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.
Alkalies. The alkalies used in the different gold baths recommended are: borax, carbonate of soda, and bicarbonate of soda. Carbonate of soda is a very strong alkali and should be used carefully. It is used for warm tones in the gold, resulting in olive tones in the platinum bath.
187. Bicarbonate of Soda is used for purple or blue tones. For toning prints in a bath, in which this alkali is employed, care should be exercised not to tone too far, unless you desire a blue black result.
188. Borax is the most universally used alkali, there being less danger of using too much of it and injuring the bath. Borax gives brown tones in the gold bath, and a rich olive-black tone in the platinum bath. Alkali is used in the gold bath for the purpose of precipitating the gold and to act as a restrainer on the highlights, holding them from over-toning and bleaching while the shadows are toning to the desired color. A formula may specify a certain amount to use, or direct that alkali be used until red litmus paper turns blue, in a given time. These directions given for the use of alkali in a gold bath formula can only be employed as a general guide, as each lot of prints may require a different quantity; a fresh bath, or fresh paper, for instance, requiring more alkali than a ripened bath or old paper. The only true test of how much alkali a bath requires is its action on the prints you are toning. If you bear in mind that the alkali in the gold bath acts as a restrainer on the highlights and prevents their bleaching, while the shadows are toning to the point desired, you will know that the proper amount to use is the amount necessary to accomplish this result. It is not a question of whether it be a drop, an ounce, or whether the red litmus paper turns in five minutes, or at once; it is the result you must look for and not the time in which it is produced.
189. Most of the trouble in toning in gold baths comes from improper use of the alkali. To illustrate this point, suppose we are making up and testing a gold bath. First, be certain that your trays are perfectly clean. Having a few prints washed ready for toning, pour the toning bath, which you have previously prepared, into the tray. Place a piece of red litmus paper in the bath and add the alkali. We will use borax in this case. Add sufficient to cause red litmus paper to turn blue in a minute or two. For your first experiment select a print showing white drapery. Place it in the bath and watch it closely. If the white tones clear up at once, before the shadows tone to the point desired, add more borax. Pay no attention to the litmus paper after this point and try another print. If the same effect shows add more alkali and try another. Keep on testing the bath with a print and adding alkali until the bath is so balanced that when the shadows are toned the highlights will be thoroughly clear, without bleaching, or the loss of any detail the print possessed when it came from the printing frame. When the shadows become perfectly clear your print is completely toned.
190. Should you find in toning that your prints refuse to clear in the highlights, the trouble is probably caused by the bath containing too much alkali or from impurities in the water. If caused by too much alkali add to the bath a little acid gold or a few drops of citric acid. If from impurities in the water, distilled or ice water should be used.