It is feasible to mix the gum directly with the acid solution, but this mixture must be made fresh again each day, as otherwise it loses much of its value. The following points are important:-- _First_: If the grease remain long on a stone that, though prepared, has lost its coating of gum, it will penetrate the surface, and according to its amount and fluidity, will sink more or less into the stone, which will retain its polish on the surface but become more inclined to take dirt. It is better, therefore, to leave a small amount of gum coating on the stone in such cases. _Second_: As only the extreme outer surface of the stone is prepared by the gum, and this is rubbed away gradually by wiping during the printing, so in the same proportion of wear and tear the original preparation would become lost, if it were not renewed from time to time, that is, if the stone were not again coated with gum. Twice a day, however, is enough. _Third_: Because of this susceptibility of the surface to injury, a prepared stone must not be rubbed strongly with fatty material, because this damages the surface and the stone would readily soak up the fat. _Fourth_: If a prepared plate is totally denuded of gum, and has been dry for a time, especially if it has already lost a part of the preparation through printing, it will incline very much to take color and smut. Therefore, when it is necessary to stop printing, it is well to coat the plate at once with gum, but only with a very thin coat. If this has not been done, and it is desired to use the plate again, great care must be taken to wet it with the very purest water, or, better still, with diluted etching fluid, for instance one part aquafortis to five hundred parts of water; and then to coat it with gum. To neglect this precaution may cause the total ruin of the plate. For safe-keeping of the plates, if they are to be used again for printing, the coating with gum is, therefore, absolutely necessary. _Fifth_: Gum can prepare only a thoroughly clean stone or one properly etched. Therefore, if the surface of the stone has even the least trace of grease, it will take color, no matter how thickly it may be coated with gum. On this fact is based the method of transferring copper-plate impressions and other printed subjects, as will be described later. _Sixth_: If the stone has had fat on its surface, and this fat has been etched away again, the power of the gum asserts itself, and the stone will be thoroughly prepared even if the fat has soaked considerably into the body of the stone.