Ammonium bichromate 3 g.
Citric acid 2 g.
Water 25 ccm.
If the sensitizer is to be applied afterwards, this latter solution may be omitted, and it will then be possible to prepare quite a stock of the pigmented gum, though to prevent it from moulding a few drops of phenol (carbolic acid) should be added. This may also be added to the stock of plain gum solution with advantage, though if the latter goes sour and becomes more fluid it is still fit for use. There are two methods of sensitizing the coated paper, either by floating or by painting the solution on, and the latter is far preferable. If the floating method be adopted, the back of the paper, not the coated surface, must be floated on:
Ammonium bichromate 30 g.
Citric acid 15 g.
Water 1000 ccm.
Float for about five minutes and then hang up to dry. For painting on, half the water in the above formula should be replaced with denatured or methyl alcohol, and the solution may be freely painted on the back of the paper.
Applying the pigmented gum to the paper is somewhat of a trick, which can be easily learned with a little practice, and that is one reason why the use of the non-sensitized mixture is preferable. It is impossible to state exactly how much mixture should be used per unit area, but one soon learns from the appearance of the paper; for when finished it should present an even coating of color and it should not be possible to see the white surface through the pigment. Two brushes will be required, a camel's hair mop, with which the pigment is applied freely with circular strokes, and a hog's hair softener, which should be used first with vertical strokes and then with horizontal ones until an even coat is obtained, when the paper may be hung up to dry. The brushes must not be used too vigorously or a plentiful crop of minute bubbles will result, and this particularly applies to the use of the second brush.
Exposure is gauged with an actinometer as in ordinary carbon printing, and the paper is rather more sensitive than printing-out paper. Development, as already stated, is effected by floating the print face down on water, and although this means more time it is the best; twenty to thirty minutes should be allowed, and the print may be lifted up and examined from time to time. A spray, as from a scent diffuser, may be used, but the use of a rose tap or pouring water direct on to the surface from a vessel is not advisable, as some of the finer details are sure to be washed away. On the completion of development the print should be immersed in five per cent alum solution for ten minutes, washed by repeated changes of water, and dried. While wet the image is extremely tender.
The second pigment is applied over the first print in the manner described, and the sensitizer also used in the same way, but allowance must be made for the slower penetration of the latter, due to the greater thickness on account of the presence of the first print.
We now come to the crux of the whole process, the registration of the images, because it is impossible to see the outlines of the objects through the pigment coat. The first thing to do is to see whether the negatives themselves can be registered by their edges or corners. For this purpose they should be placed on a sheet of glass, which should be at least an inch narrower than the negatives, so that both ends of the latter protrude beyond the glass. The latter should be supported on piles of books or in any convenient way, and have a white card under it to reflect the light. Then two diagonally opposite corners should be brought into coincidence and the images examined. Care should be taken to look straight down on them and the use of a magnifying glass is advisable. If two of the negatives are thus found to coincide, the third may be compared with either in the same way. If they all agree, further printing is much facilitated as one has merely to mark the corners of the plates on the paper, or preferably use a printing frame larger than the negatives, cut cardboard corner pieces, and fit these into the frame so that the glasses will automatically drop into register.
If the corners and images do not coincide one has a much more difficult job. Six corner pieces of card must be cut, two of the negatives registered, the corner pieces applied at opposite diagonal corners and stout needles driven through both, taking care that the glasses do not shift. The third negative must now be registered in the same way, one of those already fitted with the corner pieces being on top, and the needle must be driven through the hole already made and through the second card. The corner pieces should be numbered 1, 2, 3, to correspond to the negatives, and if they are butted into the corners of the frame and registered by the needles, the negatives will be in register. It is safer, after having once made one hole, which should not be on the extension of the diagonal of the plate, but to one side, to drive a second needle through the three cards, on the other side of the diagonal, as this prevents any possible shift. Naturally one may use paper with any surface for the support, but, except for large prints, smooth papers are preferable, as with really rough papers the irregularities of the paper cause irregularities in the depth of the pigment film.