When development is complete, the powder picture may be coated with plain collodion and washed free from greasiness ; the plate is now transferred to a 5 per cent, solution of potash alum until the yellow bichromate salt has been removed. Fixing with collodion as just described may be dispensed with when a duplicate negative is being made upon glass, all that is necessary being to expose it again to the light to get rid of all further tackiness. When, however, it is desired to fix the picture—as, for example, when it is upon an opal base—and it is not convenient to use the collodion " fixer," the picture may be washed over with the following solution : Water . . . I oz. ioo ccs.
Sulphuric acid . . 2 drms. 25 „ Methylated spirit . 2 oz. 200 „ Mix in the order given. The plate is treated with this until all the yellowness has gone, is then washed gently in water, and dried by gentle heat. The collodion fixer is preferable, as with the acid solution there is a risk of entirely losing the image. Flatness in a picture is caused by over-heating or over-exposing the plate or by developing in too dry an atmosphere ; in this connection care must be taken to keep white light from the image before fixing.
This process has been used for the production of pictures in colour. In i888 a process was introduced by Germeuil-Bonnaud which con sisted in exposing a plate coated with treacle, sodium borate and potassium bichromate, under an ordinary positive transparency. It was then
dusted with various coloured pigments which, it was claimed, adhered selectively to the differ ent parts of the picture, a claim that has not been substantiated. Dr. Miethe has worked out a variation of the idea. Glass plates are coated with the following mixture : Gelatine . . . 9 grs. 5.8 g.
Sugar candy . . 300 „ Potassium bichromate 90 „ ,, Water . . . 3+ oz. 1,000 ccs.
The plates are dried in an oven and exposed under a positive while warm. Three prints will be required from three negatives representing the three different colour sensations. Develop ment is performed by brushing on the plates suitable transparent powder colours. The yellow print is made first, stripped with collodion, and affixed to a card with gelatine solution. When dry, this is coated with a thin film of shellac, and the red and the blue prints superimposed upon it in the same manner.
A formula specially compounded for ceramic work, but which will also serve for all papers and articles, is : Gumming Mixture A. Fish-glue . . x oz. ioo ccs.
Glucose . 4 „ 25o g.
Glycerine . . io drops •5 ccs.
Water . . to oz. i,000 „Sensitising Mixture B. Am. bichromate . i oz. ioo g.
Water . . xo „ ',coo ccs. Mix together equal parts, and use as already described.