Impression

The art of taking impressions from letters and other characters cast in relief upon several pieces of metal is called letter-press printing. The impressions are taken either by superficial or surface pressure, as on the common printing press, or by lineal or cylindrical pressure as in the printing machine and roller press. The pigments or inks, of whatever colour, are always upon the surface of the types, and the substances which may be impressed are various. Wood-cuts and other engravings in relief are also printed in this manner.

Copperplate printing is the reverse of the preceding, the characters being engraven in intaglio and the pigment or inks contained within the lines of the engravings, and not upon the surface of the plate. The impressions are always taken by lineal or cylindrical pressure, the substances to be impressed, however, are more limited. All engravings in intaglio, on whatever material, are printed by this method.

Lithographic printing is from the surface of certain porous stones, upon which characters are drawn with peculiar pencils or pens, &c. The surface of the stone being wetted, the chemical colouring compound adheres to the drawing and refuses the stone. The impression is taken by a scraper, that rubs violently upon the backs of the substances upon which are fewer still in number. Drawings upon zinc and other materials are printed by this process.

Cotton and calico printing is from surfaces engraven either in relict or intaglio, but it is a branch of printing which does not enter within the scope of this work.—Having explained the scientific distinctions between the various kinds of impression, the word may be connection with its technical meaning.

Amongst the best printers there exists a great difference of opinion as to the force needed for a fine impression. By some a heavy and solid indention of the paper is considered necessary, while others insist that impression which does not indent the paper is preferable. The intendation of the paper is no test of the force of the impression. A light impression against a wollen blanket will show more forcibly than a strong impression against a paper or pasteboard tympan.

Type is worn out not so much by the direct impression of the platen or cylinder on the flat face of the forme as by a grinding or rounding impression on the edge of the type, caused by forcing of the blanket between the lines and around the corners of every letter. Every fount of worn-out type, whether of cylinder or platen-press, has suffered less from a reduction in height than from a rounding of the edges. When the type is new and the tympan hard and smooth, the impression can be made so flat that the type will not found at the edges, and the impression will not show on the paper. But this cannot be done with old type or with a soft tympan; the impression must be regulated to suite the tympan. On fine work a rounding impression should be avoided, as it not only destroys the type, but also thickens the hair lines and wears off the seriffs.

It is not sufficient that the paper should barely meet the type; there must be sufficient force in the impression to transfer the ink from the type to the paper. If there is not sufficient impression it will be necessary to carry much ink on the rollers, ad this produces two evils; the type is clogged with ink and the forme becomes foul; too much ink is transferred to the paper, which smears and sets off for want of sufficient force to fasten it to the paper. Distinction must be made between a light and weak impression, and a firm and even impression. The latter should be secured, even if the paper is indented, though that it not always necessary. A forme of old type, a poster or other solid forme, must have a firm impression, or else a very tedious and careful making ready.