The Dictionary Of Photography

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

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Gum-Bichromate Process
affording a wide choice of texture. The sensitive preparation is made as follows : -
Clear white gum ............ 4 ozs.
Water           ............... 6 ..
Soak till dissolved, and squeeze the mucilage through fine muslin.
Bichromate of potassium......... 1 oz.
Water           ... ...... ...... 9 ozs.
In a room lighted by yellow light, equal volumes of A and B are mixed, and then such moist water colours are stirred in as will give the required tint; but the coloration should only be such that, when the preparation is laid on paper with a broad camel's-hair brush, as described below, the tint appears very faint, and far short of that required to form the shadows of a print. The film should seem very thin and transparent when the paper is held up to a window, and the outline of a finger held the other side of the paper should be easily traceable. Indian ink or ground lamp black, with a little cobalt blue to modify the greenish tint, is a suitable pigment. Earth colours, like Indian red or the siennas, can be used, but a much larger quantity is required than when lamp black is the base. To coat the paper it must be sponged on both sides to stretch it; after which it is pinned down on a board by one corner, and rapidly brushed over with the sensitive mixture by means of a broad camel's-hair brush. It is best to work in one direction, and the coating must be thin - so thin that the gummy mixture must rather be rubbed over the surface of the paper than painted on, and the pigmenting must be adjusted for this kind of coating. Some practice is required in coating the paper thinly and uniformly. The best method of drying is to hang the paper up over-night in a room where there has been a fire during the day, but more rapid drying before a fire is quite allowable. Exposure should generally be longer than for a print on albumenised paper, and can be judged by the faint image of the exposed bichromated gum which is visible - or should be, if the paper is not too heavily pigmented - when the paper is viewed by transmitted light; the details or the shades being just traceable. The use