The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Gum-Bichromate Process
most cases. General criteria of quality are strength (this being equal in both directions in the case of a hand-made paper), torn edges showing long fibres, no change in tint when exposed to light at a south window for a month in the summer, and no rapid change in colour when dipped in a weak and slightly acid solution of aniline hydrochlorate. Probably the gum-bichromate will not reach its highest point in the matter of control by the worker, until amateurs learn to make their own papers - really a very easy matter when only small sheets are wanted; the chief labour being the braying down of sufficient linen rags to make a stock of pulp from which to dip. The amateur making his own paper could produce each print on a complete sheet, having the original or deckle edges, and, if need be, an approxi-mate water-mark, either for the margin or the body of the sheet. (See The Amateur Photographer, November 23rd, 1900, p. 401.)
Special Sizing or Substrattim for the Paper. Special hard sizing of the paper is generally essential when several impres-sions are to be made in succession on the same sheet of paper and absolute insolubility of the size or substratum must be aimed at. One ounce of gelatine is dissolved by heat in 20 ozs. of water, and the sheet of paper (Dr. Hofmann recommends the Whatman hand-made drawing paper or Zander's paper, a kind which appears to be well-known in Germany) is well brushed over on one side with this size, first in one direction and then in the other. The sheet should be laid on a horizontal surface to dry as, if suspended, the .paper may be put into a condition of local strain. When dry the sheets are brushed over with a mixture of one volume of commercial 40 per cent, formalin and 20 volumes of water, the paper being again allowed to dry in a horizontal position. Paper thus coated or sized should, Dr. Hofmann tells us, bear seven or eight coatings, printings, and developments, without deterioration.
The Gum Solution. Starch, sugar, and various additions to the gum are occasionally recommended, but Dr. Hofmann finds no advantage or disadvantage in such additions, and on the principle of avoiding unnecessary complications, he naturally prefers plain gum. The best gum arabic should be used, this occurring as nearly round and almost white nodules ; these nodules being full of cracks and fissures. Such nodules as are clear and glass-