5 PREPARATION OF THE EMULSION

Collodion is prepared by dissolving a cellulose nitrate compound in a mixture of alcohol and ether. The proportions of ether and alcohol may vary according to the operator or manufacturer. If too much alcohol is added, the film becomes streaky; if ether is in excess, the film becomes too contractile and liable to split when drying. Aged collodion, rather than fresh, was preferred for the making of emulsions: “Collodion, like wine, improves by keeping,” wrote W. E. Woodbury ([1898] 1979, 111) in his encyclopedia. The emulsion was sensitized by mixing soluble silver salts (silver nitrate) into collodion containing an organic acid (citric or tartaric) and a chloride (strontium, lithium, calcium, etc.) so that silver chloride and other salts (strontium nitrate, silver citrate, etc.) were formed by precipitation (Clerc 1937, 349). A few drops of castor oil were often added to the emulsion to give suppleness and avoid risk of cracking (Clerc 1937, 349). It was also believed that the oil would assist the toning and be responsible for a better glaze of the finished print (Woodbury [1898] 1979, 111).