In the case of writing-papers, as has already been stated in the description of the beating engines, a vegetable sizing made from resinous matter is introduced into the paper pulp while it is still in solution, and mixes with it thoroughly, thus filling more or less completely the pores of the pulp fibers. This is found sufficient for all ordinary book-papers, for papers that are to be printed upon in the usual way, and for the cheapest grades of writing-paper, where the requirements are not very exacting, and where a curtailment of expense is necessary. For the higher grades of writing-paper, however, a distinctly separate and additional process is required. ♦Tub sizing♦ These papers while on the machine in web form are passed through a vat which is called the size-tub, and which is filled with a liquid sizing made of gelatine from clippings of the horns, hides, and hoofs of cattle, this gelatine or glue being mixed with dissolved alum and made fluid in the vat. Papers which are treated in this way are known as “animal,” or “tub-sized.” ♦Loft drying♦ We have duly described machine-dried papers, but these higher grades of writing-papers are dried by what is known as loft, or pole-dried process. Such paper is permitted to dry very slowly in a loft specially constructed for the purpose, where it is hung on poles several days, during which time the loft is kept at a temperature of about 100° Fahrenheit. Another detail of considerable importance is that of the “finish” or surface of the paper. When paper with a particularly high or glossy surface is desired it is subjected to a separate process after leaving the paper machine, known as supercalendering. ♦Supercalendering♦