The development is effected by floating the printed surface of the paper for five or six seconds upon the " developing solution." To avoid air bubbles lay one edge of the print upon the solution near the right hand end of the dish ; then, with a sliding motion towards the left, lower the print with an even movement, without stoppage, until it is entirely in contact with the liquid, where it must remain until complete action has taken place.
A good plan is to place the prints, after removal from the printing frames, in a calcium tube, with their printed surfaces outwards, and, therefore, convex. In a short time the prints will receive and retain this curva ture sufficiently for the developing operation. To de velop take the print in the right hand (its printed sur face being downwards), lay the left-hand edge on the de veloper, and then slowly and continuously lower the right hand until the whole print is floating. The great point is to well preserve, and if possible increase, the curvature of the paper as it near the liquid.
A temperature of about 140 deg. Fahr. may be considered the standard temperature for the developer, though higher and lower temperatures may be used on occasion. To test the temperature a cheap chemical thermometer must be used. The bottom of the developing dish should be covered with the de veloping solution to the depth of at least one-half of an inch.
After the prints have been developed put the solution, without filtering, into a bottle for future use ; it should not be exposed to a strong light. When next developing the solution will be found to be nearly clear, but, of course, tinted by previous use. If this clear solution be not suf ficiente for use, add to it some of the fresh solution bf the potassic oxalate. It is a safe plan always to keep the " bath solution" up to its original bulk by this means. A little suspended matter in the bath is not of any consequence.
Sometimes, when a large number of prints (or large prints upon small bulk of solution) are developed at one operation, the bath will become so loaded with chemicals derived from the paper that it will no longer give good prints. Such a solution must at once be replaced by a new one. When working on a somewhat large scale it is especially necessary to watch for any change in the quality of the prints ; and in case of a doubt a new bath should be tried.
Very large prints may be developed without any difficulty in a trough, by being slowly and continuously pulled through the solution contained therein.
Clearing and clear the developed prints; these must be washed in a series of baths (not less than three) of a weak solution of hydrochloric acid. This solution is made by mixing i part of hydrochloric acid with 6o parts of water. The specific gravity of the acid should not be less than z' i6; if lower, more acid should be used. The acid should be colorless. Or citric acid, in the proportion of z ounce to zo ounces of water, may be used This softens the paper in less degree than does the hydrochloric acid. A white opalescence of the bath shows necessity for more acid.
As soon as the print has been removed frem the developing dish it must be immersed face downwards in the first bath of this acid, contained in a porcelain dish, in which it should remain about five minutes; meanwhile, other prints follow until all are developed. The prints must then be removed to a second acid bath for about ten minutes; afterward to the third bath for about fifteen minutes. While the prints remain in these acid baths they should be moved so that the solution has free access to their surfaces, but care should be taken not to abrade them by undue friction. It is impossible to affect the image per se by leaving the prints for a long time in the acid baths, but such treatment continued for an hour or more tends to make the paper soft and porous, and to damage its surface.
The prints should not communicate to the last acid bath the slightest tinge of color. If the bath, after the prints have been washed in it, does not remain as colorless as water when a depth of fully two inches is viewed in full daylight, the prints should be treated to yet another acid bath. The last acid bath must not in any case have been used for a previous batch of prints; after use it may form the first acid bath for the next batch, but it is better to replace all the baths by fresh ones. The object of this washing in dilute acid is to remove all traces of iron salts from the paper before it is placed in the plain water. The prints must not be placed in plain water on leaving the developer, because insoluble salts will be precipitated on the print. After the prints have passed through the acid baths they should be well washed in two or three changes of water during about a quarter of an hour. It is sometimes advisable to add a pinch of washing soda to the second washing water to neutralize any acid present in the print.