Hot-Pressing is, strictly speaking, the art of applying heat in conjunction with mechanical pressure; but it is generally understood to mean the employment of that process to paper, linen, and similar fabrics, by which they acquire a smooth and glossy surface; and the mode of operating is as follows. A number of stout cast-iron plates are heated in an oven constructed for the purpose; when they have acquired the proper temperature, they are taken out and put into a screw-press, in alternate layers, with the goods to be pressed, in the manner shown in the subjoined engraving; and when paper is the material to be hot-pressed, in addition to the heated iron plates, a highly-glazed pasteboard is put alternately between every sheet of paper; the hard polished surfaces of the pasteboards, in consequence, produce upon the soft and spongy sheets of paper that smooth and elegant appearance by which hot-pressed paper is distinguished. The great inconvenience and labour attending the drawing of the heavy hot iron plates from the oven, then carrying them to, and lifting them into the proper situations in the press, struck the editor of this work as susceptible of amendment.
With this view of the subject he devised a press from which the plates never required removal, and wherein they are constantly kept at the required temperature by hot air from a small furnace beneath. By this arrangement, fuel, time, labour, and space, may be saved. The press itself, as shown in the subjoined elevation, does not materially differ from those in general use. The bed b, and the head-piece c, must be of the most solid and tenacious materials; these parts are braced and held together by four stout cast-iron hollow columns or cylinders d d, having a longitudinal slot mortice throughout their length for the reception of the corner pieces of the plates as shown by the annexed diagram, which exhibits a plan of a plate, with its four corner pieces inserted in the slots of the cylindrical columns, the dark central parts of each being the space where the hot air acts upon the corners of the plates; the heat from which, by the conducting power of the metal, is quickly communicated throughout. A series of these plates is shown in the elevation, attached to each other by means of short pieces of chain, which allow the plates to be pressed nearly close together, or the drawing of them further apart by the action of the screw e.
The ends or corner pieces of the plates are shown by dotted lines, as inserted in the hollow columns. The furnace or fire-place is placed underneath at f; at g is the ash-pit, and at h is the hot-air chamber. The bars of the furnace are made hollow, which, being heated by the burning fuel, the cold air rushes through them, abstracting caloric in its passage, and is received at the back into the hot-air chamber, which lines the sides, top, and part of the back of the fire. The air in this chamber derives a great increase of heat by radiation through the iron partition which separates it from the fire. The hot-air chamber is of course kept closed, the front plate being merely removed in the drawing to show its form; a sliding plate should be made to fit the front ends of the hollow bars, by which the quantity of air admitted may be lessened or augmented; and by a due attention to the management of the fire, the heat may easily be regulated with sufficient exactness for the nature of the operation. The hot-air is conducted from the chamber h, by means of four iron pipes, and to each of the cylindrical pillars; two of these are shown by the dotted lines i i.
As the hot-air issues from these pipes it ascends the columns, impinging in its progress upon the ends of the plates, and heating them to the degree required for hot-pressing. For fire-proof houses, see the article Fire.