146 HANDBOOK OF PHOTOGRAPHY 



The various degrees of roughness are usually controlled by varying the composi- 

 tion and coating conditions of the baryta layer; the pattern textures are produced on 

 the baryta layer under pressure by embossed rollers. 



Weight or Thickness. — Most papers which will be used for small-size prints are 

 supplied on either single- or double-weight stock, while papers for large prints, such 

 as the professional papers and enlarging papers, particularly those with rough sur- 

 faces, may be supplied only in double weight. Some papers, such as those for post- 

 card prints, are supplied on a thin cardboard. 



Papers having the usual baryta coating cannot be folded without cracking. 

 Therefore a few papers, such as those used for document copying and for advertising 

 illustrations which will be folded for mailing, are supplied on a special thin base with- 

 out the baryta layer. 



Special Emulsion Types. — In addition to the regular black-and-white negative 

 and printing emulsions already described, there are a number of more or less special- 

 ized emulsions which, because of special requirements in the mode of use or in the 

 method of preparation, differ in properties sufiicientlj^ to justify separate discussion. 



Printing-out Papers. — At present the vast majority of photographic prints are 

 made on developing-out papers, with which the latent image produced by exposure is 

 converted to a visible silver image by the action of the developer solution. Printing- 

 out papers (POP), on which the visible image is produced by direct photochemical 

 darkening, were formerly widely used but are now used only in special cases, such as 

 the preparation of portrait proofs, and are losing ground even here. 



While the normal developing-out emulsions are prepared with an excess of soluble 

 halide, printing-out emulsions contain an excess of soluble silver salt, usually the 

 tartrate or citrate, which acts as an acceptor for the halogen liberated by the photo- 

 chemical action. 



The sensitivity is naturally very low; the papers can be handled safely in artificial 

 light or weak daylight, and for printing they require a light rich in ultraviolet, such 

 as that from a carbon-arc or a mercury-vapor lamp or strong daylight. Even so, the 

 exposures are a matter of minutes. The contrast is fairly low, the exposure scale 

 being equal to that of the softer grades of bromide paper. 



After exposure, the image can be fixed by removal of the unchanged silver salts 

 with a solvent such as hypo. This leaves the image an unpleasant yellow color, 

 which can be corrected by toning the image by precipitation of gold, to give red or 

 reddish-purple tones, or precipitation of platinum, to give black tones. Even aside 

 from the unpleasant color, the untoned image is rather impermanent because the very 

 finely divided silver is rapidly attacked by any sulphur compounds in the paper or in 

 the atmosphere. Toning can be done after fixing but gives more dependable and 

 uniform results before fixing. Portrait proofs on P O P are seldom fixed or toned. 



POP emulsions may be in either gelatin or collodion. In Europe, printing-out 

 papers supplied for amateur use sometimes have the toning agents incorporated in 

 the emulsion (self-toning papers), so that prints can be made without a darkroom and 

 can be processed merely by washing in water. 



Because of the excess of soluble silver salts, printing-out-paper emulsions are 

 rather unstable and tend to darken on aging if any moisture is present. This can be 

 prevented by keeping the paper thoroughly desiccated. 



Reversal Emulsions. — In some cases it is desirable or necessary that the final posi- 

 tive image be produced in the same emulsion as was used for the original exposure. 

 At the completion of development, the emulsion contains two images, the negative 

 image of silver and a complementary positive image of the unaffected silver halide 

 which is normally removed by fixation. Thus, if the silver image is removed by 

 treatment in a bleach liath which does not attack the silver halide, the residual silver 



