256 



NATURE 



\yuly 20, 1876 



transfer-paper is now heated in very hot water, and applied to 

 the surface. It is " squeegeed " on to it, and you see it adheres, 

 this time, however, by its "stickiness." Here is another print 

 in the same stage, but the adhering paper is dry. Raising one 

 corner of it by my nail, I can grasp it in my fingers, and the 

 finished print strips off the plate held in position by the paper. 



Such are the usual manipulations in autotype printing, and 

 the pictures produced by this method should be permanent, and 

 they must be as permanent as leather, or as the pigment which is 

 employed to give visibility to the gelatine image. As I men- 

 tioned before, there are various modifications of the process ; 

 for instance, one is to develop the picture on the permanent sup- 

 port destined to bear it, using this instead of the zinc plate. A 

 little consideration will show you that in this case the negative 

 employed must be reversed. 



We now come to a large class of printing processes known as 

 photo-mechanical. And here I should state that the term 

 photo-mechanical is applied to such processes as are independent 

 of light for production of prints, after that agency has once fur- 

 nished a plate or means of producing a plate. The first of these 

 that I shall attempt to describe is that known as the Woodbury 

 type, after the inventor, Mr. Walter Woodbury. The following 

 outline will give some idea of the methods resorted to : — 



A skin of gelatine is prepared somewhat in a similar manner 

 to that which I shall describe in the heliotype process, only for 

 this it receives a tough film of collodion on one surface. This 

 surface is placed next a negative on glass, and the light from 

 an unclouded sun or from a luminous point (such as the electric 

 light) is allowed to fall on it. Owing to the thickness of the 

 gelatine employed, this method of exposure is necessary in order 

 to secure sharpness. The print is developed as in the autotype 

 process, and we get an image in great relief, formed by the in- 

 soluble gelatine, resting on the tough collodion film. When 

 dried, this relief picture is placed on the surface of a flat, soft 

 metal plate, and, by hydraulic pressure, is forced into it, fur- 

 nishing a mould, perfect in all its parts. The wonder is at first 

 excited that the gelatine does not break under the enormous 

 weight brought to bear upon it, but when it is recollected that 

 ferns and grasses can be made to furnish similar impressions, 

 the astonishment is diminished, in that the substance employed 

 is no V in a leathery condition. 



Apparently it matters little as to which side of the relief is 

 pressed into the plate. In one case we should have to use a 

 reversed negative, whilst in the other any ordinary negative may 

 be employed. This is important to the photographer, as may 

 be surmised. 



Before us we have the negative, a relief from it, and a mould 

 taken from the relief. This mould is now placed in this press, 

 which consists of a flat plate (which can give slightly in any 

 direction, and is capable of being raised or lowered) and a flat 

 hir ged top, to which is affixed a perfect plane of glass. When 

 this lid is brought dov/n on to the mould, the lower lid gives till 

 perfect contact is got between the two surfaces ; a species of 

 clamp enables the lid to be kept in position. You see on placing 

 this piece of paper m the mould, the clamp closes with difficulty, 

 but a little mechanical contrivance attached to it causes a great 

 pressure to be brought to bear. Opening the press once more, 

 a little warm gelatine, which has been impregnated with colour, 

 is poured on the mould, and a piece of resinised paper placed 

 over it ; the press is again closed. The mass of cold metal soon 

 cools the gelatine, and on opening the lid, it is found that the 

 excess of gelatine has been squeezed out beyond the mould, and 

 on lifting off the paper, a picture is found adhering to it. This 

 image is really formed in precisely the same way that a cook 

 forms her jelly in a mould, though the colouring matter in 

 this case is somewhat different. When dry, the picture is 

 rendered insoluble in water by passing it through an alum bath. 

 At first sight, this process might seem to be slow, but when it 

 is remembered that half a dozen moulds can be made from the 

 same relief, it requires no great exercise of the imagination to 

 surmise that the pictures may be produced almost as rapidly as 

 a lithograph. I referred to the relief necessary to produce the 

 mould. From what I have described it will be seen that the 

 dried relief must be as great as the wet prhtt of the autotype 

 process in order to produce the same gradations. 



The last process I shall describe is known as the heliotype 

 process, and I have chosen it for demonstration as I am prac- 

 tically acquainted with its working at Chatham, and not 

 from any inherent superiority it may possess. It is a type of 

 all the photo-mechanical processes, if we except Woodbury, 

 type, and it is to such as these that we must look for our 



book illustrations, though I am still in hopes that we may have 

 a really good process for surface printing from a metal block, 

 capable of being set up with type. We have a promising ex- 

 ample of this lattter process in what is known as Dallastint, the 

 offspring of Mr. Duncan Dallas ; but as it is a secret process I 

 cannot say anything regarding its production. 



In the heliotype process there are various operations. 



To begin with, there is the preparation of the gelatine film on 

 which the image is printed. 



The manner of preparing it is as follows : — Gelatine is dissolved 

 in water by aid of heat, and to it is added a sensitiser which con- 

 sists of potassium dichromate, to which a small quantity of 

 chrome alum is added. Now here I must remark that this 

 chrome alum forms an important part of the process. Gelatine 

 we know ordinarily dissolves in hot water, but if it be impreg- 

 nated with chrome alum, not only d les it render the gelatine 

 insoluble, but it also tous;hens it in a marked manner when it is 

 wetted. When the subsequent operations are explained, the 

 importance of th's property bestowed on the gelatine will be 

 manifest. The solution of gelatine (with this sensitiser mixed in 

 it) is flowed over a carefully levelled glass plate to such a depth 

 that in drying it has the thickness of a piece of Bristol board. 

 The glass plate may be ground and very slightly waxed ; or it 

 may be coated with a dilute solution of india-rubber to facilitate 

 the gelatine leaving it, when it is required to be employed for 

 printing purposes. A negative (which must be what is known as 

 a reversed negative) is placed in a pressure frame, the gelatine is 

 stripped off the plate, and the surface, which was next the glass, 

 is in contact with the taken image. The necessary exposure 

 may be estimated by an aclinometer or by examining the image 

 in the printing frame. When judged to be sufficiently printed, 

 the back of the print is hardened by exposure to lighf. This 

 operation gives toughness to the gelatine and renders it capable 

 of resisting the treatment it has sut)sequently to undergo. 



The skin of gelatine is next taken, and immersed for a few 

 seconds in cool water (in practice a temperature of over 60° F. 

 is found to be the best). A pewter or other metal plate, coated 

 with india-rubber, is now placed underneath it, and the 

 film caused to adhere to it by the use of the squeegee. The 

 pressure of the atmosphere causes the adhesion as it does in the 

 autotype process. For convenience' sake the edges are now run 

 round with a solution of india-rubber in benzole and paper 

 pasted round them, to prevent the water getting beneath the 

 skin. The plate is then immersed in cold water for about half- 

 an-hour, to soak out the unaltered dichromate, and it is ready for 

 use as a printing surface after the superfluous water is blotted off. 



The gelatine skin is all in an insoluble state owing to the presence 

 of the chrome alum ; but further, the part where the light has acted 

 fully will not absorb water, whilst that which only partially absorbs 

 water has only been partially acted on by light, and the part 

 wholly unacted upon absorbs it greedily. When a roller con- 

 taining greasy ink is passed over it, those parts which contain a 

 great deal of water take no ink, particularly if it be stiff ink. 

 The parts containing a little water take the ink lightly, whilst 

 those parts which have refused to imbibe any moisture take it 

 greedily. Evidently here we have a means of obtaining a picture 

 of half-tone subjects in printers' ink. Another point is that thin 

 ink takes better in a partially exposed portion than does a thick 

 ink, hence to bring out the half tone it is customary to use two 

 or even three inks of different consistency. The printing plate is 

 generally placed in the bed of an ordinary printing-press and rolled 

 up with a soft roller or rollers, charged with the printing inks. 

 The impressions are pulled off as f.r Liter press, though more 

 force is necessary. In order to ha^•e clean margins a mask is 

 cut of the proper dimensions, and brought to certain register 

 marks. The paper, usually employed for receiving the impressions, 

 is enamelled, the enamel being formed of barium sulphate and 

 gelatine. Any ordinary paper, however, may be used, if it have 

 the power of taking up the mk. On the walls of the exhibition 

 are some photographs printed on ordinary drawing paper, and 

 they are effective in their way. 



Mr. Edwards, the patentee of this process, proposed to use 

 a series of gelatine printing surfaces from the same negative, 

 to form a species of photo-chromotype, [and I have seen 

 some specimens which are very successful. Little seems to 

 have been done, however, in this direction at present. When 

 drawing your attention to the manufacture of the gelatine 

 sl<ins there was one point to which I did not allude. You 

 may make your skin of jelly or of blanc mange. I have 

 found that a certain proportion of milk added to ttie gelatine in 

 litu of the water gives more delicate pictures than does gelatine 



