PHOSPHATE PLATES AND PAPERS Plates or papers coated with an. emulsion in which silver phosphate is practically the light sensitive salt The light-sensitiveness of this 26 substance was discovered by Stromeyer about 183o, but Fyfe, about nine years later, was the first to use it for printing out, and he salted his paper with sodium phosphate, sensitised on silver nitrate and again floated on sodium phosphate. He also used an ammoniacal solu tion of silver phosphate, and recommended merely washing with water or ammonia for fixing.
Many years later, Lyte used this salt for albumenised paper and employed nitric acid for fixing ; later (published, 1856) he used a mix ture of sodium phosphate and tartrate, Rochelle salts in sugar of milk and gelatine, and stated that there was no precipitate formed, thus antedating the later use by Meyer of silver phosphate dissolved in an organic acid. It was also tried by Hunt and Hardwich. Meyer (Brit. Journ. Phot., 1899 and 900) precipitated his phosphate and dissolved it by the addition of an organic acid, such as citric, tartaric, acetic, etc., and used it with and without a vehicle on paper. Silver phosphate alone or with excess of silver nitrate, however, gives an exceedingly long range of gradation, so that unless extremely vigorous negatives are used the prints are very flat, wanting in intensity, and without any whites. Valenta, in isloo, took up the study of the subject, and suggested a collodion emulsion by adding silver nitrate to phosphoric acid collodion, but this also possessed the same fault, though to a less degree. Further experiments
were made to increase the intensity and reduce the gradation by mixing it with varying propor tions of emulsion, or by the addition of uranyl and cupric chlorides ; and finally, in 19o5, he gave a satisfactory working formula, which can not only be printed out, but also physically developed : Raw collodion (3-3i%) 20 OZ. 1,500 CCS.
Phosphoric acid (zo%) 128 mins. 20 „ Mix, and add— Citric acid . • 384 grs. 6o g.
Alcohol . . . 640 mins. ioo ccs.
Then add— Silver nitrate (powdered) . 384-512 grs. 6o-8o g. Liquor ammoniac (.88o) q.s. q.s.
Enough ammonia should be used to form a. perfectly clear solution, and then— Absolute alcohol . 3i oz. 25o ccs.
added, the mixture being added in small quan tities at a time to the acid collodion, shaking all the time. Then add— Ether . . . 3} oz. 25o ccs.
Glycerine alcohol (1 : 128 mins. 20 „ This may be coated in the usual way on matt or glossy baryta paper. It may be fully printed out or only until the outlines of the image are just seen, and should then be developed with the following :— Metol . . • 155 grs. 35.5 g.
Glacial acetic acid . to oz. i,000 ccs.
Distilled water . . Io „ i,000 „ For use, 4o drops of this should be diluted with 31 oz. or too ccs. of water. The colour of the prints thus obtained varies from sepia to brown ish black or purple.
Recently, several makers have placed phos phate papers on the market which may be treated as outlined above.