574 PHOTOZINCOGRAPHY 



' In copying maps, engravings, mamiscripts, &c., a negative of the object is taken 

 on a glass-plate, and the silver deposit blackened with corrosive sublimate and am- 

 monium hydrosulphate ; a sun-print is taken from the negative on paper, coated with 

 gelatine and bichromate of potass, which surface when exposed to the influence of 

 light, is insoluble in water at a moderately high temperature. 



'The print is uniformly covered all over with greasy transfer ink, and afterwards 

 washed with warm water to dissolve the gelatine unacted upon by light, and so carry 

 away the ink upon it, which now remains only on the insoluble portions. In this way 

 a print in a greasy carbon ink is prepared, which has both the appearance and the 

 properties of an ordinary tracing in lithographic ink, and can be transferred to zinc or 

 stone in the usual manner. 



' The paper used for the carbon print should be tough, free from loose fibres, and 

 have a surface that will remain almost undisturbed when saturated with water, and 

 subjected to gentle friction. The double-elephant bank-post manufactured by Cowan 

 and Sons, Cannon Street, London, answers these requirements, and has been found 

 more suitable than any other paper that has been tried at Southampton. 



' The sensitizing solution is made by dissolving gelatine in hot water, and adding 

 potassium bichromate in solution in the following proportions : 



Nelson's best patent fine gelatine 3 ozs. 



Potassium bichromate 2 



Hot water . 50 



The mixture must be made and preserved in the dark. 



' When used for coating the paper, it is put into a flat dish and kept at a tempera- 

 ture of about 100 Fahr., by means of a water-bath ; the paper is floated on its surface 

 for two or three minutes, then hung up to dry by two corners, floated again for a 

 shorter period, but at a lower temperature, so as not to remove the first coating, and 

 dried in a reversed position ; by this double application, with the aid of a high tem- 

 perature maintained during the act of drying, a uniform surface is obtained. 



' The coated paper, which is very sensitive to light, is then smoothed by being passed 

 through a lithographic press, and may be used any time within a week of its prepara- 

 tion, but the best results in making copies of maps, engravings, manuscripts, &c., are 

 obtained when the sensitized paper has not been kept longer than two or three days ; 

 after a week the paper becomes useless, it being almost impossible to clear the ink 

 from the ground of the print on account of the uniform reduction of a portion of the 

 potassium bichromate. The sensitizing mixture may be preserved for further use for 

 a considerable length of time, if kept in an opaque vessel, free from organic matter, 

 and in a dark room ; an earthenware jar is a convenient receptacle, as the mixture 

 when cold becomes a jelly, and the jar can be put in hot water without injury, so as to 

 melt its contents when required for preparing more paper. 



' The time required to obtain a suitable sun-print on the prepared paper varies 

 perhaps from one to twenty minutes, with the amount of light, the age of the paper, 

 and the condition of -the negative ; it may generally be known by the exposed portions 

 of the- yellow bichromate surface becoming a dark olive colour ; the printing should 

 then bo stopped, and the paper coated with transfer ink. Paper recently prepared 

 requires rather longer exposure than that two or three days old, as, if the latter be 

 darkly printed, it is almost impossible to wash off the superfluous ink. 



' To make the transfer ink, 3 ounces of Burgundy pitch are molted in an iron sauce- 

 pan, an ounce of white wax, and an ounce of palm-oil are added, and then by de- 

 grees 1 ounce of finely -powdered bitumen of Judaea ; the mixture is stirred and heated 

 over a fire until it commences to burn ; after the flame is extinguished by the lid of 

 the saucepan, 8 ounces of best lithographic printing ink, rubbed up with 4 ounces of 

 middle lithographic varnish are gradually added, and when uniformly mixed, the 

 composition is ground in small portions on a hot slab with a stone muller. 



When this ink is required for use, a little of it is spread upon a stone and thinned 

 with turpentine, according to the consistency required for the nature of the work about 

 to be transferred. To coat the paper, a lithographic stone is uniformly inked with a 

 printing roller charged with the composition, the sun-print is laid face downwards on 

 the stone, and passed through a lithographic press in a dark room ; it is detached from 

 the stone, reversed in position, and passed through the press a second time to ensure 

 a sufficient and uniform coating of ink. After the print has been inked, it is floated 

 face upwards on water at about 90 Fahr., for a few minutes, when by an unequal 

 swelling of the gelatine the detail of the image can be distinguished. To prevent 

 stains, that would inevitably spoil fine work, it is necessary that no water be allowed 

 to fall upon the face of the print while it is floating. After a few minutes, the paper 

 is raised from the surface of the water, and laid evenly, with the inked face uppermost, 

 on a slightly-inclined surface of glass or earthenware, and a fine soft sponge dipped in 



