The solution of Nitrate of Silver becomes after a time discoloured by the Albumen, but may be used for sensitizing until it is nearly black. The colour can be removed by Animal Charcoal,[41] but a better plan is to use the "kaolin," or pure white china clay. This substance often contains Carbonate of Lime, and effervesces with acids: it must in such a case be purified by washing in vinegar, or the Bath will become alkaline, and dissolve off the Albumen. It has been stated that an addition of Alcohol to the Nitrate Bath prevents it discolouring with Albumen.
[41] Common Animal Charcoal contains Carbonate and Phosphate of Lime, the former of which renders the Nitrate of Silver alkaline; purified Animal Charcoal is usually acid from Hydrochloric Acid.
Sensitive albuminized paper will usually keep for several days, if protected from the light, but afterwards turns yellow from partial decomposition.
Formula II. Preparation of plain paper.—Take of
| Chloride of Ammonium or Sodium | 160 | grains. |
| Purified Gelatine | 20 | grains. |
| Iceland Moss[42] | 60 | grains. |
| Water | 20 | ounces. |
[42] Iceland Moss is recommended because the writer finds that Positives so printed stand the action of destructive tests better than prints on plain paper, and equal to prints upon Ammonio-Nitrate paper.
Pour boiling water upon the Moss and Gelatine and stir until the latter is dissolved, then cover the vessel and set aside until cold; add the salt, and strain.
Use Papier Saxe or Towgood's paper,[43] floated upon the salting Bath in the same manner as directed for Albumen at [p. 243].
[43] The writer does not recommend the Positive paper of De Canson, having noticed that prints upon that paper do not withstand the action of sulphuretting agents so well as others (?).