Home >> Cyclopedia Of Photography >> Printing In Clouds to Vignetters And Vignetting >> Self Developing Plates

Self-Developing Plates

lens, exposure, operator, camera and acid

SELF-DEVELOPING PLATES Dry plates carrying the developer in or on the film, or on the glass side, and requiring only to have water applied for the developing action to take place. An early form was that introduced by Dr. Bmcklandt, who soaked dry plates in a solution containing salicylic acid and pyrogallic acid. The systems by which the developer is in the form of a dried paste on the glass side of the plate are far more satisfactory. Thomas Bolas patented (in 1907) a dry developer, to be applied to the glass side of the plate or used as a separate sheet. In addition to claiming the distribution of different portions of the developer in different parts of the area, the use of hydroxyl amine and an ammonium salt is named, also the use of acid sulphite, and the use of bicarbonate as an alkali. The acid constituent (A) may con tain the reducing agent :— Metol . . . . • i part Hythoquinone . . . 2 parts Milk sugar, mannite, or other sugar-like preservative . ij „ Sodium bisulphite . . . ij „ Starch, partly boiled and partly in grains . . . . 6 „ and water in sufficient quantity to give a paint ike consistency on a thorough incorporation or ;rinding of the ingredients. Instead of metol and hydroquinone, other reducing (developing) agents may be employed. The alkaline accele rator (B) may contain the following ingredients :— Sod. carbonate or bicarbonate • 5 parts Gum arabic . , . . x part and water in sufficient quantity to give a paint like consistency to the mixture upon grinding. The inert or slightly acid separating material (C) may contain the following ingredients :— Sulphate of lime or sulphate of baryta . . . . 4 parts

Gum arabic . . . . i partand water as before.

When there are no bright objects in the back ground and the duration of exposure is of little or no importance, a small stop may be inserted in the lens in order to make a comparatively long exposure necessary. The operator may then open the shutter or uncap the lens and take up his position at a suitable spot previously arranged opposite the camera, stay there while the ex posure is being completed, and then go back to the camera and close the lens. The brief period during which the operator is not in position will make no appreciable difference to the plate if the stop used is small enough, the plate slow enough, and the background dark enough. Another and a better plan, suitable for outdoor groups when the operator wishes to be included, is to make the exposure through the agency of black thread. Two lengths of black thread, long enough to reach from the camera to the operator should be taken. One is attached to the lens cap (a shutter cannot be used), and the other is tied to one corner of the focusing cloth, the latter being folded on the top of the camera in such a way as to be easily pulled over the lens. When all is ready for the exposure the thread attached to the cap is pulled, the cap then falls to the ground and the exposure begins ; when suffi cient exposure has been given the thread at tached to the black focusing cloth is pulled and the cloth falls in front of the lens ; the operator then hastens back to the camera and caps the lens.