The latter operation is not only very difficult but very slow, particularly when the painting possesses artistic qualities that must not be impaired by the removal of the varnish. This first operation successfully accomplished, the artist steps in and proceeds to help the work, say of such and such a school, to resemble the painting of the master of this school as much as possible. The process is naturally executed by the aid of a more or less complete collection of photographs of the work of the master the faker intends to imitate. The retouching may follow the most varied methods. To take the most common case, that of oil painting, the new work can be carried out with oil colours previously kept on blotting-paper to drain off the oil which is then substituted with turpentine to give the colours their lost fluidity; it may also be effected with tempera colours or with colours the fluid element of which consists only of varnish. The use of tempera is preferred by restorers because, although it presents the extreme difficulty of changing hue when varnished and consequently demands no little experience to judge the requisite hue or tone, still once laid down it is not likely to change with time as oil retouching on old paintings generally does. The mixing of colour with varnish alone has the advantage of keeping the proper tone from beginning to end. This method is extremely useful not only in the painting of missing parts but also to veil and tone what has been painted in tempera if this is not entirely harmonious with the rest after varnishing. Needless to add, those colours the fluid part of which is supplied by varnish are unalterable as they do not contain any oil whatever. One of the difficulties in handling these pigments is the lack of fluidity, hence turpentine may be added with advantage. However, as the above methods of retouching are not proof against chemical tests, alcohol being the proper solvent with which to do away with added touches to old paintings which have been done with either oil or varnish colours, the shrewder fakers either mix amber varnish with the colours or give the fresh touches a solid coating of this varnish, which when well prepared is supposed to be insoluble and not easily acted upon by solvents. Although more than one special work on the art of restoring gives recipes for the preparation of this varnish, in practice very few know how to prepare it in the proper way. We have here presupposed that the picture was in good order, that there were no missing parts of importance, or rather that, with panel or canvas unimpaired, the work only required to be retouched by the artist, a rare case, as when the paint has vanished the preparation of the panel or canvas has generally vanished with it, on account of its adhesiveness.