GLASS, CLEANING Glass for photographic purposes must be scrupulously dean. Glass upon which prints are squeegeed for glazing is best cleaned by soaking in very dilute nitric acid and scrubbing with soap and water ; after drying, it is dusted over with French chalk and polished. When glasses are to be used for coating and sensitising, as in the wet collodion process, rouge or whiten ing or fine tripoli powder mixed with methy lated spirit to the consistency of thick cream is recommended ; the paste is rubbed over the glass, rinsed off with water, and a final polish is given with chamois-leather.
A good mixture for thoroughly cleaning glass for ordinary purposes is Soft or rain-water . . i part Powdered pumice stone Chalk or whitening Liquor ammonia . . . Apply with a piece of flannel, and polish with a soft rag, chamois-leather, or soft paper crumpled up.
To dean films off old negatives, soak in hot soda-water for a few minutes, and scrub with a brush ; or, if they are not varnished, soak them for an hour or two in water made slightly acid with nitric acid ; the films can then be rubbed off with a strip of wood, or placed in hot soda water and scrubbed. When the negatives have been varnished, it is necessary to use a strong and hot solution of common washing soda or caustic potash, and leave the negatives in this until cold, when the films will leave the glass. Do not let
the caustic potash solution touch the fingers.
In process work, in which collodion and collo dion emulsion are used so largely, it is necessary to take great pains in cleaning glass. New glass is best soaked in a 5 per cent, solution of hydro chloric acid contained in grooved lead - lined troughs ; and it is then rinsed with plenty of water and polished with methylated spirit 20 oz., tripoli io oz., iodine 2 drams. Prepared chalk or whiting may be used instead of tripoli, and equal parts of methylated alcohol and ammonia instead of iodine.
For removing collodion films from old nega tives, nitric acid is generally used in the pro portion of a oz. acid to 6 oz. water. A good film-removing pickle, free from fumes, consists of Sulphuric acid . . 4 oz. 200 ccs.
Potassium bichromate . 4 22o g. Water . 20 I ,000 ccs.
After treatment, swill and put in the draining rack. Next, with a linen rag charged with whiting of the consistency of thick cream, rub well both sides of the partially dry negative glasses ; then put into a clean bath composed of nitric acid 4 oz., water 4o oz. Finally, swill under a tap and then albumenise.