Limestone treatments

[kboyd]

I have a simple gray limestone base that I would like to darken in some way so that it will better contrast with a white alabaster sculpture. Can anyone suggest a chemical or other treatment that would darken the limestone somewhat, yet maintain that grainy, limestone look?

[fritchie]

kboyd - My background is chemistry, as well as sculpture, so I may be one of the best qualified to give at least a theoretical answer, if not a practical one.

Limestone is a very soft stone, and chemically is more or less pure calcium carbonate. That means it will be attacked, or partially dissolved, by any acid, such as even vinegar. I’m not aware of any darkening agent that won’t attack the stone, so I would suggest a pigment (paint) of some sort, unpalatable as that may be.

Possibly a pigmented wax polish, such as shoe polish or furniture polish, might give the effect you want. Whatever you do, I would suggested you seal the limestone first, so the darkening is reversible. I’m not sure even how to seal limestone in a reversible manner. Perhaps a clear varnish , shellac, or contemporary polymer coating would do this job. If you do use such a seal, possibly a second, different coating with pigment added would get the effect you want.

Good question! I’m sure the solution will be of interest to many, so keep us informed on what you do.

[Sysiphus]

hello kboyd. i have done some stuff in limestone. depending on just how porous and what color you are attempting to acheive, you could try boiled linseed oil. i've used it on a couple of very grainy limestone sculptures. depending on how 'polished' the limestone is, this would give it a 'wet' look which is a bit darker. my limestone pieces are usually left outside - i like the aged look. you would definitely want to test this on a sample piece as the result is unpredictable. good luck.

[jvc stone]

If the stone is dark enough when wet, try using a "color enhanser" on it. This is a type of impregnating sealer for stone, and is used to darken the stone. We have also used things like tea, coffee, tobacco juice, and some other things not so mentionable to darken light colored limestone. As with any treatment, please test any and all suggestions prior to using on a finished piece, and be aware that what works for one stone will not be so effective on another.
John VanCamp / JVC Stoneworks

[kboyd]

Just remembered finally that I should post back about what I did on this project. I ended up just using a wood varnish, which did have a little beige-y sort of color to it, but it looked nice on the limestone -- didn't color it too much. It creates a good contrast with the alabaster. My only complaint is that it was kind of thick and goopy and so it didn't go on as smoothely as I would have hoped.