DEVELOPMENT OF THE COLLOTYPE PLATE This simply consists of washing out the soluble or unexposed portions in cold water. The plate should be kept in gently running water, or given occasional changes, for about two hours, it being necessary to remove all the bichromate. By this time the image will be invisible, but will probably possess a slight amount of tives are those which are rather thin, but possessing a full range of gradation and detail ; a hard black and white negative is perfectly useless if good results are desired. A printing frame with wedges or screws is to he preferred, so that close contact may be ensured. The progress of printing is judged by looking at the back of the plate, a faint brown image being visible, with full detail in the high lights, when the exposure has been sufficient. The majority of workers, however, prefer the guidance of an aetinometer as in car bon printing, and with metal plates this is, of course, imperative. The length of exposure varies according to the nega tive, the light, and other factors, and can relief. If all has been cor rectly clone, the plate should have a scarcely perceptible brown colour when looked through, and should show a ground glass or matt sur face by reflected light. It may now he
stood up in a rack to dry, and is better if left a day or so before printing from.
THE " &cu." The next proceeding is to treat the Dilate with a deliquescent or moisture retaining mixture known as the " etch," with the object of keeping it in a damp condition. The term is altogether a mis nomer, since no etching takes place, the plate being simply rendered moist in varying degrees, according to the different action of the light on each part, which has rendered some portions comparatively non-absorbent, while others are scarcely affected. It is, of course, this greater or lesser moisture of various portions of the plates which causes the greasy ink to be attracted or repelled. A suitable formula for this purpose is Glycerine ... ... 24 ozs.
Water ... 16 ozs. Sodium chloride (common salt)... 40 grs.
The plate is placed on a level table and the solution poured on, care being taken that it is equally distributed. It is left on for a time varying with the correctness of the exposure. An over-exposed plate,