“The paper can be prepared in quantities beforehand. To use it I apply upon each sheet a solution of citric acid at 30 or 35 per cent. of water[44]—which may be done by daylight—and let them dry in the dark.”
“Exposed under a negative of the ordinary intensity, the paper is impressed in sunshine in a few minutes; in the shade it requires about the same time as chloride of silver paper.”
“After exposure the image is not visible, and without being obliged to shelter it from light, I immerse the print in a solution containing about 1 per cent. of silver nitrate. This solution can be used over and over again, by adding to it a little of the silver salt. It does not become turpid by use; it simply turns slightly green from formation of iron nitrate. The image appears soon and rapidly becomes vigorous; in half an hour it will be completely developed. When the exposure is sufficient the color is deep sepia, but not so intense if the quantity of citric acid is feeble. No fixing is necessary; it suffices to wash in several changes of waters.”
“The image can be toned with great facility by a weak solution of gold or of platinum chloride, or, better, by a mixture of these two salts. If the impressed paper be treated by a very diluted solution of potassium ferrocyanate, one obtains very pretty blue proofs.”
“A weak solution of gold chloride develops a violet image. A solution of platinum chloride has no effect.”
“All the various phases of this printing method can be followed in full (diffused) light; there is only the desiccation of the paper when sensitized with citric acid, which requires to be done in the dark.”