This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Note. Should there be any portions of the print which you desire to have dissolved away more than others, you may dip up some of the warmer water in a small cup, and gently pour it on those portions which you desire to have dissolved away.
Hardening Bath. The hardening bath should have been prepared previously to starting to develop the print. This bath is composed of 1 ounce of potash alum to 30 ounces of water. Nothing but plain potash alum should be used for this purpose. Allow the print to remain in this bath for five minutes in order to thoroughly harden the pigment image. When this is accomplished, remove the print from the bath and rinse off in clear water for five or ten minutes; then, with photo clips, hang up to dry. Should the whites appear slightly yellow or muddy, add one grain of sulphite of soda to the alum bath; the sulphite will purify the whites.
What The First Print Teaches. As soon as the print is placed in the alum bath it may be regarded as finished, and upon its appearance you will be able to judge whether or not the exposure has been correct. Assuming that the negative was a good one-a vigorous negative is essential for carbon work-if the print has a pale, washed-out look, printing was not carried far enough; if it is dark and heavy, printing has been carried too far. By looking at it and noting the appearance of the guide print made on the printing-out paper, you will be able to decide how deep to carry printing the next time. The second attempt should result in giving a print which will please in every respect; if you have made a careful study of your first experiment. The first guide print may be kept handy, in the dark, and compared from time to time with the second guide print.
A Possible Source Of Trouble. There are a few sources of trouble, which, although possible, are not probable, yet we desire to point them out in order that you may avoid them. Always procure pure potassium bichromate, for if the impure chemical is used, or if the tissue is dried in air contaminated badly with fumes from burning gas, the coating may become insoluble without any exposure to light at all. This generally manifests itself by the tissue refusing to adhere to the transfer paper. To make sure that the tissue is in perfect condition before printing, you may cut a narrow strip off one of the pieces and put it in cold water for a minute or two until quite limp. If you then place it in warm water and the coating dissolves entirely, it is in good condition. If it does not dissolve, the tissue is useless. It is not at all likely that this will occur, but we mention it here in case a beginner should be so unfortunate as to find that his first attempts are failures from such a cause. If you have tried the tissue in this way and find it to be in good condition, and still your print will not adhere to the transfer paper, either it is very much overprinted, or, what is more likely, you have left it soaking too long in water before squeegeeing it to the transfer paper.
168. Should you be troubled with blisters, you will find that they are caused from one of two things-either too hot water or too strong an alum bath. Be sure that the water is not too hot, and if the alum bath is too strong reduce it by the addition of a little more water.