Repair and restoration of fine art is a specialized and very technical field, but artists often get asked to do it, and frequently need to make minor repairs to their own work. When working with stone, as far as possible, always test the proposed repair procedure first on a sample of the same stone-- glues and fillers can give surprising results for many reasons, including the porosity and translucence of the particular stone.
Repair techniques for interior and exterior stone are quite different from repairs to marble. Exterior stone is subject to precipitation, large changes in temperature, and a variety of chemicals, both organic and inorganic. In addition to this, non-igneous stones are permeable, with water carrying various dissolved salts and minerals passing in and out of the stone. For this reason, exterior stone is usually repaired with lime-based cements and grouts, which tend to have properties more like stone. These cements and mortars are also designed to be physically weaker than the substrate they are repairing, so that future repairs are not made more difficult. Epoxy and acrylic based cements tend to be impermeable to water, resulting in a variety of destructive dynamics.
Even completely sound marble is very fragile--it breaks as easily as a clay pot. Pushing a chisel into a tight space is a common cause of breakage.
Marble and other carbonate stones are also prone many kinds of flaws that may or may not be visible on the surface. There can be a single well defined cleavage line; bands of weak crystallization; inclusions of foreign matter; or regions that are riddled with invisible cracks. Inspect the stone very carefully before starting, wetting the surface with a spray bottle as you go, to find any flaws that reach the surface. Even otherwise invisible cracks will suck in water, while the rest of the stone has a sheen. Weak spots can sometimes be spotted by slight milky tone, or by an area of the surface that absorbs dampness. Just as one can hear that a bowl or plate is cracked, tapping on the stone and listening for irregularities in the tone can reveal flaws.
Minor detached pieces can often be simply glued back on with cyanoacrylate "super-glue.' These glues work well for breaks that mate perfectly, and leave almost no glue line. Make sure the broken surfaces are absolutely free of any dust or loose chunks that may interfere with a perfect fit. The attached piece should be clamped lightly in place overnight. The clamp should be elastic pressure like rubber bands. It does not require a great deal of pressure, but it is important that the pressure be even.
The glue will stick quickly, but it won't be truly set hard for a while. A tight joint in an area that has not had a final sanding and polishing will be all but invisible when the surface is finished. Don't use a chisel on the repaired piece. Try to limit work on the repaired area to filing and sanding.
For worse injuries, where some stone is missing, filling can be used. In former times, wax and various lime-based plasters were used as fillers. Modern materials give better results, but there are several considerations. Fillers must match not only in color, but in translucence, luster, etc.
The first major choice is between an organic and an inorganic binder. The organic binders are typically epoxies, polyesters, or other resins that form a matrix to bind together particles of bulking agent pigments. They may harden by either evaporation, e.g., methacrylate in acetone; catalyzed reactions, e.g., epoxy and polyester; polymerization, e.g., acrylic modeling paste; or thermosetting , e.g., polymethyl methacrylate (PMMA) resin. For outdoor use it is important to confirm that the resin is light-fast, as some resins are subject to yellowing and/or deterioration in sunlight. Use resins formulated for art repair--hardware store epoxies are risky, because for one thing, they are not necessarily light fast.
The inorganic binders are typically plaster-like, being based on gypsum plasters like plaster-of-Paris or Hydrocal, or or lime cements, like white Portland cement. The inorganic binders are usually used in architectural and other outdoor restorations.
Many kinds of filler materials can be used, depending upon the stone and the nature of the repair. Dust ground from the same stone is the obvious base for a filler, but probably will not match particularly well, because of refractive properties of the plastic binder, and the altered physical structure of of the mineral itself. Even stone from the original will probably have to be tuned up with other additives to adjust the appearance of the filler. The mix may include fumed silica, glass microspheres and micro balloons, chalk, titanium white and other inorganic pigments, and pulverized natural minerals such as mica, to provide a variegated appearance. There is no one recipe--every stone will require experimentation to get a good match. For consistency of test and final result, unless you are weighing your ingredients with Heisenberg-like precision, as you get close to a match, make the full batch, mixing it all very carefully, and adjusting the entire batch, testing only a sample.
For repairs to marble, organic binders will usually be chosen, usually epoxy or polyester. A transparent organic binder will probably also be best for for granite and any other hard stones that have visible structure. For limestone and other opaque stones, you may do better with a lime-based filler.
Translucent fillers like micro balloons will work better for marble and other translucent stones; more opaque fillers such as fumed-silica and chalk may not give the needed translucence. By the same token, opaque fillers should work better for limestone. For granite it may be necessary to add macroscopic aggregates to simulate the original. Eleanora Nagy's [nagy 98] laboratory report on the issues provides a good overview of the properties of various fillers.
No matter what the choice, finish sample repair all the way to completion, including polishing. Color differences, differences in the transparency, and other properties of fillers and substrates may not show up until the final polishing. For very small repairs, acrylic modeling paste works well.
Epoxies and polyesters may soak into the surrounding stone leaving a band of discoloration. To prevent this, you can seal the boundaries of the fill with a non-migrating sealer such as Paraloid B-72. Test the sealer too, before applying it in a visible area.
For major repairs, particularly to outdoor work, the elastic properties of the patch and resistance to cycles of freeze and thaw, etc., may be important. These concerns are beyond the scope of these notes and must be researched in the literature it they apply.
Conserving, restoring, and repairing art is a fast-evolving field, and techniques and materials improve constantly. Therefore, modern conservators are concerned that repairs be reversible. For this reason, it is advisable that the repair material be physically weaker than the substrate so that it can be removed mechanically. This may or may not be of concern to the working artist. Griswold and Uricheck [Griswold 98] give an excellent overview of the issues. CathedralStone, in Hanover Maryland, sells a wide variety of high-tech materials for repair and stabilization of stone sculpture, particularly exterior work.
For large pieces, say, a head separated at the neck, you may need to to insert one or more metal pins. Use steel, rather than brass or aluminum dowels, because the rate at which steel expands and contracts with temperature change most closely matches that of marble. For outdoor use stainless steel or titanium may be more suitable.
Slight differences in the expansion rate of the two materials, measured by a quantity called the "coefficient of thermal expansion" (CTE) will be absorbed by the elasticity of the stone and adhesive, but combining materials with CTEs that are too different could result in opening the break if the piece is later exposed to extremes of temperature. Corroding iron can expand enough to break the surrounding stone.
It is difficult to get a precise alignment of the holes because of the irregular surfaces; fortunately, perfect precision not really necessary. To locate the holes, drill tiny pilot-holes first in the less easily moved piece. A nail with the head clipped off is set in the hole with the point sticking up very slightly. Align the other piece and press down to make a mark. Now drill the real holes, aligning them by eye as well as you can.
The holes in what will be the bottom when you reassemble it should be a little bigger than the dowel to allow a loose fit, while the holes in the top can be the right size.
Test fit with dowels in place to insure that the alignment is good. If necessary, enlarge the hole that will be on the bottom when you make the final assembly. This need not be the on the bottom with respect to ultimate orientation.
Fix the dowels in the upper piece (the snug fit) first. Be sure you have wetted both surfaces and leave no epoxy pushed up above the surface to interfere with a perfect fit.
Let the epoxy cure. This will prevent the epoxy from draining out when you do the final assembly. Unlike many glues, epoxy fills gaps well, hardening into a tough, rigid plastic.
Re-test to make sure the new fixed alignment still allows the pieces to come together perfectly. You may still need to ream out the hole(s) slightly and blow them clean again. Make sure everything is prepared in advance--the work piece should be blocked up solidly in a convenient position, sandbags or other blocking should be handy for securely aligning the piece to be attached.
Don't try to get this stuff together after you mix the epoxy!
When you're ready to re-attach the piece, wet both surfaces of the dowel and hole with enough epoxy to fill the spaces, but not enough to gush out a large volume. Wet the surfaces of the stone that will touch. You want the bottom hole to fill, but not overflow, when you let the pieces come all the way together. As you set the piece down slowly, add or remove epoxy as necessary with a small brush as the pieces close. Epoxy does not leave the invisibly thin glue line of cyanoacrylate glue. If possible, it is best to use cyanoacrylate cement to glue the surfaces and reserve the epoxy for seating the dowel. If epoxy is to be used to adhere the surfaces, it must be a filled epoxy. Pure transparent epoxy filling the gap will show as a black line, because you can see into crack. Filler diminishes this effect, but may not eliminate it entirely, so a real test requires joining sample pieces, not just preparing dabs for a color test. Missing chips at the edge of the repair can be filled. Leave a little above the surface to shave down, but beware that the filler and the stone will not be equally hard, so care must be taken when finishing the surface.
Marble and limestone are porous, absorbing and transmitting liquids and gasses like a sponge. And like a sponge, it's difficult to remove every trace, once a stain or pollutant has penetrated.
Marble should be cleaned with cotton balls, slightly dampened with distilled water and a 2% mixture of non-ionic detergent. Non-ionic detergents, such as Kodak Photo Flo are used because they are of neutral pH, and thus do not produce salts when in contact with minerals. They are effective, yet low-foaming.
A small amount of artists-grade mineral turpentine, a.k.a, white solvent, a.k.a. naphtha, can be added. Do not use "gum" turpentine, which is a very different kind of solvent made from pine sap. This solvent acts as a degreaser, removing oils and waxes that are too resistant for the detergent. Older pieces can have a patina that should not be disturbed--wipe gently, rather than scrub, and rinse repeatedly.
Don't use any other household cleaning products, as they often contain either abrasives or chemicals that are bad for the stone. Ajax and similar cleansers contain powdered quartz which is much harder than marble, and will literally grind away the surface. Even Bon Ami contains feldspar, which is softer than quartz, but still harder than marble.
Removal of stains should not be attempted without expert knowledge. Chemicals capable of bleaching out the stain are likely to be destructive, and chemicals that dissolve the stain may also move and spread it. Resistant dirt and stains can be removed with "poultices", i.e., pastes of absorbent clay and other materials to soften and draw out stains. Prolonged exposure to bright sunlight will bleach out many stains naturally.
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