Scraping marble which has been blackened or turned green by air and damp is dangerous to the design; whatever precautions may be taken, the work is always scratched more or less, and it is impossible to clean the carved parts without breaking the sculpture, or causing incongruities between the designs in relief and those which are sculptured. Soiled articles, which have not been tarnished by exposure to the open air, may be cleansed by potash water, then wash them in pure water, finish with water containing a dash of hydrochloric acid. Soap and water is often sufficient, spread on with a brush, and introduced into the sculptured parts by a somewhat stiff pencil.
(a) Take two parts of soda, one of pumice, and one of finely powdered chalk. Sift these through a fine sieve, and mix them into a paste with water. Hub this well all over the marble, and the stains will be removed; then wash it with soap and water, and a beautiful bright polish will be produced. (b) Clean with dilute muriatic acid, or warm soap and vinegar; afterwards heat a gallon of water, in which dissolve 1 1/2 lb. of potash; add 1 lb. of virgin wax, boiling the whole for half an hour, then allow it to cool, when the wax will float on the surface. Put the wax into a mortar and triturate it with a marble pestle, adding soft water to it until it forms a soft paste, which, laid on marble, and rubbed, when dry, with a woollen rag, gives a good polish.
Mix up a quantity of the strongest soap lees with quick-lime to the consistence of milk, and lay it on for 24 hours; clean it afterwards with soap and water. (See also ii. 125.)
Heat the edges of the marble before a strong, clear, charcoal fire, avoiding dust or smoke, until the marble is sufficiently hot to take small pieces of shellac. Then choose a sufficient number of thin pieces, of such a size as not to project above the surface of the marble, and apply them along the edge of each piece to he joined; but in such a manner, that the bits of lac on each piece of marble will come between those on the other. Then just before applying them together, a hot iron must be passed along each piece at a sufficient distance to fuse the lac, but not to make it run. The pieces of marble must be well forced together.
The slate having been reduced to a perfectly level surface, a coating of colour is applied according to the stone it is intended to imitate. For black, tar varnish is used with good effect. The slab is then thoroughly baked in an oven heated at 130° to 250° F., for 12 to 48 hours, according to size. The colours, say grey and white, are then floated on to the surface of a cistern of water, over which they float naturally into the shapes of the streaks of colour seen in marble. The slate, with its black ground now burnt in, is dipped into the surface of the water, and receives from it a thin coat of colour. The slate again has to go into the oven, and when sufficiently hardened, a coating of enamel is applied. Another baking to harden the enamel, and the slab is then pumiced to reduce it to a level surface. Baked again, it is once more pumiced, and this time goes into the oven with the pumice wet on its surface. If necessary this last operation is repeated. The slab is then ready for polishing, which is effected firstly by woollen cloths and fine sand, next by the finest and softest French merino, and lastly, by the hand and powdered rotten-stone. The dipping process is not applicable to imitations of all stones.