BROMOIL PROCESS A process of obtaining pictures by bleaching and pigmenting bromide prints ; suggested by E. J. Wall in 1907. Working details were first published by C. Welborne Piper in August of the same year, and the modified process, as now worked, was introduced in the following month.
Prints for the ordinary oil-pigment process (which see) are made by contact, and as the pig ment process is generally more suitable for prints of fairly large size it follows that users of small plates must make an enlarged negative before they can produce an ordinary oil print. It was to obviate this that the bromoil process was devised. Essentially, it is a means by which a bromide print (which, of course, may be an enlargement from a small negative) can be pre pared for pigmenting. Special bromide papers for this purpose are obtainable, although many of the ordinary brands are equally suitable. The print should not be too old, and it should possess certain qualities. A flat print, or one whose development has been curtailed, will seldom give a satisfactory result. The exposure should be so adjusted that when the developing action has been carried to its fullest extent the result is a clean, strong print, rather more vigor ous than would be desirable if it were intended to remain unaltered. Amidol is a good developer for the purpose, a suitable form being so grs. of dry amidol added to 20 oz. of water in which oz. of sodium sulphite has been dissolved. The print is fixed in plain " hypo " solution, and very thoroughly washed. At this stage the prints may preferably be dried. Subsequently they are re-soaked for a few minutes to facilitate the even action of a bleaching solution. A suggested formula is : Citric acid . . grs. r 2'5 g.
Potassium bromide . 120 „ 12'5 „ Potassium ferricyanide 12o „ 12.5 „ Potassium bichromate 240 25 „ Alum ,) so „ Water to . . . 20 OZ. 1,000 CCS.
Dissolve in the order given. It is well to crush the crystals, dissolve in hot water, and use when cold. This is applied to each print separ ately until the black image is entirely changed to a yellowish brown colour. The prints are then washed until the bichromate stain is removed, and they are then placed for about five minutes in a bath containing I oz. pure sulphuric acid to 20 oz. of water. This is pre pared by adding the acid slowly to the water, and as considerable heat is generated it should be made some time before use, or blistering of the print will result.
After a short washing, the prints require five minutes' immersion in a solution of 2 oz. of " hypo " crystals and I oz. of sodium sulphite in 20 oz. of water. A further washing to elimin ate most of the " hypo " brings the prints to the stage where pigmenting may be proceeded with.
There are many variations of the routine of preparing the print. For example, many workers prefer to dry the prints again, and re-soak them for pigmenting. Others omit the acid bath from the preliminary stage, and use it immediately before this second soaking instead. When the prints are only soaked in water before pigment ing it is generally advisable to see that the temperature of the water is at least 65° F. (about 18° C.) in order to secure the necessary swelling and relief in the gelatine. (For an outline of the method of finishing the prepared print by pig menting, see the heading " Pigmenting.") A modification of the bromoil process has been made for the preparation of lithographic transfers, especially in large sizes for poster work. The bromide paper used should be the so-called " carbon " or velvet surface, and should be of a good substance. An enlargement of a half tone negative is made on to the paper, which is then developed with amidol, though some workers prefer pyro-ammonia. The print is fixed and washed as usual. It is next bleached in the usual bromoil bleaching solution at a temperature of 75° F. (about 24° C.) for two minutes ; soaked in a 5 per cent. solution of sulphuric acid for six minutes ; washed in several changes of water for five minutes ; fixed in " hypo " or toned with sodium sulphide for one minute ; washed in water for five minutes ; and dried thoroughly. It is inked all over with a composition roller charged with lithographic printing ink thinned with turpentine, until a thin even coating is laid upon the surface ; five minutes or so is allowed for the turpentine to evaporate completely, and the print is then immersed in water at about 7o° F. (about 21° C.), and after soaking for about half an hour the transfer may be developed in exactly the same way as an ordinary bichromated gelatine transfer.