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Collodion Wet Process for

grains, water and nitric

COLLODION WET PROCESS FOR POSITIVES.—Very little difference in the manipula tion is necessary for the production of positives by the wet process than for negatives as described under Collo lion Wet Process. The chief object is to obtain the deposit of silver as white as possible. The collodion is the smile as that recommended for the production of negatives. The silver bath should be weaker. A good formula is : Silver nitrate (recrystallized) 25o grains Nitric acid }( minim Water to ounces For development the pyrogallic developer recommended for negatives can be used. Abney recommends the following ; Ferrous nitrate i to grains Ferrous sulphate 6o grains Nitric acid. 20 minims Alcohol quant. suf. Water. 4 ounces The ferrous nitrate may be prepared by taking barium nitrate 130.5 grains, and dissolving it in two ounces of water. A solution of 76 grains of ferrous sulphate, dissolved in two ounces of water, is next added to it. A precipitate of barium sulphate falls. This must be filtered out, and 110 grains of ferrous nitrate are left in solution. The nitric acid should be dropped in, diluted with half-an-ounce of water. The alcohol is added after the 6o grains of sulphate of iron

have been dissolved. The effect of the nitric acid is to cause the silver to deposit with a white lustre by reflected light. The exposure required is less than for a negative. The cyanide fixing solution should be used. As soon as dry the film side is varnished with a black varnish. The image is then seen from the back in its proper position. A receipt for a good black varnish will be found under Varnish.

Instead of glass, ferrotype plates may be used. These are thin iron plates enamelled with black or chocolate-brown enamel. They are first cleaned by the application of a little dilute potash or dilute nitric acid, and finally washed in distilled water, and polished with a silk hand kerchief. They are then coated with collodion, sensitized, exposed, developed and fixed in the manner described. The dried picture only requires to be coated with a protected transparent varnish. It must be remembered, however, that with positives of this kind the image is re versed.