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August 31, 1883.] THE photographic news. 549 sun will remedy the defect. Should they take too much ink and be too heavy, a slight wash with acid will lighten them ; but it must be used with great care to avoid washing away the fine lines, because the insoluble gelatine will not stand the prolonged action of any acid. When the stone is perfectly cleaned, and in good order for printing, it is printed off just in the same way as an ordinary drawing on stone. It is said that 1,500 impres sions may be taken from a stone without sensible deteriora tion. The same method is applicable to zinc, but the zinc, being less porous and capable of imbibing moisture, is less suit able than stone for these processes. It has, however, the advantage of being easily handled, and of rendering it com paratively easy—if thin plates be used—to obtain perfect contact between the negative and plate during exposure to light. The plates of zinc used are thin, and of the kind used for roofing purposes, but carefully planished, so as to have a perfectly even surface. Captain Hannot recommends that, instead of being grained, the plates should be polished with emery, and finished with a piece of fine emery paper applied on a small block of wood covered with a piece of thick cloth. Senhor Rodriguez, of Lisbon, working a similar process, prefers to grain the plates with powdered pumice stone. All impurities are removed from the surface by a wash with caustic soda, and the plate is then coated as thinly as possible with the following solution :— Gelatine 2 parts Bichromate of ammonia 2 „ Water 100 „ Or, according to Rodriguez — Gelatine 3 parts Bichromate of ammonia 1 part Water 100 parts The sensitive coating may be dried off over a stove or lamp, and the plate exposed to light under a reversed nega tive in an ordinary pressure frame, care being taken to obtain perfect contact. The length of exposure is usually about five minutes in the sunshine. After exposure, the plate is covered all over, by means of a leather or india-rubber roller, with a mixture of 4 parts of litho printing ink and 1 part of transfer ink. It is then washed in warm water to remove all the unaltered gelatine. When all is clear, it is rinsed in cold water, and etched with a solution of— Gum-arabic 40 Sulphate of copper 2 Gallic acid ... 5 Nitric acid ... | Water 1,000 This is allowed to dry on the plate. The subsequent operations of proving and printing are as described in the last chapter. In an ordinary lithographic scraper-press a plate will only furnish about 300 to 400 impressions ; but more may be printed by using a roller-press or vertical pressure. Senhor Rodriguez soaks his plates in cold water for two or three hours, then passes a lithographic roller over the plate to develop the image, dissolves the unaltered gelatine by washing in warm water, and finally etches with the same solution as given above. Senhor Rodriguez does not seem to recommend the use of this process, preferring to employ the image on zinc for transfer purposes only. The zinc plates, previously washed over with a ten per cent, solution of caustic potash, are coated very thinly and evenly on a turning-table with a solution of— Gelatine ... ... ... ... 8 parts Bichromate of potash 3 „ Water 100 „ When dry, the plate is exposed to light for eight to fifteen minutes in the sun; then well washed in cold water, and dried. It is then damped lightly with a sponge, wiped with a soft cloth, aud inked-in with a good lithographic roller charged with a mixture of equal parts of printing and transfer inks. An excellent image is thus obtained, which can be transferred to stone or metal. M. Gey met has giveu a very full description in his Photo- lithographie of a direct process on stone. He recommends, as a sensitive coating for the stone, a preparation made by taking the whites of two eggs, adding six grammes (about 90 grains) of bichromate of ammonia in powder, and beating up the whole to a froth. After settling, the clear solution is applied to the stone. Albumen is very suited for fine work, and gives more im pressions than gum, but the latter is easier to work. The proportions of gum and bichromate are not of much consequence. A quantity of gum is placed in a suitable vessel, and j ust covered with water. When dissolved, a suffi cient quantity of the thick solutionis taken and mixed with one-third its volume of a saturated solution of bichromate of potash. The mixture is strained through muslin. With a brush or cloth some of the solution is spread all over the stone without regard to the evenness of the coating. With out waiting, the stone is wiped with a clean cloth, and well rubbed, so that while the excess is removed, the sensitive solution is made to penetrate the pores of the stone. The operation is continued with a soft cloth free from fluffiness till the surface appears bright and the stone becomes dry. It is essential that no free gum should remain on the surface; it must only be in the pores of the stone, and the surface should appear quite bright. An even coating given in any other manner will not answer. The prepared stone is allowed to remain for about a quarter of an hour in the dark, inorder to become thoroughly dry, and may be found to hasten the operation. It is then ready to be exposed to light. M. Geymet exposes his stones to light under a reversed negative in an ordinary pressure frame, with its sides deepened sufficiently to hold the lithographic stone. The back of the frame is fixed in its place by hooks, and the pressure given with screws. The exposure is very short, about half a minute in the sun, and two to four minutes in a good diffused light. The image should not be visible. After exposure, the stone is inked in the dark with an ink composed of transfer ink and varnish. For this operation two lithographic rollers will be required, and if new, they should be prepared as described in Chapter IX. The stone is uniformly inked all over with one roller, and is then plunged bodily into a vessel containing water mixed with 2 per cent, of nitric acid, and the same quantity in weight of gum. It is taken out again immediately, aud rolled, while wet, with the second roller, which lifts the ink from the parts which should remain white, while the altered parts retain the ink, and thus the image is developed. The stone is then washed under a tap and dried, though sometimes a second inking is required if the ink appears to leave the lines. Corrections can now be made, and the stone is gummed. If the image is under-exposed, the lines will appear broken and pale, and the operation must be done over again. If much over-exposed, there will be a difficulty in clear ing up the image ; but if only slightly so, a brisk and pro longed treatment with the roller, and a second application of the gum and acid bath, will generally clear it, and pro duce a good result. It is, therefore, better to slightly over expose than not to expose enough. It is recommended to defer the gumming of the stone till the day after development, and then to let it rest for some hours before applying the acid. The gum is washed off, and the stone is treated with water acidulated with nitric acid, so as to be just acid to the taste. The acid is allowed to act for a minute, or less if it seems to act too strongly,