The Dictionary Of Photography

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

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Gum-Bichromate Process
like should be rejected, as these pieces are often but imperfectly soluble, giving rather a slimy mass than a true solution. Two parts of gum and five parts of water form a good standard mucilage. The mucilage should be strained, and to each half-pint add 20 drops of the pure crystallisable carbolic acid.
Choice of Pigments. This is very wide, the one limitation being the exclusion of those which are affected by chromate salts. The following list includes all that are likely to be required : - Lamp black, ivory black, burnt umber, Venetian red, Paris blue, dark burnt ochre, sepia, gamboge, Pereira's green, and Cassel brown When white is to be used on a dark ground (see Carbon Printing) baryta white is suitable.
Stock Gicm and Colour " Solutions." These may be prepared by rubbing together in a mortar about one part of the moist water colour, as sold in collapsable tubes, with four parts of the mucilage or gum solution, but as some colours are more intense than others, no absolute rule can be laid down.
Mixing and Sensitising the Colour "Solutions!" In using the standard "solutions " of gum, and colour to the required tint, the old dictum of the apothecary " shake the bottle " must not be forgotten. A mixture of one part of mixed or adjusted colour solution and one or one-and-a-half of saturated solution of ammonium bichromate tends to hardness of the print and a rendering of the deep shadows ; while the use of a mixture con-taining one part of the colour solution and two parts of a saturated solution of ammonium bichromate tends to greater sensitiveness, flatness of the picture, and the rendering of the fainter gradations. A sheet of paper 12 inches by 15 inches will ordinarily require a little more than a quarter of an ounce of the mixture, but this estimate is subject to much variation, as explained below. See above for instructions as to coating the paper. Dry as recommended above.
Exposures for Multiple Printing. When several impressions are to be made on the same sheet to build up a picture, the question of exposure becomes somewhat complex. Let it be assumed that there are to be three printings. The first may be for the higher lights, and a range including all the darker shades. The paper being allowed to dry, and having been once more coated, the second exposure is for middle tones and shades, while the third coating and exposure may be merely to add to