292 Albumen, Collodion, etc. 



but salted paper, though well suited for the artist who may 

 wish to retouch the picture, is, on account of the way in which 

 paper is manufactured, liable to several imperfections. 



The nature of the material used, the mode of sizing, etc., 

 modify the photographic results. The metallic stains, produced 

 by abrasion of the instruments used in preparing the pulp, are 

 brought out ; maize paper, being made without the machinery 

 required for tearing the rags in pieces, is free from particles of 

 metal. A picture produced on paper, by means of nitrate and 

 chloride of silver, may be effaced by washing with a solution 

 of corrosive sublimate, so that, unless iodide is present, no 

 trace of it will remain ; it will, however, be revived by hypo- 

 sulphite of soda, or caustic ammonia. 



Albumen, Collodion, etc. When the silver salt is diffused 

 through the whole substance of the paper, the solidity of the 

 picture is augmented, on account of its penetrating to a greater 

 depth, but there is a serious waste of a costly material. To 

 prevent this, the paper is coated with albumen, glue, isinglass, 

 etc., and only the film produced by these substances is sensi- 

 tized. Albumen is spread with difficulty, and is very liable to 

 dust. It is a mistake to suppose that it can be coagulated by 

 the same means as white of egg in its normal state. White of 

 egg is coagulated by a temperature not exceeding about 146° 

 Fahr. ; dry albumen is scarcely coagulated at all by mere heat, 

 and is not altered by a temj)erature of nearly 400° Fahr. Since, 

 however, its coagulation is very desirable, for the purpose of 

 economizing the nitrate of silver, many, attempts have been made 

 to render it easy ; metallic salts have been found very effective 

 for the purpose. When albumen is used, the picture contains 

 three superimposed images, one formed by the albuminate of 

 silver, another by the chloride, and a third by the nitrate ; the 

 two first, when in good proportion, secure a proper balance be- 

 tween the lights and shades ; if there is too much albumate, 

 the picture will be red, purple, brown, and finally metallic, with 

 a red. shade, which the toning bath will with difficulty, if at 

 all, correct. The nitrate augments the intensity, and secures 

 rapidity. 



Waxed paper affords positives which are free from the im- 

 perfections due to the transmission of light through negatives 

 of irregular texture and more or less yellow in colour, but it 

 is not always uniformly translucent ; and as it is with difficulty 

 permeated by liquids, it contains but a very small quantity of the 

 silver salt. To obviate the latter inconvenience, paper rendered 

 transparent, by means of gelatine, certain resins dissolved in 

 alcohol, etc., are sometimes employed, but a plate of glass has 

 much the advantage of all other media ; and the collodion with 

 which it is coated may be considered as a species of paper 



