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 (?). Render sensitive by floating for two or three minutes upon a solution of Nitrate of Silver, 40 grains to the ounce. Thirty grains to the ounce, or less, will be sufficient if the sample be pure; but in that case occasional additions of fresh Nitrate of Silver must be made, as the Bath loses strength. _A second Formula for plain paper._--Take of Chloride of Ammonium 200 grains. Citrate of Soda[44] 200 " Gelatine 20 " Water 20 fluid ounces. [44] This salt may be obtained at the operative chemists; or it may be prepared extemporaneously by neutralizing 112 grains of pure Citric Acid, free from Tartaric Acid, with 133 grains of the dried Bicarbonate or "Sesquicarbonate" of Soda, used for effervescing draughts.