PHOTOZINCOGEAPHY 575 



tepid water is repeatedly passed over it with a light hand, plenty of water being used ; 

 this removes the gelatine and bichromate that have not been affected by light, and 

 they carry off with them the ink that was on their surface, leaving it only on the parts 

 of the print that have been rendered insoluble. Very gentle friction should bo used, 

 and during the process of sponging, the print should be soaked face downwards in 

 warm water for a few minutes, to assist in loosening the ink and removing the gelatine. 

 Soaking in this manner, for some little time, has a great effect in preventing blemishes, 

 in lessening the amount of friction which is sometimes liable to break up the finer 

 parts of the print, and in the case of sensitive paper, over-exposed to light, of im- 

 proving the resulting transfer. When the chromo-carbon print is sharp and clear, it 

 should bo well washed with tepid water so as to remove every trace of soluble gelatine, 

 as if any intervened between the inky surface and the zinc plate (or lithographic stone) 

 with which it is to be pressed in contact, there would be a flaw in the transfer. After 

 this washing, the print is dried and transferred to zinc 'or stone, in the same way as 

 an ordinary tracing in lithographic ink, only from the ink not having been long 

 exposed to the atmosphere, it is seldom necessary to pass it more than once through 

 the press. 



' The same number of impressions can be obtained as if the transfer had been effected 

 by means of a tracing in greasy ink. 



' In reproducing a large-sized map by photozincography, a series of rectangles are 

 drawn on it suitable to the size of the camera, and negatives are taken which shall 

 overlap each portion, the chromo-carbon prints from these being joined together before 

 they are transferred to the zinc-plate. To ensure a good result, the negatives should 

 be as nearly as possible of uniform density, the prints finished with a corresponding 

 thickness of ink on their surfaces, and the joining effected by cutting away the over- 

 lapping portions in such a manner as not to interfere with the names or prominent 

 objects on the map. In this way photozincographs exceeding 3 feet in length by 2 

 feet in breadth have been successfully prepared. 



'Photozincography has also been applied to the representation of buildings, natural 

 objects, scenery, &c., with more or less success, according to the strength of the con- 

 trasts of light and shade presented by the object ; the difficulty being to obtain fine 

 gradations of shade or half-tone. In this application of the art, the glass negatives 

 taken are left in the same condition as they would be used for making ordinary silver 

 prints of views and scenery, for if they were intensified with corrosive sublimate and 

 ammonium hydro-sulphate, as described in copying maps and engravings (where sharp 

 lines are required), all gradations of shade would be destroyed. 



' Chromo-carbon prints are made from these negatives in the manner before described, 

 and transferred to zinc, the only difference in their preparation being that the paper 

 coated with bichromate and gelatine is kept from four to five days after preparation 

 before being printed upon, and the transfer-ink is a little more burnt, when being 

 prepared, than the ink that is used for prints without gradations of shade ; considera- 

 ble experience is necessary in washing the prints so as to bring out the full effect of 

 the shadows, and much of the success of the process also depends on the skill and care 

 of the zinc-printer. An application of glycerine to the zinc-plate is found to assist 

 materially in keeping the transfer clean, and preventing the fine portions from being 

 clogged with ink ; 1 ounce of glycerine is added to 1 pint of freshly-dissolved gum 

 Arabic of the usual consistency, made neutral to test-paper by the addition of lime- 

 water, and 2 ounces of this mixture added to 12 ounces of water are used for wiping 

 over the surface of the plate ; the addition of the glycerine seems to keep the surface 

 moist for a longer time than the simple application of water, and renders the roller 

 less liable to ' catch.' Glycerine is also used in this manner in ordinary zincographic 

 printing, when a very large number of impressions are required from a plate. 



' Photozincographic views of natural objects generally have a better effect when 

 printed on tinted and enamelled paper than on an ordinary white ground, and paper 

 so coated adds materially, by lifting the ink better than a plain surface, in keeping 

 the transfer in good condition on the zinc. A paper of an agreeable tint may be pre- 

 pared with the following composition : 



, Size melted in 10 pints of water ... . 20 oz. 



Zinc white, ground in water and weighed wet . . .14 Ibs. 



Oxide of iron 3 oz. 



Chrome yellow 200 grains. 



Vermilion 30 



' The mixture is brushed while warm over the paper, any streaks being obliterated 

 with a flat camel's-hair brush ; and when the coating is dry, a second layer is applied in 

 a similar manner.' 



