PAPER (Fr. and Ger., Papier) The raw material of paper is linen, cotton-wool, hemp, flax, esparto grass and wood pulp ; but for photographic purposes the absence of the last two substances is essential, save in such processes as carbon, where the paper only forms a tem porary support for the tissue. The raw material is mechanically torn up, bleached, and then mixed into a pulp with water and the pulp spread on fine-meshed wire, so that the water can drain off and the paper pulp or felt be dried. All paper, except blotting and filter paper, is sized to prevent the undue absorption of water, and very frequently, in addition, filling is used to render the paper more opaque and give it a greater glaze, but at the same time lessening somewhat its strength. The filling may be either silicates, such as kaolin, the carbonates of lime, barium, zinc or lead, or sulphates, such as those of calcium and barium.
Paper contains as a rule about 5 to ro per cent. of water, and is always more or less hygroscopic, and according to the sizing so it will absorb a greater or less amount of water. Photographic papers are almost always sized in the stuff or mass, but in some cases, especially with the so called drawing or water-colour papers, the sizing is so poor that it is necessary to remove it by treatment with acid and subsequent washing, and then to re-size with a colloid-like gelatine, agar-agar, etc., in order to keep the photographic image on the surface.
Paper for photographic work must be free from wood pulp, which rapidly yellows on expo sure to light, and spoils the emulsion coating or decomposes the free silver salts of printing-out papers. Equally important is the absence of metallic particles—iron, brass, copper, etc.— which might be introduced from the paper-mak ing machine ; such particles cause black spots in the case of papers containing free silver nitrate, and generally white insensitive spots in the case of development, though often in the latter case the little white spot will be seen to have a minute black centre, when examined micro scopically. The quality of the water used in pre paring the paper pulp is also of great importance, as this may introduce foreign matter which acts prejudicially on emulsions. Naturally, too, the
absence of " antichlor " is all-important, as in every case this spoils the sensitive film and usually results in spots.
Many photographic papers are primarily coated with an emulsion of barium sulphate (see " Baryta Paper "), which prevents the sensitive film from penetrating the paper, and gives the prints a special surface. The preparation of this coating is as important as that of the paper, and it may also be the cause of peculiar markings and spots, but even this will not allow of a poor quality raw stock being used. One important point in connection with baryta paper is the homogeneity of the coating, for if the coating is not absolutely even in thickness or hardness, the sensitising material, especially in the case of printing-out papers, will penetrate more or less deeply into the same, and the results are that the prints show a peculiar and characteristic defect known as measles or mealiness.
In process work, the expansion and contraction of paper is of serious importance. For instance, in photo-lithography by paper transfers it affects the scale in such work as map reproduc tion, and makes the joining up of sections diffi cult. To overcome this it is usual to allow a definite amount for expansion whilst the paper is damped for transferring, the sheet being measured between two points by means of a trammel rule provided with needle-points. The paper is further damped if it does not reach the required measurement. In colour work the risk which paper undergoes often affects the registering of the colour, and has to be allowed for. As the expansion is greatest in the direction of the web, and is fairly uniform, it is a good plan to mark this direction and use it always the same way, when several pieces have to join or to register.
(For the preparation of the various kinds of papers, see under separate headings. The coating of paper is described under its own heading.)