FIXING-HARDENING BATHS Solutions for hardening the films of plates and papers and at the same time fixing the image. Their use does not yield such permanent results as the use of separate fixing and hardening baths. (See " Hardeners.") The best known formulae are : Alum " Hypo " Alum (saturated sol.) 20 OZ. 100 CCS.
Sodium sulphite (sat urated solution) . 6 3o " Hypo " sol. (I in 5) 20 100 Chrome Alum "Hypo" Add Strong sulphuric acid 6o drops cc.
Water . . . 2 oz. 50 CCS.
to Sodium sulphite . 2 OZ. 50 ccs.
Water . . . 6 15o and pour the mixture into " Hypo " . . . i6 oz. 40o ccs.
Water . . . 48 1,200 Finally add Chrome alum . . I oz. 25 ccs.
Water . . . 8 200 "Hypo" Acid Sulphite " Hypo " (in powder) I lb. 25o g.
Sodium acid sulphite 2 oz. 32 Water . >, i,000 ccs.
The second and third baths, being in an acid condition, must not be used for printing-out papers, and are suitable only for plates and bromide and gaslight papers. The " hypo "
and plain alum bath may, however, be used after toning.
Another form of fixing-hardening bath, which is largely used in hot countries and in England during hot weather, is the following : Sodimn hyposulphite . 2 oz. 220 g.
Potas. metabisulphite 28 Chrome alum . . 28 Water . . . IO 1,000 ccs.
First dissolve the " hypo " in 5 oz., half of the water made hot, next the metabisulphite in half the remaining water (cold), and add to the " hypo " mixture ; lastly dissolve the chrome alum in the remaining water (cold), and add to the hypo-metabisulphite mixture. Careless mixing causes turbidity, whereas the solution should be clear and of a greenish colour. The above being acid is suitable for negatives and bromide and gaslight prints, but not for P.O.P.