386 HANDBOOK OF PHOTOGRAPHY 



of sodium sulphate is a typical temporary hardener. The hardening produced by 

 such substances is reversible, i.e., gelatin will subsequentlj- absorb water and swell. 



Permanent hardening is characterized by a reduced absorption of water (swelling) 

 by the gelatin during subsequent washing. Various materials maj^ be used for 

 permanently hardening gelatin, such as formalin, quinone, tannin, organic developer 

 oxidation products, and certain inorganic compounds. Formalin, quinone, and 

 developer oxidation products harden gelatin only in alkaline or neutral solutions, 

 and their application is therefore limited to use in developers or to hardening of com- 

 pletely washed film. 



Of the inorganic compounds, the salts of iron, chromium, and aluminum exert, the 

 most powerful hardening action on the gelatin. Salts of aluminum are, perhaps, the 

 most satisfactory hardeners because thej^ are colorless, are readily soluble in water, 

 and do not form colored compounds with the common developing agents, either in 

 acid or alkaline solutions, but they give satisfactory hardening provided the wash 

 water is not above 75 to 80°r. Sodium and potassium alum are equally efficient, but 

 with the ammonium alum an evolution of ammonia takes place after the fixing bath 

 becomes alkaline, and this tends to produce dichroic fog. 



Composition of Fixing Baths. — Bj'- way of summary it may be stated that (1) plain 

 fixing baths contain only a solvent for the unexposed silver halide grains, (2) acid 

 nonhardening fixing baths contain the silver halide solvent as well as an acid anti- 

 staining agent and preservative or sulphurization inhibitor, while (3) acid hardening 

 baths contain a gelatin hardening agent in addition to the ingredients alreadj^ enumer- 

 ated for (2). 



The following tabulation shows some of the materials which have been used in 

 fixing baths of the various types enumerated : 



1. Silver solvent : 



Sodium thiosulphate (sodium hyposulphate, or "hypo") 

 Ammonium thiosulphate 



2. Antistaining agent. Practically any acid may be used, although organic acids are 

 preferred because they show less dissociation. Suitable antistaining agents are: 



Acetic acid 



Citric acid 



Tartaric acid 



Oxalic acid 



Lactic acid 



Malic acid 



Maleic acid 



Sodium sulphite 



Sulphuric acid 



Sodium bisulphite ) . , , 



Potassium metabisulphite ) 



3. Preservative or sulphurization inhibitor 



Alkaline sulphites (sodium sulphite, for example) 

 Alkaline salts (sodium acetate, for example) 



4. Gelatin-hardening agents 



Potassium alum 



Chrome alum 



Sodium sulphate (temporary hardener) 



Formalin (formaldehyde) ) 



Quinone j-in alkaline or neutral solutions only 



Tannin \ 



