The paper at this stage has a dull coated surface, which is somewhat rough and unfinished, and a high polish is imparted to it by a machine known as a supercalender. The supercalender consists of a number of alternate steel and cotton or paper rolls placed vertically in a stack one above the other. When the coated paper is led through this machine the friction of the alternate steel and cotton rolls produces a high finish on its surface. An art paper coated on both sides is manufactured by passing the paper through the coating machine twice. Machines have been devised for coating both sides of the paper at one operation, but these are not in very general use. Tinted art papers are prepared in the same manner, the desired colour being obtained by the addition of pigments or aniline dyes to the mixture in the trough containing the coating materials. When the two sides of such tinted papers are coloured differently, they are often described as duplex coated papers. _Imitation Art Papers_ are prepared by quite a different process, although they have the appearance, more or less, of the coated paper. They are merely esparto papers very heavily loaded, containing frequently as much as 25 to 30 per cent. of mineral matter prepared as follows:-- Bleached esparto half-stuff is beaten together with any suitable proportion of chemical wood pulp in an ordinary beating engine, and a large quantity of china clay is added at the same time. The beating is carried out under conditions which favour the retention of as much china clay as the pulp will hold while being converted into paper on the Fourdrinier machine. After the paper passes over the drying cylinders of the machine it is passed through the calenders in the usual way, but the surface of the paper is damped by means of a fine water spray just before it enters the calender rolls. The result is that a "water-finish," so called, is imparted to the paper, and a close imitation of the genuine art paper is obtained, the effect of this peculiar treatment being to compress the fibres and bring the clay up, as it were, to the surface. A paper containing such a large proportion of mineral matter intimately mixed with the fibre is naturally very weak. It easily tears, and if moistened with water goes all to pieces. At the same time it is a cheap substitute for high-class art paper, being suitable for circulars, temporary catalogues, and similar printed matter. In an "art" paper the nature of the fibrous constituents is too often regarded as a matter of secondary importance, because in the process of printing the ink does not come into contact at all with the paper, and an impression is produced merely on a layer of clay which is bound together by the glue.