In the case of marble, stone or other hard material that has to be chiselled, the faker generally starts his work along the lines of the sculptor, that is to say, he models the original in clay, casts it in plaster and transfers it to the marble by the usual methods. Then when this artistic part has been accomplished successfully, the marble or stone must be given the appearance of antiquity and the patina belonging to age. This is generally effected by two distinct operations, one relating to the form, the other to the colour and the whole peculiar harmonization of tone and polish called patina. As regards the form, modern sculpture being somewhat too precise and sharp-edged, the chief aim of the operation is to destroy these qualities, as well as to confer upon the object the abuse that is supposed to be traced upon an antique during its long pilgrimage through the ages. The marble is therefore skilfully chipped here and there with mallet and chisel, sand and acid are applied to dull the over-sharp tooling, and sometimes to cause corrosion, etc. The principle accepted, it is easy to understand that ways of ageing sculpture are multiplied, and vary according to the illusion the faker intends to convey. The fact that old Greek and Roman work is not identical with Renaissance productions in appearance, as the former are generally excavated while the latter come down to us through a long succession of owners, is sufficient to show that there are slight differences which must be taken into consideration. For colouring marble and stone, a general tone is usually given at first which is intended to destroy the crudeness of the new material, especially in the case of marble. One of the most common ways is to wash the object with water containing a certain quantity of green vitriol. When applied before the stone has lost its permeability, this solution penetrates deeply, particularly in marble, and the colouring is not easily destroyed or washed out by long exposure to atmospheric action. Some use nitrate of silver also when a different hue is to be given, but the solution mentioned first, which confers the proper ivory tone to the marble, is the most common. Naturally, a tone given by these means is too uniform and monotonous to be taken for the colouring of old age, so the artist calls his talent and experience into play to produce the desired variation; there is, in fact, no other teaching but experience and taste. It is to be noted that in the colouring of stone, and particularly marble, the artist has an almost complete palette at his disposal, for in this branch chemistry supplies nearly every hue possible.