260 PHOTOGRAPHIC FACTS AND FORMULAS 
It is important that the paper base should be free from 
wood pulp, preferably sized with starch, and with a hard 
surface. It has been suggested to add white dextrine to the 
sensitiser, but this lowers the keeping quality. The paper 
should be exposed preferably to sunlight until the ground, 
corresponding to the whites of the negative or plan, appears 
a dark olive-green; it should then be thoroughly washed with 
running water, and the lines should appear white on a deep 
blue background. With stale paper or with excess of or 
impure ferricyanide, the lines will appear more or less tinted. 
With underprinting the colour is more or less pale. Fre- 
quently, by somewhat prolonging the washing, the lines will 
clear up; or this can be much curtailed by using a 0.01 per 
cent solution of ammonia or sodium carbonate. This gives 
a violet tinge to the blue, and this can be corrected by subse- 
quent treatment with an acid, 5 per cent of hydrochloric, 
oxalic or sulphuric acid, or alum solution of the same 
strength. A much more stable and contrasty image is ob- 
tained if the print is developed with a 0.2 per cent solution 
of potassium ferricyanide, and well washed for four or five 
minutes. 
Numerous methods have been suggested for toning blue 
prints, but, as a rule, the results are extremely unsatisfactory. 
Immerse the dry print in: 
Ammonia (sp. gr. 897) 12.5 ccm 96 mia. 
Water 1000 ccm 16 oz. 
As soon as the blue color disappears, wash for a few minutes, 
and then immerse in a 2 per cent solution of tannin; this 
gives a blackish colour, but the outlines may become blurred 
from spreading of the colour into the whites. Or the print 
may be well washed in distilled water, and bleached in yellow 
light in: 
Silver nitrate 20 g 154 gr. 
