Page:The American Cyclopædia (1879) Volume XIII.djvu/875

 PRINTING 851 is then covered with a woollen blanket, and placed in a press the bed of which is moderately heated, and the press screwed down. The heat soon dries the matrix, which is removed from the type and placed in the casting box, the interior of which is curved so that the plate when cast will conform to that of the cylinder of the rotary press upon which it is to be print- ed. The melted metal is poured in, and the plate cooled by immersing in water the box which contains it ; the edges are trimmed, and the back is planed down to the required thick- ness, when it is ready for the press. Thirty- five minutes is sufficient for the whole process, but if necessary it can be completed in half the time ; and any requisite number of duplicates can be made. For other purposes the plaster process of stereotyping was almost universally used in this country until about 1850, when it began to be superseded by the far better pro- cess of electrotyping. For the scientific theory of this, see GALVANISM, vol. vii., p. 601. The practical operation is as follows : Upon a form, prepared as for stereotyping, is laid a sheet of wax covered with a coating of graphite (black lead), secured in a shallow pan called the mould- ing pan. This is submitted to a strong pres- sure, which forces the face of the type, and even the most delicate lines of a woodcut, into the soft wax, thus forming an absolutely perfect mould. This is again black-leaded, which gives it a conducting surface, upon which copper will be deposited from a solution of blue vitriol (sul- phate of copper) by means of an electric cur- rent from a galvanic battery. The mould is placed in a tank of this solution, and the elec- tric current established, the mould forming a part of the circuit ; the copper set free from the solution attaches itself to the graphite on the mould. Silas P. Knight of New York has introduced a more expeditious process. He pours a solution of sulphate of copper over the graphite of the mould, and then dusts upon it fine iron filings. Decomposition and recompo- sition take place immediately ; the acid leaves the copper and unites with the iron, forming a solution which runs off, while a film of the lib- erated copper is instantaneously deposited al over the surface of the mould; this is then placed in the bath, where it remains until a de posit of sufficient thickness, usually about tha of ordinary drawing paper, has been formed This shell looks as though the letters had been formed by punches upon a thin sheet of cop per, being in relief on one side and in intaglic on the other. As it would be crushed in the pressure of the printing press, it must b backed up with metal. The shell is placej face downward in a casting pan, and its back washed over with a solution of chloride of tin t make the metal adhere ; a sheet of tin foil i then laid on, and the pan is heated to abou 450, when the tin melts ; melted metal i then poured in, and a solid plate is formed When the batteries are employed to their i capacity, the cost of an electrotype does no xceed that of a stereotype plate. An elec- rotype plate will wear much longer and give more perfect impressions than a stereotype, nd is less liable to injury from careless han- ling ; it also reproduces more perfectly the fine "ines of type and of woodcuts, impressions from lectrotypes of the latter being practically as good as those from the original engravings. D rinting Presses. It is probable that the ear- iest impressions were taken by a mallet and )laner (a smooth-faced block of hard wood used for levelling the type before printing), as roof slips now often are, or by a brush in the Chinese manner ; but presses were soon in- vented for the purpose. There are engravings epresenting the press as it existed about 1520. ^t was large enough to print only two folio >ages, and for this two pulls were required. Che force was applied by a simple screw and ever. About 1620 Blaeuw of Amsterdam pro- duced a greatly improved press, which with FIG. 2. Franklin's Press. little alteration continued in use for more than a century and a half. The press upon which Beniamin Franklin worked in London m 1725 is preserved in the patent office at Washington ; it is a clumsy structure, almost entirely of wood, known as the Ramage press, of whicl many were still in use more than a centurj later. Iron was subsequently used for some of the parts; and about the beginning of the present century Earl Stanhope invented a press 5