Shared here with permission from Mr. Michael Trinkley, Director of the Chicora Foundation, Inc. - his statements in a discussion of the subject requesting more information about using the most favorable adhesives for repairing tombstones.
*****As a reminder, if you are not sure, please consult with a professional who has hands-on experience and training using materials approved by the Association for Gravestone Studies.
Director at Chicora Foundation, Inc.
April 4, 2013 - "Linkedin" - "Gravestone and Monument Preservation" Group:
"There are a few
epoxies that are UV stable and used in conservation. Most are used by
objects conservators for repair of clear glass where the least bit of
yellowing is immediately noticable (and the object is clear, so
yellowing will always be seen). Such epoxies, however, are very
expensive and not used (to my knowledge) by any stone conservator.
There are virtually no epoxies commonly used in stone conservation that are UV stable. This, however, isn't a big deal since epoxies for stone should never be visible and hence never exposed to UV light. I don't recommend tinting and infilling with epoxy. While this is done by some commercial monument companies for granite (who presumably don't know better) and often done by kitchen counter companies (where it really doesn't matter), it is a poor match for stone exposed to the elements and will eventually fail. An appropriate infill must have a strength matched to the stone (and no greater); it must be breathable; and it must contain no synthetic polymers. Epoxy fails at each turn.
In addition, the effort to "glue" broken stone together is likewise doomed to failure in most cases. Yes, it is simple; but like most simple solutions to complex problems it is inappropriate and frought with problems. For more, read http://www.chicora.org/pdfs/AGS%20Conservation%20Talk%20-%20Simple%20Epoxy%20Repairs.pdf . For most breaks, good, long-term repairs require pinning.
Where epoxy is appropriate there are several choices that must consistently be made. First, do you need a hi-mod or low-mod epoxy. Mod, very basically, refers to strength. Second, most used in stone conservation will be moisture insensitive, meaning that the stone need not be perfectly dry, although no epoxy can be applied where there is standing or puddling water and acetone or alcohol should be used to both dry the stone and remove grease as well as stone dust. Third, a suitable viscosity should be chosen that prevents the epoxy from running (especially if you ignore this advice and insist on "gluing" stones together). Another consideration is where a thickner is needed, even if a "knife-grade" epoxy is used. As for a specific brand, there are many suitable. Each conservator will have personal choices, but that shouldn't be taken as a recommendation. I do not recommend fast-curing epoxies. They tend to allow no mistakes and, more importantly, they tend to be far weaker than epoxies with longer cure times.
Polyesters are unsuitable for most exterior use, not only for the reasons cited earlier, but also because they tend to swell in areas of high moisture (meaning, perhaps, they may be more suitable for the arid southwest, although I'd stick to epoxies.
We've had this discussion before, but "cookbook" approaches to conservation rarely (perhaps never) work. Each object is different and conservation ethics require that each object be reviewed and decisions regarding treatment made based on the specifics of that particular situation.
There are virtually no epoxies commonly used in stone conservation that are UV stable. This, however, isn't a big deal since epoxies for stone should never be visible and hence never exposed to UV light. I don't recommend tinting and infilling with epoxy. While this is done by some commercial monument companies for granite (who presumably don't know better) and often done by kitchen counter companies (where it really doesn't matter), it is a poor match for stone exposed to the elements and will eventually fail. An appropriate infill must have a strength matched to the stone (and no greater); it must be breathable; and it must contain no synthetic polymers. Epoxy fails at each turn.
In addition, the effort to "glue" broken stone together is likewise doomed to failure in most cases. Yes, it is simple; but like most simple solutions to complex problems it is inappropriate and frought with problems. For more, read http://www.chicora.org/pdfs/AGS%20Conservation%20Talk%20-%20Simple%20Epoxy%20Repairs.pdf . For most breaks, good, long-term repairs require pinning.
Where epoxy is appropriate there are several choices that must consistently be made. First, do you need a hi-mod or low-mod epoxy. Mod, very basically, refers to strength. Second, most used in stone conservation will be moisture insensitive, meaning that the stone need not be perfectly dry, although no epoxy can be applied where there is standing or puddling water and acetone or alcohol should be used to both dry the stone and remove grease as well as stone dust. Third, a suitable viscosity should be chosen that prevents the epoxy from running (especially if you ignore this advice and insist on "gluing" stones together). Another consideration is where a thickner is needed, even if a "knife-grade" epoxy is used. As for a specific brand, there are many suitable. Each conservator will have personal choices, but that shouldn't be taken as a recommendation. I do not recommend fast-curing epoxies. They tend to allow no mistakes and, more importantly, they tend to be far weaker than epoxies with longer cure times.
Polyesters are unsuitable for most exterior use, not only for the reasons cited earlier, but also because they tend to swell in areas of high moisture (meaning, perhaps, they may be more suitable for the arid southwest, although I'd stick to epoxies.
We've had this discussion before, but "cookbook" approaches to conservation rarely (perhaps never) work. Each object is different and conservation ethics require that each object be reviewed and decisions regarding treatment made based on the specifics of that particular situation.
There's another AGS column that you might be interested in, at:
AGS is a great organization and, if you aren't already a member, I encourage you to join."
