Another, but not quite so effective a manner of sizing although sufficient for the cyanotype, is the following, employed by Mr. Pizzighelli for the paper used in the platinotypic process:
Ten parts of arrowroot are powdered in a mortar with a little water and then mixed by small quantities to 800 parts of boiling water. After a few minutes 200 parts of alcohol are added and the mixture filtered. The paper is immersed for two or three minutes in the warm solution and hung up to dry.
_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.
_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 20 parts citrate Water 100 parts B. Potassium 15 parts ferricyanate (red prussiate) 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:
Take hold of the paper by the two opposite corners and fold it into a loop, lay it on the iron solution, the center of the sheet first placed in contact with the liquid, and then gradually spread it by lowering the corners with a little pressure. No solution should run over on the back of the paper; it would be a cause of stain. This done, and without allowing the liquid to penetrate _in_ the paper, immediately take hold of the two corners near the body and withdraw the paper by dragging it over on a glass rod for this purpose fixed on the edge of the tray. Now pin up the paper to dry, which should be done rapidly, and sensitize a second time in proceeding in the same manner. If this second sensitizing be found objectionable, let float the paper for no more than ten seconds; of course this method of sensitizing is not applicable to prepare larger sheets of paper. In this case the paper is pinned by the four corners on a drawing board or any other support, lined with blotting paper and quickly brushed over with a sponge sparingly imbued with the sensitizing mixture, so as to wet the paper with a very small excess of liquid.