PHOTOGRAPHY 



151 



inercially. Ami after being exposed to light beneath 

 a negative in a printing-frame for aliout one-third 

 of the time necessary in the case of a mint on 

 silvereil paper, its lemon-yellow tint is found to 

 change, where the light has reached it, to a pale, 

 dirty -gray colour. Development is conducted in an 

 iron enamelled tray, beneath which a .spirit-lamp or 

 bunsen burner is placed, so as to keep the oxalate 

 solution at a temperature of about 175 F. Under- 

 exposed prints will l>eiiefit by this temperature 

 being exceeded, whilst those which have received 

 more light than they should have had can be 

 advantageously treated with a much colder de- 

 veloper. The print is floated on this warm bath, 

 which turns the faint image of the picture to a 

 dense black, and fixation follows by placing the 

 picture in a series of water baths made slightly acid 

 with hydrochloric acid ; these remove the iron from 

 the paper ; a simple rinse in plain water completes 

 the operation. The <lvelo|r can be mixed in 

 bulk, for it keeps well, ami the same quantity 

 will develop a large number of prints one after 

 the other. The platinum prepared paper will keep 

 well if damp lie excluded. Fur this reason it is 

 sold in tin tubes, which have at one end a small 

 quantity of calcium chloride, a salt which is so 

 greedv of moisture that it will absorb all in its 

 neighbourhood. The favour with which this pro- 

 cess has been received, liecause of its permanence 

 and its quick results, is well indicated at photo- 

 graphic exhibitions, where a large proportion of 

 the picture* shown are invariably platinotypes. 

 Mr Willis, the inventor, has introduced a cold 

 bath platinum process, in which the metallic salt 

 i.s contained in the developer, and this modified 

 method is said to present many advantages. The 

 developer thus prepared is not of a lasting kind, 

 ami only enough must be mixed to meet the exist- 

 ing demand. Another modification of the platinum 

 process, made known by Willis, but more gener- 

 ally associated with the name of Captain l'i//i 

 chelli, yields a dark image in the printing frame. 

 The only necessary after- treatment is a bath of 

 weak arid, followed by washing in plain water. 

 There is every reason to suppose that platinum 

 will I* the printing process of the future, but 

 unfortunately the price of the metal, which in 

 IS'.HI went up more than 100 per cent., is calculated 

 'fur the present to limit its use. 



l\i'h ruinated Gelittine I'roress. So far back as 

 Is.'tll Muiigo 1'onton announced that paper steeped 

 in bichromate of potash and dried changed its 

 colour when exposed to light. It was subsequently 

 discovered that light not only alters the composi 

 tion of the bichromate, bill also oxidises the size 

 (gelatine) of the paper, (iiini, starch, and albu- 

 men were nlso found to become, like gelatine, 

 insoluble when excised in contact with the bichro- 

 mate of potash or ammonia to the action of light. 

 If ordinary gelatine l>e soaked in cold water, it 

 alisorbs the water and swells, and then if heated, 

 or if hot water lie poured on it, the gelatine melt.-. 

 If some bichromate of potassium or ammonium 

 had been added to the cold water, the gelatine 

 would absorb the chemical along with the water. 

 If now the gelatine be dried and exposed to light 

 until the stain imparted by the bichromate is 

 altered in colour, it will no longer swell in col. I 

 water, neither will it dissolve in hot water ; the 

 action of light has made the bichromated gelatine 

 insoluble. It is to gelatine thus chemically modi- 

 fied that we owe the 'autotype' or 'carbon ' more 

 correctly ' pigmented gelatine process. TheCollo- 

 t\pe, Woodburytype, and some forms of photo- 

 /inco engraving and photogravure, also certain kinds 

 of ' phantom ' photograph-, ami one method for vitro 

 enamels, depend on the same principle. 



77" .1 Hint I/in', ('urban, or 1'iijin fitted Gelatine Pro- 



cess. Pigmented gelatine paper is an article of 

 commerce, and the Autotype Company supply this 

 ' tissue ' sensitised ready for printing. The tissue 

 consists of a thick coat of gelatine, with which has 

 been intimately mixed a certain amount of per- 

 manent pigment in very fine powder if for a black, 

 Indian ink may be employed ; other colours are 

 added to modify the tint. The paper so coated is 

 sensitised with ammonium bichromate, and then 

 exposed under a negative till it is supposed to be 

 sufficiently printed. The image is not visible as in 

 a silver or iron print, therefore some indirect plan 

 of telling the proper time of exposure, such as by 

 the use of an actinometer, must l>e resorted to. 

 The change which takes place in the gelatine 

 film is this : the surface next the negative has 

 teen rendered insoluble wherever the light has 

 acted, and that to a depth corresponding to the 

 intensity of the light. It results from this that 

 almost the whole of the surface of the gelatine 

 has been rendered insoluble to the greatest 

 depth where the light has acted most strongly. 

 Soluble portions, however, remain enclosed between 

 its surface and the paper. No picture is visible till 

 these are removed. To overcome this difficulty 

 the removal of the soluble portions imprisoned 

 between the insoluble skin and the paper at the 

 back took many years of experimenting, and all 

 sorts of devices were resorted to. One plan was to 

 expose the back of the tissue to the negative, thus 

 pnnting through the paper, but the grain of the 

 paper showed offensively. Fargier spread the pig- 

 mented gelatine on glass, exposed it thus under a 

 negative, and then coated the film with collodion. 

 On subjecting the whole to the action of warm 

 water, the latter penetrated the collodion and 

 softened the gelatine, which eventually floated off 

 the glass, being held together by the collodion. 

 This was now supported on paper (collodion side 

 down), and washed from the back with warm 

 water, and so the first half-tone photographs in pig- 

 mented gelatine were obtained. 



Swan experimented on similar lines, and in 1862 

 he took out a patent for pigmented gelatine films 

 spread upon collodion supported on glass. When 

 dry the whole was stripped from the glass, ami 

 thus the first tissue was made. The difficulty of 

 removing the entangled pigment still remained, 

 and this Swan overcame by coating the collodion 

 surface of his tissue with india-rubber solution, and 

 applying it to a piece of paper similarly coated. 

 ' When both were dry the whole was passed through 

 a press, and then soaked in warm water ; thus the 

 soluble portions could be removed, leaving that part 

 of the film acted on by light untouched, and pro- 



J'ecting in relief according to the varying action of 

 ight as it passed through the gradations of the 

 negative. By this method and a subsequent modi- 

 fication Swan sent out a number of fine pigment 

 prints, but it was so troublesome, expensive. a.nd 

 unsatisfactory in the hands of the average photo- 

 graphic printer that printing by bichromated gela- 

 tine never liecame popular until J. R. Johnson, 

 aliout the close of the year 1868, discovered that 

 the pigmented gelatine paper ( ' autotype tissue ' ), 

 when sensitised by the bichromate and correctly 

 exposed to light under a negative, only required to 

 l>e soaked in cold water, and then evenly applied 

 to ftntf xttrfac.e iin]n't'i't>mx i (ttt\ such as glass, 

 zinc, oilcloth, &c. , when the gelatine surface would 

 adhere very tenaciously to the support after the 

 principle of the school boy's sucker by atmospheric 

 pressure alone. Then by soaking in hot water the 

 paper at the back came'ott', carrying with it much 

 of the unaltered pigment and gelatine, and by 

 laving the image remaining on the support with 

 the hot water the picture in all its delicate grada- 

 tions appeared clean and perfect. It was this 



