PHOTOMECHANICAL PROCESSES 337 
Rinse in cold water and dry. If etching is to be done at 
once, flow over with equal parts of methyl alcohol and water 
and dry on whirler. 
Varnishing. The back and edges of plate should be 
painted with a 20 per cent solution of bitumen in benzole or 
Mogul varnish thinned down. 
Etching Solutions. Anhydrous ferric chloride is used. It 
is advisable to add to this 6 per cent of freshly precipitated 
ferric hydroxide, just cover with distilled water, and make 
solutions up to 43, 40, 38, 36 and 33 degrees Beaume with 
distilled water. The following strength solutions correspond 
to the above: 43° = 46 per cent; 40° = 41 per cent; 38° = 
39 per cent; 36° = 36 per cent; 33° — 33 per cent. To each 
of these the ferric hydroxide may be added, well shaken and 
allowed to settle for use, or the whole solution made up very 
strong, heated with the hydroxide and then diluted to the 
above strengths. The weaker the solution the stronger the 
etching. As soon as etching is complete, rinse the plate, 
immerse in 10 per cent solution of caustic potash and rub 
with swab of cotton. Then rinse, dry, remove grain with 
benzole, clean plate with 1 per cent nitric acid and finally 
with washed whiting and ammonia. 
PHOTOLITHOGRAPHY.—Transfer paper should be sensitised 
with a 4.5 per cent solution of potassium bichromate, rendered 
alkaline with ammonia in winter and with the following in 
summer : 
Potassium bichromate 22.5.2 175 gr. 
Manganese sulphate Sls) & 24 gr. 
Water 1000 ccm 16 oz. 
Temperature of bath 15° C. (60° F.), time of immersion 
5 minutes. It is as well to squeegee the paper to plate glass 
so that it may dry with a polished surface. After printing 
place on rolling up board and apply the ink in a very thin 
