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or Bleaching Powder

solution, prints, water, blisters, blistering and fixing

BLEACHING POWDER, or calcium hypochlorite, is prepared by passing chlorine over slaken lime. It decomposes rapidly and in different ways by keeping. Its uses in photography are based upon the fact of its containing chlorine in such a loose form that any acid will liberate it. See Calcium Hypochlorite.

BLIND instantaneous or time shutter, working in the form of a blind, passes across the lens or the plate. See Shutter.

BLISTERS.—Blistering of the film is an annoying defect which occurs with plates and papers coated with gelatine, collodion or albumen. Films of this nature expand on being wetted, so that the probable cause of the formation of blisters is due to the non-adherence to the glass or paper of certain portions of the film. It is also probable that blistering is due to the generation of carbonic acid gas underneath the film when treated with the hypo bath.

In the albumen process blisters generally make their appearance in the prints while in the fixing bath or in the first washing water, sometimes due to a strong saline solution being shut in between the albumen and the paper support, and which is afterward brought into contact with water. Papers which have received a double or treble coating of albumen are most liable to blister. Blistering in albumen prints is often caused by rough washing, in which the prints are subjected to severe handling or tumbling, and also to a gas being formed by the decom position of organic matter. The best known remedy is the use of all solutions used in the toning, fixing, and washing operations at a certain temperature, and the addition of a little common salt to the first washing water If the blistering is very bad, the prints should be plunged immediately after fixing into a saturated solution of common salt, or the following : Table salt i pounds Ammonia ....36o grains Water t gallon In this solution the prints should be floated face downward for about ten minutes, and afterward thoroughly washed. W. H. Sherman recommends the use of a salt solution followed

by immersion in a 5 per cent. solution of carbolic acid.* The following remedies against blistering of albumen paper are given : Do not dry the paper by excessive heat. Avoid acidity in solutions. Moisten the prints before washing with a sponge saturated in alcohol. Immerse the prints before fixing in a weak alum bath. Add a trace of ammonia to the fixing bath. Add one-tenth part of alcohol to the ordinary toning bath.

Gelatino-bromide papers are also liable to blister. If during the summer, the tempera ture of the solutions must be lowered. Blistering with this kind of paper can be cured by laying the prints for a few minutes in a solution composed of io grains of chrome alum dissolved in 4 ounces of water, and another 4 ounces of alcohol added. If the blistering is not very bad, the prints may be laid in methylated alcohol only. The addition of i drachm of the following to each ounce of fixing solution is also a preventive of blisters : Sulphate of aluminium 3 ounces Water 24 ounces This solution should be well shaken and then allowed to stand until clear.

Blisters are often caused by the presence of foreign substances, such as lime, in the water. In this case the water should be for about fifteen minutes before using.

With develqped papers of this kind the method of adding a little methylated spirit to the developer (about 5 per cent. will suffice) may often be the means of preventing blisters.

Blisters with dry plates mly b2 avoided by immersing the plate immediately upon the first signs of their appearance into methylated spirit, or into a saturated solution of chrome alum, or a weak solution of formic aldehyd.