We have stated that the screens are ruled with varying degrees of fineness. Fifty lines to the inch is not too coarse for the daily newspaper tossed through the press at lightning speed, though some of the illustrated dailies print with 85, and even 100 line blocks in a very creditable manner. Fine blocks on art paper vary from 175 to 250, the former degree being sufficient for nearly all subjects. However, Mr. W. Gamble, the leading authority on photo-engraving, has produced blocks of still more delicate character, and is of opinion that, with the present high quality of letterpress printing, the 500 line block ought not to be regarded as an impossibility. In such blocks, the screen effect will have entirely disappeared, and the result be equivalent to the continuous tones of the silver print.

Half-Tone Etching

What has rendered this fine degree of tone possible on metal has been the substitution of the fish-glue film for bitumen or albumen as the acid resist. Sixteen parts of fish glue and six parts of pure albumen are dissolved in the equivalent of their bulk of water, and sensitised with ammonium bichromate. The exposure of the metal plate does not usually exceed two minutes in sunlight, after which the superfluous glue is washed away, and the film is heated almost to charring point, when it forms an excellent resist. Copper is now employed in place of zinc, and the biting-in process is either performed with perchloride of iron, or Dutch mordant - the latter a very-weak solution of hydrochloric acid, with a little chlorate of potash.

Payne Type

Constant improvements and variations are inevitable in the photo-engraving processes; one that is worthy of record is the direct process of Mr. Arthur Payne, which is known as Paynetype, and does away with the necessity of a negative. The metal plates are to be purchased ready for use, coated with a thin gelatino-bromide emulsion like ordinary dry plates, and are placed in the dark slide (behind the ruled screen if for half-tone work). After exposure the plate is developed in the dark room for 2\ minutes with a glycin developer, rinsed under the tap, and then immersed in a 5 per cent. solution of potassium bichromate, which has the well-known effect of hardening the gelatine in the presence of the silver image. On the application of hot water the soluble gelatine is washed away, as with a carbon print, leaving the negative image on the plate. Reversal is either obtained by rolling up the plate with litho ink mixed with varnish, or by the electro-deposition of a thin copper film on those portions of the zinc plate that are not protected by the image, after which the etching process may proceed in the usual manner, and the whole manipulations, as described, need not occupy more than about 15 minutes, a consideration, especially in newspaper work, apart from the saving of expense. For line work, and for all half-tone work up to the 85 or 120 screen, Paynetype possesses many salient advantages, and will greatly simplify the reproduction of up-to-date photographs in the daily press. When a negative is available the block can be made even more rapidly, because, as a positive is obtained from the negative, there is no need for reversal, and the original image on the zinc will form the basis of the resist.

Photogravure

The Talbot-Klic process, a modification of that originally devised by Fox-Talbot, is not a very intricate one for the amateur with leisure and a little mechanical skill. A very carefully cleaned copper plate, so finely polished that not even the minutest scratch is discernible, and cleaned from grease by immersion, first in a solution of caustic soda, and then in very dilute nitric acid, is, when dry, prepared to receive an etching grain. For this purpose a dusting box is required, such as engravers use. Any closely made box will do, provided it is more than twice as large in area as the copper plates to be etched, and has a narrow, hinged door, or flap, extending along the bottom of one side. About a pound of very finely ground bitumen and colophony is put into the box, and the door closed, when it is shaken vigorously and turned upside-down several times, until the atmosphere within is heavily charged with dust. Then the copper plate is inserted through the hinged flap, and the dust allowed to settle upon it for about five minutes.

The nature of the grain depends upon the subject to be etched. If a fine grain is wanted the box must be left for a minute or so before the plate is inserted; if this is done immediately the box comes to rest, coarser grains are obtained. Exact degrees of grain will be ascertained after a little experience; it can be examined, on removal from the box, under a reading glass. Next the plate is laid on a copper heater or hot plate, in order to cause the grain to adhere firmly on the plate; when a sort of bloom appears it will be a sign that a sufficient amount of heat has been applied, and the plate should be removed and allowed to dry. If left too long, the resin and bitumen will have melted and formed a solid coating on the plate; when this happens the whole must be cleaned off with benzole, and we must begin again.

The next proceeding is to secure a negative carbon of the subject, which is generally made from a positive transparency. Some workers can make the carbon negative direct by sensitising the tissue in a solution of ferric chloride rendered acid by the addition of citric acid; but this is far from easy, and the positive transparency is the quickest method in the end. If a light-coloured tissue is adopted, printing is visible. The usual "safe edge" will give us a black border, and if this is not desired, a resisting border may be secured by masking the tissue after printing with a card or piece of orange paper rather smaller than the transparency, and exposing once more to light for a short time.