For transferring photo-litho copies to zinc plates Albert gives the following method: - The copy is printed, developed with fatty ink, and dried as usual. The drawing is afterwards dusted in with a finely pulverised mixture of 10 parts asphaltum and 1 of pure beeswax, and all superfluous asphaltum is removed by means of a fine camel-hair brush and a tuft of cotton, and slightly heated over an alcohol lamp to melt the asphaltum together with the fatty ink. The copy is then passed through the saturated alum bath to keep it uniformly moist, laid between sheets of moistened blotting-paper, and finally transferred upon a zinc plate.
The zinc plates for either process are sold ready polished, and differ only in thickness, the zinco block being four times as thick as those used for drawing upon. The zinc for drawing - upon is not ready for graining until it has been washed with strong potash and rinsed in clean water, and the graining must be done with sand and water, in a manner similar to that adopted for the stone. If for crayon work, it must be very sharp in the grain, or it will not take the crayon. If for ink, in line or stipple, an inferior grain will do. In case of drawing anything that may require erasing, the zinc must on no account be scraped, as it would roll up solid black. It is better to take out with a piece of clean rag dipped in benzine anything that is wrong, and let it dry, when the crayon or ink may be used without fear over the same part. For convenience of working, the white portions may be stopped out as on stone, but the gum used should have a few drops of glacial acetic or nitric acid mixed with it to the strength one would use for a "strong etch" on stone.
When the drawing is completed, take a solution made as follows, and etch the plate with it for 10 minutes, not longer. Put 24 nut-galls into a saucepan - preferably one glazed with earthenware - and cover them with 1 pint water. Simmer over a slow fire until it is reduced to 1/2 pint or rather less. Strain through fine muslin into a clean vessel, and let it stand until cold; or it may be kept in a stoppered bottle. Take of strong gum and the above tincture of galls equal parts, and add a drop or two of glacial acetic or nitric acid. The former is preferable, as in washing the zinc acetate is more soluble than the nitrate.
This solution should be rapidly passed over the plate, whether chalk or lime, just as the "etch" over a stone. After 10 minutes' etching, wash off with a clean sponge and plenty of water, and roll up in the usual way, bringing it up with a roller. Some prefer to allow the plate to dry all over during the rolling up, and keep rolling until the whole plate is one black mass, when they wash out the job with "turps" and water and roll up again. Others are careful to prevent the plate drying in the white or clear parts by wiping very frequently with a very slight etch of gum and acetic acid, feeding the job with the roller all the time in between. If a transfer is required, a few impressions should be run off before again washing out, when the job will be found strong enough to roll up in retransfer (litho) ink, and the transfers pulled may be put down upon polished zinc for the bath.
Zinco blocks for letterpress printing must be polished, unless, perhaps, for coarse poster work. The grained surface would not answer in the printing. To polish the zinc, take the ordinary pumice powder, very fine, and, with a piece of soft, preferably linen, rag and a little water, rub it down till an even polished surface appears; after which, with the same powder, dry, complete the polishing till the surface reflects like a mirror. Immediately put the transfer down exactly as if it were stone; then, before rolling up, dip it in a very weak bath for a minute or so, and rinse and dry without heating. It should not be washed out, but rolled up in litho ink, and may then be put in the trough and rocked in the usual way, being heated from time to time, and rolled again with the ordinary varnish.
Sufficient care is not usually exhibited in England in biting up these plates for zinco blocks. In Paris the plates are carefully examined through a strong magnifying glass; and if any tendency to undermine the lines is shown on arriving at a certain depth, the operator takes a varnish brush and prote:ts the shelving sides with it, and also touches up any parts of the surface which seem feeble.- If any specks of "scum," or " dirt," adhere to the sides of the lines or among the chalking, he takes a graver and cuts them away, taking care to touch each graver out with varnish, so as to prevent the subsequent bath from undermining the line. To this care is due the superiority of the French, and for that matter, the American, process work.
Some houses, before subjecting the transfer on zinc to the weak bath, etch it with the tincture of galls and gum for 5-6 minutes, which will clear away all scum, and then rinse off with cold, clean water, and immerse in the bath.
Almost every operator has his own favourite mixture of ink for rolling up the zinco block during the biting up. which he pretends to keep a profound secret. But anything which will feed the job, and prevent the acids in the trough from impoverishing it, will answer the purpose satisfactorily. Cobblers' wax, rosin, and white or yellow wax, all of which are rendered fluent by the heated plate, in various proportions of admixture, form the bases.