Solution of nitrate of silver penetrates marble deeply, communicating to it a deep red colour. Solution of nitro-muriate of gold produces a very fine violet colour. Solution of verdigris penetrates marble 1/12 inch, giving a fine light green colour. Solutions of gum dragon and of gamboge also penetrate it; the first produces a fine red, and the second a yellow colour. To cause these two substances to penetrate deeply, the marble should first be well polished with pumice, after which the substances should be dissolved in warm alcohol, and applied with a small brush. All the wood dyes made with alcohol penetrate marbte deeply. Tincture of cochineal, prepared in this manner, with the addition of a little alum, gives a fine scarlet colour to the marble, penetrating it 1/5 inch. Artificial orpi-ment, dissolved in ammonia and laid on marble with a brush, quickly produces a yellow colour, which becomes more brilliant when exposed to the air. To all the substances employed, add white wax; this, when placed on the marble in a melted state, soon penetrates it.
If the verdigris is boiled in wax, and then laid melted upon the marble, it will be seen on its removal, when cold, that the design has penetrated the surface to the depth of 1/3 to 1/2 inch.
When several colours are to be successively used without blending them, proceed in the following manner. The dyes obtained by spirits of wine and the oil of turpentine should be laid on the marble when it is heated, particularly in the execution of delicate designs, but the dragon's-blood and gamboge may be used cold. For this they must be dissolved in alcohol, and the gamboge used first; the solution of this gum is quite clear, but soon becomes troubled and gives a yellow precipitate, which is used to obtain a brighter colour. The lines drawn by this solution are then heated by passing a chafing dish filled with lighted charcoal closely over the surface of the marble. It is then left to cool, after which the lines which have not been penetrated by the colour are heated in the same manner. When the yellow colouring has been applied, the solution of dragon's-blood, which should be concentrated as much as possible, is employed in the same manner as the gamboge; and while the marble is warm, the other vegetable tints which do not require so strong a degree of heat, may also be applied.
The design is completed by the colours mixed with wax, which should be applied with the utmost care, as the slightest excess of heat will cause them to spread, for which reason they are less suited to delicate designs. In colouring marble, the pieces should be well polished, and free from any spots or veins. The harder the marble, the better it supports the heat necessary to the operation; alabaster and common soft white marble are not suitable for the purpose. Marble should never be heated to a red heat, as the fire then alters the texture, burns the colours, and destroys their beauty. Too slight a degree of heat is also bad; for though the marble takes the colour, it does not retain it well, and is not penetrated deeply enough. There are some colours which it will take when cold, but these never fix so well as when heat is employed. The proper heat is that which, without reddening the marble, is intense enough to cause the liquor which is on its surface to boil. The menstrua which are used to incorporate the colours should be varied according to the nature of the colour employed; a mixture made with urine mixed with 4 parts of quick-lime and 1 of potash, is excellent for certain colours, common lye of wood ashes is good for others; for some, spirits of wine, others require oily liquors, or common white wine.
The colours which succeed best with the different menstrua are the following; blue-stone dissolved in six times its quantity of spirits of wine or urine, and litmus dissolved in a lye of pearlash; the extract of saffron and sap green succeed very well when dissolved in urine or quick-lime, and tolerably in spirits of wine. Vermilion and cochineal dissolve well in the same liquids. For dragon's-blood use spirits of wine, which is also used for Cam-peachy wood. For alkanet-root the only menstruum is turpentine. Dragon's-blood in tears gives a beautiful colour when mixed with urine alone. Besides these mixtures, certain colours can be put on dry and unmixed; such as the purest dragon's-blood for the red, gamboge for the yellow, green wax for a kind of green, common sulphur, pitch, and turpentine, for a brown colour. For all these the marble must be considerably heated, and the dry colours then rubbed upon the block. A beautiful golden colour is produced by equal quantities of the crude salts of ammonia, of vitriol, and of verdigris, the white vitriol is the best for this purpose. Grind these together, and reduce them all to a very fine powder.
All the shades of red and yellow may be given to the marble with the solutions of dragon's-blood and gamboge, by reducing these gums to powder and grinding them with spirits of wine in a glass mortar. When only a little is required, mix one of these powders with spirits of wine in a silver spoon, and hold it over a heated brazier; this extracts a fine colour, and, by dipping a small brush in it, the finest veins may be made upon the cold marble. By adding a little pitch to the colouring, a black shade, or all the varieties of dark red, can be given. Archil diluted in water and applied when cold to the marble gives it a beautiful blue colour; by putting on the colouring in proportion as it dries, it becomes very fine in less than 24 hours, and penetrates deeply. If the paste of archil is used, which is a preparation of the plant with lime and fermented urine, the colour obtained Will be more of a violet than blue; to obtain a perfect blue it must be diluted in lemon juice; this acid will not injure the marble, as it has been weakened by its action upon the archil. Large blue veins may thus be formed upon white marble; but as this colour is apt to spread, it will not be exact unless the coloured parts are instantly touched with dragon's blood, wax, or gamboge, which checks it.