Sizing, with Gelatine.—Dissolve at a temperature of about 140 deg. Fahr. (60 deg. C.) 10 parts of good gelatine in 800 parts of water, then add 200 parts of alcohol and 3 parts of alum dissolved in a little water. Filter and prepare the paper by immersion as above directed. The gelatinized paper when dry should be prepared a second time and dried by hanging it up in the opposite direction in order to obtain an even coating.
THE CYANOTYPE OR BLUE PROCESS.
This process gives white impressions on a blue ground with diapositives or drawings on transparent or semi-transparent materials, and blue impressions on a white ground from negatives. It is commonly known under the names of “blue print process,” “negative ferrotype process” and “ferro-prussiate process.”
The process is indeed exceedingly simple. A sheet of paper, impregnated or sensitized, as it is termed, with a solution of ferric citrate and ferricyanate is impressed under a cliché,[5] then immersed in pure water, whereby the image is developed and at the same time fixed. It is on account of the great advantages offered by its simplicity that this process is generally preferred by civil engineers and architects for the reproduction of their plans.
The sensitizing solution is prepared in mixing by equal volumes the two solutions following:
| A. | Iron, ammonio citrate | 20 parts |
| Water | 100 parts | |
| B. | Potassium ferricyanate (red prussiate) | 15 parts |
| Water | 100 parts |
Although the mixture keeps pretty well for a certain period in the dark, it is best to prepare only the quantity wanted for actual use.[6]
The paper is preferably sensitized in operating as follows: