BARYTA PAPER (Pr., Papier baryte ; Ger. Barytpapier, Kreidepapier) Good raw paper stock coated with an insoluble emulsion of baryta in gelatine and used princi pally for coating paper intended to take a gela tino-chloride emulsion. A good baryta paper must be coated with three films, the first two serving to prevent the emulsion from coming into contact with the paper and the third giving the particular surface and tint desired. Both glazed and matt surfaces, tinted red, blue or violet, or white, can be obtained. It is also used in surfacing paper for collotype and Wood burytype.
Its preparation requires costly machinery. The baryta is either precipitated by the paper coater, or bought in the form of powder and mixed in a kneading machine with one-third its weight of water till a perfectly uniform cream is obtained, which is carefully sifted, divided into two equal parts, and thoroughly mixed with gelatine solution. To one part of the baryta emulsion is slowly added, with constant stirring, a solution of chrome alum, which causes the emulsion to become ropy, and at the moment of this appearance the other half of the baryta emulsion is added. The mixture is again passed through sieves, some glycerine added, and then coated on the paper. The first coating is so adjusted that there is about 12 to 15 g. of dry baryta to every square meter (17 to 21 grs. per square foot). The emulsion is picked up by the paper, which is coated in long rolls, by passing through a trough, or it is wiped on by a roller, and it then passes over a rubber coated cylinder ; evenness of coating is obtained by seven brushes with reciprocating motion, the first being hard and the others gradually in creasing in softness until the seventh is of the very finest and softest badger hair, about 2 in.
long. The paper is then hung in festoons to dry, rolled up, and given the second coating in precisely the same way. Sometimes it is then calendered, but usually again coated by the same machine with another baryta emulsion containing glycerine and chrome alum, and more or less gelatine, according to the surface desired. To this emulsion is also added the colouring matter—Paris blue, ammoniacal car mine solution, etc., being used. After drying, the paper is passed through calendering machines, which are provided with heavy steel rollers, which may, or may not, be heated, and exert a pressure of about io,000 lb. The paper is sprayed with water before passing through the calender rolls, and the surfaces of the latter are highly polished, roughened, or grained respec tively, so as to impart a special surface to the baryta film.
The commercial baryta paper is usually obtain able in rolls of about boo metres in length, and is graded according to surface—matt, glossy, rough or grained—and according to the weight in grammes per square meter.