trained professionals with proper equipment, and should not be undertaken by volunteers without training from said professionals. Never undertake restoration without express permission of the cemetery/burial ground management)