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.
Introduction. Of the various simpler processes which admit of practically any kind of paper being prepared and sensitized by the individual worker, the process known as kallitype embodies a method of printing which, although known and used for many years, deserves a much greater list of users. The three strongest points in its favor are: First, the extremely small expense connected with the process; second, the close imitation which the final print bears to the more expensive platinum process; and third, the simplicity of sensitizing, printing, developing and fixing.
533. The rapidly increasing scarcity of metallic platinum makes it necessary for the amateur worker to obtain something to take the place of platinum paper. It is not necessary to look further than the kallitype for results quite as good as will be found in platinum. The permanency of the kallitype is just as good as any other silver process. The richness of deposit in the metallic particles usually associated with the platinum image can equally be claimed for the silver image of the kallitype, the basis of both being ferro-oxalate. In fact, kallitype at its best cannot be distinguished from platinum.
534. In the first experimental stages of this process it was customary to coat the paper with a solution of ferric-oxalate (an iron salt) and develop the image which was secured by printing through a negative in strong daylight, in a bath of silver nitrate. This method was soon discarded for another, in which the iron and silver were combined in the sensitizer and the faint image which was secured in printing brought to full strength in a solution of rochelle salts, borax, or a combination of the two. In fact, this is one of the simplest methods of procedure, and we give here the process in its simplest form.
Theory Of Process. Upon exposure to light the ferric-oxalate is reduced to a ferrous salt, which when dissolved by a suitable solution has the power of reducing silver nitrate to a metallic silver, and this reduction takes place in that degree in which the light has altered the ferric salt.
536. A Satisfactory Formula.
This bath may be prepared in any quantity, but in the same proportions.
537. The ferric-oxalate should be dissolved in the water first and care must be exercised at this stage. It requires warm water to dissolve it, but this must not be too hot, or it may spoil the oxalate. After it is dissolved, the liquid should be cooled and filtered and the silver nitrate added. The solution should then be quite clear.