The great enemy of stonework is salt. In Egypt this permeates the soil so that nothing is free from it; and any object near the surface has much salt accumulated in it by evaporation. The effect of salt is to disintegrate the stone, and make it flake or fall away in powder. If there is the faintest taste of salt on a stone slab it should be laid to dry, face down, on the ground; for I have seen a fine block of sculpture entirely destroyed by being left for a single day face upward. When the stone is once dry it is safe in Egypt, but in a damp country it may begin a course of slow destruction by continual recrystallization of salt. Sculptures have been entirely wrecked by being cemented into the wall of a museum; the wet of the cement brought all the salt to the face and ruined it. The only treatment for salt in stone or any other material is long soaking in water. If a canal is at hand, stones may be sunk in it for some weeks, face down. Or barrels or zinc trays may be used, and the water changed every two or three days, for five or six times. After such soaking the stone must be left to dry face down, so that all the remaining salt will come out on the back. Where there is not much salt it would be best to lay the stone back upwards to dry, brush off any salt which comes out, and then wet the ground below, so that more water may be drawn up to evaporate on the back. If this was continued until no salt appeared the stone would be cleaned, and the face could not be injured. Sometimes a face is already flaking, and then the stone must be kept quite flat in soaking and drying, so that each flake will be left in place, and can be stuck down afterwards. Granite is often entirely disintegrated into separate crystals, if it has lain near the surface. It is then even impossible to turn the block over to copy it, as there is no cohesion left in the mass. The only salvation possible for such a block would be to make a thick plaster or cement coat to the exposed parts, under cut, and turn the whole over with a board beneath it, and then saturate it hot with paraffin wax.
The face of limestone is often in tender condition, and will not bear wet brushing to clean it. Dry picking and brushing is then the only resource. If long exposed to damp, limestone dissolves throughout the body of it, so that it becomes spongy, and like putty with the contained water. A large sarcophagus lid in this state at Denderah was brought up to the house, then covered with 3 or 4 inches of sand, and left to dry slowly for some weeks; otherwise it would have cracked to chips by contraction on the face. When quite dry it was very porous, but in safe state for copying and transport. I have seen a slab of limestone in perfect condition, reduced to a shapeless paste by a few minutes of sharp rain.
The original stucco facing often remains on limestone, and also the colour. If the carving has been fine it is best to remove the stucco, which is generally much less detailed. But if the stucco is an improvement on the carving, and especially if there is colour, it must be preserved. This is best done by fixing it with thin tapioca water, just so thick that it will soak into the stone without leaving any glair when dry. This treatment also does for limestone with a rotten face.
The same tapioca water may be used for fixing colours on stucco, as I did on the Tell el Amarna pavement ([Fig. 48]); and the thickness must be graduated to the porosity, so that it will just soak entirely into the material. Any film left on the face will peel away.
Frescoes, Tell el Amarna.
Fig. 48. Plants and animals.
Frescoes, Tell el Amarna.
Fig. 49. The two princesses.