236 Photographic Processes. 



Collodion negatives may be obtained with less than half 

 the ordinary time of exposure by plunging the plate, after it 

 has been sensitized with the nitrate of silver, into a concen- 

 trated solution of acetate of silver, and developing with pyro- 

 gallic acid. 



If, after developing a collodion negative as much as possible 

 by the ordinary method, a solution of sulphuret of potassium 

 and a solution of protosulphate of iron are poured alternately 

 upon it, water being used abundantly in washing it after each 

 solution is employed, it will become so opaque as to be abso- 

 lutely black and white. 



Collodion proofs may be developed positive, by means of 

 an alcoholized solution of sulphate of iron containing acetic 

 acid and nitrate of potash, which makes the lights of a dead 

 white ; or with an alcohohzed solution containing less iron and 

 acetic acid, but nitrates both of silver and potash and nitric 

 acid, which gives the lights a brilliant metallic appearance, 

 The free acid in each mixture tends in a special manner to 

 preserve the shadows. Without the alcohol the mixture would 

 not run freely over the plate. Cyanide of potassium is used 

 for fixing. The presence of a small quantity of copper in the 

 sensitizing bath used with the paper for positives, causes it to 

 afford vigorous and effective pictures from feeble negatives, 

 but does not answer so well with good negatives. As, however, 

 it renders the process tedious, it is objectionable to the profes- 

 sional photographer. If, after developing, the plate is drained, 

 and coated with glycerine, it may be left for some days Without 

 being finished. The glycerine prevents the oxidation of the 

 iron, and increases the adhesion of the collodion to the glass ; 

 it has the property of continuing moist, and is easily removed 

 with water. 



Dry Collodion Process. The glass plate having been coated 

 with collodion, sensitized with nitrate of silver, and well 

 washed, it is brushed over with a bromo-iodated solution of 

 albumen, which preserves it from the decomposing action of 

 light, so that it may bo kept for two years or more. It must 

 be sensitized anew at least one or two days beforo being used. 

 Dry collodion requires four times as long an exposure in the 

 camera as moist. 



A preparation of malt, of malt and tannin, or of tannin 

 and glycerine, has bees used with great success instead of 

 the albumen. Also ammonia has been employed with excellent 

 effect in the development of dry collodion negatives; the time 

 required, both for exposure and development, being greatly 

 shortened. Ammonia, for some unknown reason, has no effect, 

 occasionally : the development is effected in such cases with 

 caustic potash. ... 



