REASONS FOR VARNISHING.
The reasons for varnishing the negative are as follows : (a) as a protection against damp ; (b) as a protection from abrasions of the film, or to prevent the retouching being disturbed ; (e) as a support for fur ther retouching. (a) If the negative is printed from without varnishing, there is always the danger of staining. Free silver nitrate or citrate contained in the paper is liable to enter into combination with the gelatine, if in the presence of moisture ; which moisture may be due either to the state of the atmosphere or to the presence of citrate of soda in the paper. Either of these may lead to the production of orange - red stains, which are exceedingly difficult to remove. Per haps the best plan is to treat them with a solution of thiocarbamide or a mixture of nitric acid and ammonium sulphocyanide. Prevention is better than cure, however, hence the necessity of varnishing. (b) The
constant friction arising through placing paper upon the film of the negative must in time remove some portion of the work ing up, which may have a disastrous effect on the picture. It is easy to conceive that this might he worse than removing the whole of the retouching, and it will cer tainly be the finer and lighter work which will be the first to suffer. (c) When re touching a negative which requires a large amount of work, it will generally be found that the deepest shadows require considerably more lead than can he per suaded to stay on the medium, unless a very soft pencil is used, which is not always convenient. The only way out of the difficulty is to give a coating of varnish, which will form a basis for a further application of medium, upon which more retouching can be clone.