76. =Sandpaper.=--As regards other mechanical means, be distrustful of tints obtained by rubbing the copper with sandpaper; these tints generally show in the proof as muddy spots, and are wanting in freshness. Avoid the process, because of its difficulty of application. Only a very skilful engraver can put it to good uses.
77. =Sulphur Tints.=--I shall be less afraid to see you make use of _flowers of sulphur_ for the purpose of harmonizing or increasing the weight of a tint. The sulphur is mixed with oil, so as to form a homogeneous paste thick enough to be laid on with a brush.
By the action of these two substances the polish on the plate is destroyed, and the result in printing is a fresh and soft tint, which blends agreeably with the work of the needle.
Differences in value are easily obtained by allowing the sulphur to remain on the plate for a greater or less period of time. This species of biting acts more readily in hot weather; a few minutes are sufficient to produce a firm tint. In cold weather relatively more time is needed. The corrosions produced in this way have quite a dark appearance on the plate, but they produce much lighter tints in printing. If you are not satisfied with the result obtained, you can rub it out with charcoal, as the copper is corroded only quite superficially.
Owing to this extreme slightness of biting, the burnisher may also be used to reduce any parts which are to stand out white.
This process, as you see, is very accommodating; but it is too much like mezzotint or aquatint, and, furthermore, it can only be applied in flat tints, without modelling. I have, nevertheless, explained it to you, so that you may be able to use it, if you should have a notion to do so, as a matter of curiosity, but with reserve. It is better to use the dry point, which has more affinity to the processes natural to etching.
78. =Mottled Tints.=--You may also make use of the following process (but with the same restrictions) in the representation of parts of old walls, of rocks and earth, or of passages to which you desire to impart the character of a sort of artistic disorder:--Distribute a quantity of ordinary etching-ground on a copper plate sufficiently heated; then take your dabber, and, having charged it unequally with varnish, and having also heated your etched plate, press the dabber on the passages which are to receive the tint; the varnish adheres to the plate in an irregular manner, leaving the copper bare here and there. Now stop out with the brush those parts which you desire to protect, and bite in with pure acid; the result will be a curiously mottled irregular tint (see Pl. V. Fig. 2). Properly used in the representation of subjects on which you are at liberty to exercise your fancy, this process will give you unexpected and often happy results.