In the process of conserving a gravestone, it may become necessary to add infill, to replace or crumbling stone. Infill refers to a mix formulated to fill a void or gap, in masonry, and ideally to have it blend in with the rest of the stone, or substrate. Infill of various types has been used since antiquities to restore damaged sculpture, gravestones or masonry structures. Unfortunately few advances have really occurred in this field in the last few thousand years.
Stone infill most often consists of a mix or cement of some kind, sand, and pigment. A crushed stone may be added to help with coloration. The problem with infill is it will always be a patch. As they say, a patch is a patch is a patch. The best you can hope for is to match the color as closely as possible. This in itself is very problematic, due to the vast range in coloration even among the same stone type. In addition, stones weather in an outdoor environment at varying rates, with random soiling patterns.
When formulating a cementitious, (cement based), patching compound it must first be determined if the stone in question is going to be cleaned. If so, the patch must be accordingly. I have worked with many various patching compounds. Some are custom mixed to order, and can be quite expensive.
Jahn’s Restoration Mortar is considered one of the better products available in the field, but requires a special three day training seminar to enable ordering. A sample of the stone to be matched can be sent to the North American distributor, Cathedral Stone Products, and they will formulate a custom patching compound to order. Jahn’s Restoration Mortar does not contain any polymers or Portland cement. The biggest problem I had found encountered when working with Jahn’s, is regarding the darker colors. They tend to fade, and over time leave less then an ideal color match.
A two hundred year old French Lime producer, named St. Astier has recently entered the custom mortar patching field with a product called Lithimix. It may be custom mixed in a similar manner as the Jahn’s by sending them a sample. Lithimix contains little or no cement. Instead it is composed from Lime and pigments. It sets up very slowly, as you would expect from a lime mortar.
Edison Coatings also makes a custom patching compound. They prefer to include a polymer to strengthen the patch. The mortar is mixed with milk like liquid as tile setter use with grout. Some in the field refuse to use any polymers in the infill they install. I have not had enough experience to review the Edison patching product yet. I do know of others who have high regard for it. Recently Edison has started offering a new patching product based on Rosendale cement.
A Dutchman is a stone patch made out of stone. This is the ideal way to replace damaged stone on a historic masonry building, providing you can find a match to the original stone used in its construction. I have been asked; why not use this same technique.
When patching a gravestone or monument. In order to place a Dutchman stone first has to be cut away. In order to cut away a part of gravestones damage is likely to occur, and regardless, the original fabric will be diminished in the process.