J. C. S. MUMMERY, A.R.I.B.A., F.R.P.S.
Past-President of the Royal Photographic Society.
The process of printing in gum bichromate is one of the most interesting at the command of the amateur and the following brief description may be of assistance in helping the beginner over some of the initial difficulties of the process.
There are several methods of working, and that described is the one adopted by the writer, who has found it simple and satisfactory.
It consists of coating a piece of paper with a compound of gum pigment and bichromate of potash, which is sensitive to light when dry ; exposure under a negative and development ; the operations being repeated a second time if necessary to obtain strength or transparency. The materials and apparatus essential to the process are few and simple, and it may be well to consider them in detail as follows : PAPER. Practically any paper may be employed, but such as the O.W. Paper and Arts Co.'s machine-made, Joynson's papers, or hard sized cartridge will work better than the softer varieties, some of which may require sizing to keep the pigment upon the surface and prevent it from staining
J. C. S. Mummery, A.R.I.B.A., F.R.P.S.
GUM SOLUTION. The gum arabic (gum acacia) should be obtained in lumps or " tears," and the solution made by tying up 4 ozs. of gum in a small piece of linen and suspending the whole in 10 ozs. to 12 ozs. of cold water in a wide-necked bottle. The gum will dissolve out in a day or two, and is then ready for use, and the linen with the impurities contained may be thrown away. The gum solution should be corked up ; it will gradually become acid, but so long as the acidity does not proceed so far as to render the gum thin, it will not interfere with the working.
The bichromate of potash should be prepared in a saturated solution.
PIGMENTS. These may be finely ground powder colours, obtained from an artist's colourman ; in this form they are easily measured, but, of course, moist tube or pan colours may be used.
Lamp and ivory black, red ochre, burnt sienna and many others are suitable, but if browns are required they will be better if made with ivory black and red or burnt sienna, as these mixtures will work with greater ease than the prepared browns.
BRUSHES. A large camel-hair mop brush and a 4-inch hog-hair " grainer's softener are required for coating the paper, and some sable or camel-hair water-colour brushes for working upon the print, if this is contemplated.
ACTINOMETER. As the exposure cannot be properly judged by the appearance of the image, a Sawyer's or Wynne's, or some similar actinometcr will be required, and may be worked with pieces or strips of P.O.P.
MULLER AND SLAB.
A piece of ground glass as a slab and a small glass muller arc desirable for properly mixing the powder colours, and a palette or other flexible knife .
Other accessories consist of a board upon which to coat the paper, drawing pins, saucers, muslin for filtering, and the ordinary graduates and dishes.
COATING THE PAPER.
To prepare the paper take the following materials : -
Gum solution ............
Bichromate of potash solution
(Should lamp black be preferred, 18 grains will be sufficient.)
This will give a warm black, suitable for two or three coatings, and may be taken as a basis for the beginner to work and experiment upon, rather than as a formula for the process.
Should a lighter tinted pigment be used, a greater quantity will be required, and perhaps a slight addition to the gum. Place the powdered pigment upon the glass slab with a portion of the gum, and mix them together with the palette knife, afterwards grinding with the muller for two or three minutes to secure an intimate mixture ; then transfer the mixture to a saucer and add the remainder of the gum and the bichromate solution ; stir well, and filter through a piece of muslin into a graduate and turn out into a clean saucer. The mixture is now ready for coating, and the sheet of paper, which should be somewhat larger than the negative to be printed, is placed upon a sheet of absorbent paper on the board and secured with drawing pins at the four corners.
Now, taking the mop brush, stir the pigment mixture thoroughly, and striking off all superfluous mixture on the edge of the saucer, proceed to lay a thin wash over the paper with long horizontal strokes, afterwards crossing this with vertical strokes, and finishing off with the dry softener, by a succession of horizontal and then vertical strokes, keeping the brush in a vertical position and using only the end of the hairs.