Potassium Bromide . . . . . . 18 Potassium Bichromate . . . . . . t8 Alum Ammonia . . . . . . . . 36 Hydrochloric Acid . . . . . . . 6o Water We give this formula of Mr. A. I-I. Garner's, in order to show the chemical reaction.
It has been claimed, however, that the above is a breach of Mr. T. Manly's patent, and it would perhaps be better to use the following variation, as no one should begrudge this gentleman the lawful reward of his investigations : Ozobrome solution . . . . . . . 4 oz.
Potash Alum to% solution . . . 4 ,, Citric Acid to% Water . . . . . . . 20 after which it is soaked in a 5 per cent. solution of sulphuric acid until the high lights of the picture appear in relief, which will generally happen within five minutes. The image is then fixed out in an ordinary fixing bath, but this is not absolutely necessary, unless the pigments to be employed are of so light a colour that the silver when darkening would spoil the effect. In summer the image will occasionally show itself in relief without the use of the acid bath.
After washing, the bleached print is ready for pigmenting, which is done in much the same way as with ordinary oil prints, or even with the roller employed for papyrotints.
Ordinary Robertson's medium may be mixed with tube colour and dabbed, smudged, or " hopped " on with the usual smooth brush, or with a soft muslin or cotton-wool dabber according to the skill of the particular artist. With finer brushes it is not difficult to touch in light effects in colour. The bromide image remains in faint yellow as a guide, and we have seen some excellent bromoils of spring flowers and the like subjects of not too complicated nature executed in two or three colours.
If any mistake is made and the effect is unhappy we can wipe the picture off with a little benzole, wash in water, and start afresh, so long as no ink has been allowed to get at the paper on the back of the print. Elaborate directions as
to the laying on of the oil pigment, of devices for increasing light and shade, and of other instruments which may be enlisted into service, we are not called upon to append here. Bromoil, like gum-bichromate, is an art process, and one which affords the individual worker an opportunity to mature devices for himself and to apply them in his own way. A little practice, and perhaps a few spoilt prints, are not a heavy price to pay, if the experience gained eventuates in productions combining success with the higher quality of originality.
Manufacturers are beginning to label the bromide papers most suitable for conversion into bromoils. Ilford rough, Ilford, or Wellington carbon ; Barnet ordinary or rough present little difficulty. The latest advice can generally be obtained when purchasing the ozobrome solution from the Ozotype Company, who from time to time publish new directions for the more economical management of the process. A simplified bleaching bath lately suggested by the company consists of Ozobrome solution . . . . . part Hydrochloric Acid (pure) t per cent. solution . . 5 parts Water 4 » 1 To make up the hydrochloric acid solution take 2 drams fluid (sp. gr. i6) to 25 oz. water. In one to three minutes the print will become a faint yellow colour and may then be placed direct into the fixing bath : . . . . . . . . . 2 oz.
Ammonia . . . . . . . . . i dr.
. . . . . . . . . . 20 oz.
where it must remain from two to six minutes according to the original hardness of the bromide emulsion, which can be gauged with sufficient accuracy by the time the image takes to bleach. After a wash of three to five minutes it is ready for inking up.
According to experts, the bromide prints which make the most successful bromoils are those developed with amidol, or hydroquinone combined with metol.