The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

Home | About | Photography | Contact

small tuft of rag, a few drops of very thin gum-water being used. The amount of moisture thus communicated to the block need not be more than in the usual process of facing with white pigment; indeed, the operation is practically the same. Before* exposure, the sensitiveness of the oxalate surface may be in-creased by fuming with ammonia (see Fuming). The progress of the printing, which should be under a reversed negative (see Reversed Negatives), cannot be watched in the usual way, but if negative and block are carefully adjusted into one corner of a deep printing-frame, it becomes practicable to remove the block from time to time and to replace it in exact register. Another method is to sensitise a strip of stout paper in the same way and at the same time as the boxwood block is sensitised, and to expose this as a guide strip under a negative of similar density to that used for the print on the block. If the engraver is to work by lamplight and immediately, no fixing will be required ; but in other cases a partial fixing - or, rather, a partial desensitis-ing - may be effected by moistening a few sheets of blotting-paper with a saturated solution of ammonium chloride, or sal ammoniac, and laying this on the surface of the block, contact being established by a plate of glass and a weight. Ten minutes is generally sufficient, and the moisture communicated to the block need not be so much as in making the usual drawing in washes of Indian ink. The oxalate of silver used should be precipitated in contact with excess of silver nitrate; or, better still, it should, after precipitation, be infused as a solution of silver nitrate, con-taining about 28 grs. to the ounce.
Woodburytype. This is a very beautiful photo-mechanical process, and consists of exposing a thick film of bichromated gelatine to light under a negative; and when fully exposed it is washed to dissolve the unacted-upon soluble portions, and after being soaked in alum is dried. When dry, the gelatine print, which at this period looks like a delicate piece of silk with the image in relief, is placed on to a bed of metal, and a pressure of from four hundred to five hundred tons brought to bear on it. This forces the gelatine into the metal, and makes an impression the same as a seal on hot sealing wax, the film of gelatine itself being unharmed and used over and over again. The metal sheet bearing an impression now becomes a mould, and this is placed