[48] The Print must be well washed in cold water, to remove the Hyposulphite, before using the hot water; or the half-tones will be liable to be darkened, or changed to incipient yellowness, by sulphuration. This point is important as regards the permanency.
The size may also be effectually removed from the Print by the common Carbonate of Soda used in washing, although the former process is recommended as the most secure. Dissolve about a handful of the Soda in a pint of water, and when the milky deposit, if any occurs, has subsided, immerse the washed Positives for twenty minutes or half an hour. The Soda renders the paper quite porous, but produces no alteration of tint. If the process be properly performed, ink will run in attempting to write upon the back of the finished picture. After removal from the Soda Bath a second washing will be required, but the time of the first washing may be proportionally shortened. Here a difficulty will occur with many kinds of water; the Carbonate of Soda precipitating Carbonate of Lime, in the form of a white powder which obscures the picture. To obviate this, use rain water until the greater part of the alkaline salt has been removed, and do not allow a stationary layer of liquid to rest too long upon the Print. The New River water supplied to many parts of London, being comparatively soft, answers perfectly, and produces no white deposit, if the proofs are moved about occasionally.
When the Prints have been thoroughly washed, blot them off between sheets of porous paper and hang up to dry. Some press them with a hot iron, which darkens the colour slightly, but does so in an injurious manner when Hyposulphite of Soda is left in the paper.
Albumen proofs when dry are sufficiently bright without further treatment; but in the case of plain paper, salted simply, the effect is improved by laying the Print face downwards upon a square of plate-glass and rubbing the back with an agate burnisher, sold at the artists' colour-men's. This hardens the grain of the paper and brings out the details of the picture. Hot-pressing has a similar effect and is often employed.
Mount the proofs with a solution of Gelatine in hot water, freshly made; the best Scotch glue answers well. Gum water, prepared from the finest commercial gum, and free from acidity, may also be used, but it should be made very thick, that it may not sink into the paper, nor produce an unpleasant "cockling up" of the cardboard, which is caused by the damp and expanded print contracting as it dries.
Caoutchouc dissolved in mineral Naphtha to the consistence of thick glue or gold-beaters' size, is employed by many for mounting Photographic Prints; it may be obtained at the varnish shops, and is sold in tin boxes. The mode of using it is as follows:—with a broad brush made of stiff bristles, apply the cement to the back of the picture; then take a strip of glass with a straight edge, and by drawing it across the paper, scrape off as much as possible of the excess. The print will then be found to adhere very readily to the cardboard, without causing expansion or cockling; and any portion of the cement which oozes out during the pressing may, when dry, be removed with a penknife without leaving a stain.
REMARKS UPON THE WANT OF CORRESPONDENCE BETWEEN THE FORMULA OF DIFFERENT OPERATORS.
The formulæ for Positive printing given in the works on practical Photography exhibit great variety; and it has been proposed to attempt to reduce them to more uniform proportions. This cannot however easily be done, both on account of the difference in the structure and preparation of the various Photographic papers, and also because the mode of applying the solutions is not always the same.
Take as an illustration the following process, which has long been recommended for its simplicity, and which is in every respect a good one:—Dissolve 40 grains of Chloride of Ammonium in 20 ounces of Distilled Water, and immerse about a dozen sheets of Towgood's Positive paper, removing air-bubbles with a camels'-hair brush. When the last sheet has been placed in the liquid, turn the batch over and take them out one by one, so that each sheet, remaining in the liquid at least ten minutes, may be thoroughly saturated. When dry, excite by brushing with a 40 or 60-grain solution of Ammonio-Nitrate of Silver in the usual way.
Now this formula contains less than one-fifth of the amount of salt often employed, and if a thick foreign paper sized with starch, such as Canson's Positive, were floated upon such a salting Bath, it would be difficult to obtain a good picture. By immersing however a paper sized with Gelatine like the one recommended, a much larger quantity of salt is retained upon the surface, and the film is sufficiently sensitive. There are three modes of applying solutions, viz. by brushing, floating, and immersion. The quantity of solution left on the paper varies with each, and consequently each requires a different formula. Immersion in a strong salting Bath tends to give a coarse picture wanting in definition; whereas the plan of brushing a weak salting solution, produces a paper deficient in sensitiveness, and yielding a pale red image without proper depth of shadow.