572 PHOTOGRAPHY 



Protostdphate of iron, which was first introduced as a photographic agent in 1840 

 by Mr. Kobert Hunt, may be employed, instead of the pyrogallic acid, with much 

 advantage. Generally the collodion portraits are now developed by the iron salt. The 

 following are the best proportions : 



Protosulphate of iron 1 ounce. 



Acetic acid 12 minims. 



Distilled water 1 pint. 



This is used in the same manner as the former solutions. 



Fixing of image. This is simply the removal of iodide of silver from the surface of 

 the plate, and is effected by pouring over it, after it has been dipped into water, a 

 solution of hyposulphite of soda, made of the strength of 4 ounces to 1 pint of water; 

 At this point daylight maybe admitted into the room, and indeed we cannot judge 

 well of its removal without it. We then see by tilting the plate to and fro the iodide 

 gradually dissolve away, and the different parts left more or less transparent, according 

 to the action of light upon them. 



It then only remains to thoroughly wash away every trace of the hyposulphite of 

 soda, for should any salt be left, it gradually destroys the picture. The plate should 

 therefore either be immersed with great care in a vessel of clean water, or what is 

 better, water poured gently and carefully over the surface. After this it must be 

 placed upright to dry or be held before a fire. 



The fixing processes. The most important part of Photography, and one to which 

 the least attention has been paid, is the process of rendering permanent the beautiful 

 images which have been obtained. Nearly all the fine photographs with which we 

 are now familiar are not permanent. This is deeply to be regretted, especially as 

 there appears to be no necessity for their fading away. In nearly all cases the fading 

 of a photograph may be referred to carelessness; and it is not a little startling, and 

 certainly very annoying, to hear a very large dealer in photographic pictures declare 

 that the finest pictures by the best photographers are the first to fade. This is, no 

 doubt, to be accounted for by the demand which there is for their pictures, leading to 

 a fatal rapidity in the necessary manipulatory details. 



There is no necessity for a photograph to fade if kept with ordinary care. It should 

 be at all events as permanent as a sepia drawing. Hyposulphite of soda is the true 

 fixing agent for any of the photographic processes, be they Daguerreotype, calotype, 

 collodion, or the ordinary process for producing positive prints. It should be under- 

 stood, whichever of the salts of silver are employed, that by the action of the solar 

 rays either oxide of silver or me,tallic.silver is produced, and the unchanged chloride, 

 iodide, or bromide can be dissolved out by the use of the hyposulphite of soda. 



The photographic picture on paper, on metal, or on glass, is washed with a strong 

 solution of the hyposulphite of soda, and the silver salt employed combines with it, 

 forming a peculiarly sweet compound, the hyposulphite of silver ; this is soluble in 

 water, and hence we have only to remove it by copious ablutions. The usual practice 

 is to place the pictures in trays of water and to change the fluid frequently. In this 

 is the danger, and to it may be traced the fading of nine-tenths of the pictures prepared 

 on paper. 



Paper is a mass of linen or cotton fibre ; howsoever fine the pulp may be prepared, 

 it is still full of capillary pores, which, by virtue of the force called capillarity, holds 

 with enormous tenacity a large portion of the solid contents of the water. If we make a 

 solution of a known strength of the hyposulphite of soda, and dip a piece of paper into 

 it, it will be found to have lost more of the salt than belongs to the small quantity of 

 water abstracted by the paper. Solid matter in excess has been withdrawn from the 

 solution. So a photographic picture on paper holds with great tenacity one or other 

 of the hyposulphites. By soaking there is of course a certain portion removed, but it 

 is not possible by any system of soaking to remove it all. 



The picture is, however, prepared in this manner, and slowly, but surely, xinder 

 Ihe combined influences of the solar rays and atmospheric moisture, the metallic silver 

 loses colour, i. e. the photograph fades. 



The only process to be relied on demands that every picture should be treated 

 separately. First, any number may be soaked in water, and the water changed ; by 

 this means the excess of the hyposulphite of silver is removed. Then each picture 

 must be taken out and placed upon a slab of porcelain or glass, and being fixed at a 

 small angle, water should be allowed to flow freely over and off it. Beyond this, the 

 operator should be furnished with a piece of soft sponge, and ho should maintain 

 for a long time a dabbing motion. By this mechanical means he disturbs the solid 

 matter held in the capillary tubes, and eventually removes it. The labour thus 

 bestowed is rewarded by the production of a permanent picture, not to be secured by 

 any other means. 



