BLISTERS Blisters appear at times upon all makes of plates, films, and papers in the manufacture of which albumen or gelatine is employed, but the papers most subject to the trouble are albumen and bromide. The principal cause of blisters is the use of a too strong "hypo" bath, rapid washing, excess of alkali in the developer, and the difference in temperature between the developing, toning, or fixing solutions and the washing water. The blisters usually appear when the plates or papers are being washed after, fixing. It is a curious fact that fewer blisters appear where ordinary tap water is used than where soft water is employed. It is, however, the fixing bath that usually needs attention when bromide and gaslight papers are prone to blister. The fixing bath, as freshly made with cold water and " hypo," should not be used immediately, the temperature of such a solution dropping almost to freezing point, and of course many degrees lower than the temperature of the washing water used before and after ; hence the expansion of the wet gelatine, which is very susceptible to temperature, in the form of blisters. If a fixing bath is needed quickly it should be made with hot water and used when the temperature has fallen to the level of the washing water ; or, if cold water is used, the bath should be mixed some considerable time before use in order that it may have time to rise in temperature. When care is not taken about the temperature of the bath, blisters may be prevented by allowing the washing water to run gradually into the fixing bath while the fixed prints remain therein. The water gradually replaces the "hypo," the differ ence in temperature (if any) is gradually made up, and the expansion of the gelatine is too slow to do any harm ; this method, too, largely prevents blisters due to the use of a strong fixing bath.
The use of an acid fixing bath is widely advo cated for the prevention of blisters on bromide papers and negatives. Any formula will serve, but that containing " hypo " and metabisulphite will be found best. Another plan is to soak the prints previous to, or immediately after, fixing in a 20 per cent, solution of formaline and then to wash well. There is really no satis factory cure for blisters when once they have appeared ; pricking the paper at the back with a pin for the purpose of allowing the air to escape from the bubble has been advised, hut the loosened film never becomes properly attached to the paper, and frequently peels off when dry. Another plan is to squeegee the
blistered print upon cleaned ground glass and strip when dry, but, as in the previous remedy, the blisters invariably scale off later.
Information given above on preventing blisters applies equally well to negatives and bromide and gaslight papers. Blisters but rarely appear on negatives, but when they do the negatives should be soaked in methylated spirit and dried ; a more general trouble with plates is " frilling " (which see).
Albumen paper, as a rule, blisters very badly if carelessly manipulated, the cause being the unequal temperatures of the solutions employed. Rives paper, which is now almost universal, is thought to blister more than the Saxe paper, which is tougher, but not now so widely used. Preventive measures are (a) to use solutions of as even temperatures as possible ; (b) to soak the prints in hot water or methylated spirit, afterwards washing well, previous to toning ; (c) to remove the prints from the ordinary alka line fixing bath (acid baths must not be used for albumen paper) to water to which has been added one-tenth its weight of common salt, allowing them to remain for ten minutes and finally washing well.
Carbon prints show minute blisters when the water used is too hot. Blisters may be also due to free air in the water, greater trouble in this direction being experienced when water cornea direct from the main than when it comes through a cistern. It is advisable to boil sufficient water for the bath in which the tissue is to be soaked before squeegeeing to the temporary or final support. Fifteen minutes' boiling should be sufficient to expel all the air, and it is then in a jug in order that only a small surface may be exposed to the air, of which water can absorb a large quantity.
Gelatine (P.O.P.)prints rarely blister, but when they do the cause is a too strong " hypo " fixing bath, or the unequal temperatures of solutions and water. An acid fixing bath must not be used for P.O.P. ; if hardening is thought necessary, formaline should be used.
The German name for flashlight powder. It is occasionally used in English and American literature. In the United States, a powder having this name contains the mixed nitrates of barium and strontium 5 oz., metallic magnesium 2 oz., and amorphous phosphorus 12o to i 8o grs.