Historically numerous substances have been used, in an attempt to make stones last longer when placed outdoors. Olive oil, whale oil, turpentine, and wax represent just a few of the liquids applied to masonry in a mostly vain effort to inhibit the future deterioration of stone.
When epoxy and fiberglass resins were invented in the mid-1900s, they were at first thought, to be ideal alternatives to naturally derived products for the preservation of masonry. Attempts were made to coat gravestones and sculptural elements and seal them off from the elements. As it turns out, sealing is not what was desired.
Anything placed outdoors must withstand a wide range of conditions in order to survive.
The thinking behind sealing a stone from the weather was simple, and seemingly flawless. If you could seal out the water, snow and ice, then nothing could penetrate the stone, so it would last forever.
Just as people need to breath so to do stones. If a stone is sealed, it will trap moisture inside along with soluble salts. When the moisture moves towards the surface during the evaporation process it carries a certain amount of mineral and salts along. This mineral migration accounts for a great degree of the case hardening often seen on softer stone types. Case hardening is when a harder protective crust forms on the outside of a stone, with a weakened interior beneath it.
If a stone is sealed the minerals and salts will still migrate towards the surface, but they will be unable to escape, and the stone will never dry out. This effect may continue without being noticed, but later may surface with a massive surface failure of the facade.
If that were not enough of a problem in itself, there is yet a bigger issue regarding tablet style gravestones. A tablet stone is monolithic, meaning it is one piece. It has to be partially underground by its very nature. A tablet stone acts exactly like a kerosene lantern. It will wick up what ever is underground. Through capillary action the buried part of the tablet will effectively be a wick. If the above ground section of a tablet stone is sealed, it will still wick up ground moisture and salts, but it will have no way to get out and evaporate.
Many materials historically used to construct gravestones and sculpture has not endured as well as their makers had advertised. Purveyors of marble during the 1800s claimed their products were permanent, and would stand the test of time. They are not fully to blame; there was no way to anticipate the havoc acid rain would reek on the built environment. But, even without acid rain, nothing lasts forever. Some types of stone just last much longer then others. The important thing to remember is the rate of deterioration is variable based on the stone type. The granites used today just degrade at a very slow rate.
Luckily there are a few products now produced, which can effectively treat and help protect some of the weakest and most decayed stones. Instead of sealing a stone completely, a consolidate penetrates deeply beneath the surface and re-bonds the molecular structure on a cellular level. It finds the voids and fills them. It is not a sealer as the stone will still breath, but it will limit the absorption of moisture to a slower rate, then that of an untreated stone.
Prosoco distributes a product called Conservair which is produced in Europe. Like much of the preservation filed they tend to be more advanced in there technologies in Europe, then we are here in America. Conservair is a stone consolidate which actually helps strengthen a friable or weakened stone. It is considered a stone strengthener and works great on sandstone. I have had very positive results treating and strengthening Connecticut’s Portland Brownstone, which is notorious for its crumbling and delaminating problems. It is applied in repeated treatment cycles, until the stone being is unable to absorb any more, or to the point of rejection.
With the use of a pre treatment it is now also possible to treat calcium carbonate based stones such as limestone and marble. This is a great step forward, as until recently there was really no effective way to treat and protect calcium carbonate based stones. Treating large numbers of marble gravestones, monuments and cemetery sculpture has thus far proved cost prohibitive, as most cemeteries and graveyards do not have adequate funding to undertake extensive preventive measures. It is my hope that as the technologies improve, consolidation products can be made more environmentally friendly, and less expensive.