ftteaching Powder Blocks for Printing
brush is shown in the subjoined sketch, fig. 15. (See also Buckle's Brush.)
Bleaching Powder. The so-called chloride of lime of the shops, the active ingredient of which is probably calcium hypo-chlorite, and this in contact with acids liberates the powerfully oxidising hypochlorous acid.
Bleaching Prints or Engravings. This is often necessary before making a photographic copy. Agitate \ oz. of fresh bleaching powder (see preceding article) in a pint of water, and filter. The print is first soaked in water and then transferred to this solution, and ten minutes or so should be allowed for com-plete penetration. The print being removed, a dozen drops or so of hydrochloric acid are stirred in and the print returned to the bath, where the discoloration should gradually disappear ; but failing this, the print should be once more removed and acid again added. When the bleaching is complete, a very thorough removal of the bleaching agent by long soaking in many changes of water is essential.
Blisters. One of the worst troubles of an amateur, whether working with plates or paper. On the former, blistering is usually the precursor of a general Frilling (q.v.~). Blisters invariably make their appearance on prints, either in the fixing bath or the first washing after. It is more generally a fault with papers that are heavily charged with albumen and salt, and in this case it is most likely due to exosmose action. The remedy is to plunge the prints immediately after fixing into a saturated "solution of salt, and use all solutions at the same temperature. Blisters are frequently caused also by an accumulation of gas behind the film of albumen, and in this case it would seem to bethe action of hypo upon a partially decomposed albumen. A cure for this is to dip the print on the first sign of blistering into a bath of methylated spirit. In the case of some kinds of gelatino-bromide papers, which seem particularly liable to blisters, a bath of chrome alum 2 grs., water 1 oz., methylated spirit 1 oz., will be found efficacious. Formalin, properly used, is an almost certain preventive. See Formalin.
Blocks for Printing. The many methods of making printing blocks by photographic means are treated of under special heads,