(a) According to Volkner (Photographic Times), the original for a heliographic reproduction must be extremely sharp in outline. A reversed photographic negative upon glass, 4/5 or 3/4 the original size, is first made. This reduction renders the lines still sharper and more delicate than in the original. A gelatine solution to which is added sugar, lamp black, alcohol, ammonia, and creosote, is then prepared, and with it a sheet of photographic paper is coated, and laid upon a plate glass carefully levelled. As soon as the gelatine mixture has set, the coated sheets are hung upon cords, and after drying, are kept in a perfectly dry room. The quantity of the pigment is proportional to the character of the original to be reproduced, one-tenth of it being the maximum, one fortieth the minimum, the former for fine and deli-cate drawings in line manner, the latter for other work. The prepared pigment paper is sensitised in a bichromate of potash bath, 1-15, and laid face down upon a carefully-cleaned plate-glass, and dried in a strong current of air. Immediately before using it the paper is detached from the glass. The exposure under the reversed glass negative is made in an ordinary printing frame, the time adjudged by a Vogel photometer.
After printing the pigment paper is taken to the dark room, and in a cold-wate.r bath, transferred to a silvered copperplate, picture side down. Removed from that bath, the paper is squeezed and dried with blotting paper. After 5 minutes another cold-water bath is used to wash the bichromate from the non-exposed parts. In 1/2 hour it is taken up, well rinsed, and subjected to a warm water bath o,f 30° to 35° C. to dissolve the gelatine not acted upon, that is to develop the gelatine relief upon the silvered copper plate. After a short time the water penetrates the pores of the paper, dissolved black gelatine oozes from the film, indicating the beginning of the development; in about 30 minutes the paper is detached, floats upon the surface of the bath, or can be lifted up with ease. The rest of the gelatine is dissolved in a few minutes, and the relief, a copy of the original, appears gradually upon the copper-plate. The development is continued in another bath of warm distilled water, till all remaining fog or dirt is totally removed from the interstices of the relief, and the picture stands out clear and distinct upon the plate.
Finally, it is well rinsed with water and dried spontaneously.
To make this relief plate conductive, powdered graphite is spread over it with tampon and a soft brush; afterwards it is placed on the kathode of a Daniell trough apparatus, and a zinc anode used to accelerate the precipitation of copper. In an hour or less the heliographic relief plate is sufficiently covered, the plate is taken up, cleaned and rinsed, and again placed in the trough. To promote the force of chemical action, caused by the electric current, an iron anodo is inserted. After 20-24 days the plate will be of the desired thickness, and is, therefore, then taken from the trough, rinsed with water, and dried; the edges are filed off, and the patrix removed from the matrix. Both plates are well washed. Adhering parts of the gelatine relief are carefully removed.
If the plane of the gelatine relief has been perfectly clean, and free from any tone, the intaglio plate is also smooth, lustrous, and printable. Matt spots are wiped off with oiled flannel and rotten-stone; tone and impurities are scraped and burnished; other defects, which but rarely occur, are retouched with needle and graver.
The first impressions made from heliogravure plates are always rough, and retouching them should not be undertaken till a number of prints have been made. They are in every way equal to engraved copper-plates, and with them there is a great, almost incredible, saving of time. Heliogravure plates, maps of the Austrian Empire, made in fifteen years, would have taken generations to engrave. Before large editions are printed galvano-plastic reliefs are taken from the intaglio plate. From them new copies can be made, in case the original should suffer in course of time.
An important substitute for multiplying copper-plates by galvanoplasty is to face them with steel. It is used for plates like objects of art, which never require correction. When, a copper-plate is placed on the cathode suspended in a solution of sesquichloride of iron, and subjected to the action of the galvanic current, it will in a short time be covered with a delicate and lustrous cuticle of iron, hard as steel. There is no difference seen in the prints taken from plain copper or steel-faced plates. The iron gives the plate an extraordinary durability, and many thousands of prints can be made from it. In case the steel cuticle has been unsound, it can be easily taken off and renewed. The plate is laid in sulphuric acid, diluted so much that it will not attack copper; but it loosens the steel, which blisters and comes off in scales. The plate is again washed before another steel-facing. The process is carried on in a peculiar dark trough, with a 3 cell zinc carbon battery; the electrodes are placed vertically. The suitable iron solution is made by the current. One part of chloride of ammonium is dissolved in 10 parts of water, and in it are placed iron plates, as kathode and anode.
When the circuit is closed a chemical action takes place, the chlorine of the chloride of ammonium unites with the iron of the anode, forming sesquichloride, which remains dissolved in the bath. Within 1-2 days the bath assumes a greenish colour, its surface turns red, owing the formation of hydrate of oxide of iron from contact with air, and metallic mirror appears on the kathode. The bath is then sufficiently saturated, and in place of the iron kathode the copper plate to be steeled is inserted. The plate must, of course, be absolutely clean, and is therefore washed in caustic potash, rinsed, and any possibly adhering alkali neutralised with sulphuric acid, washed again and dried. Scamoni, of St. Petersburg, makes very durable plates by first precipitating nickel upon the silvered relief, and then allowing the copper to build up till the desired thickness is reached. His bath is: 45 parts water; 5 sulphate of nickel, and 1 to 1 1/2 of chloride of ammonium.
A plate of zinc or any other metal is coated with a mixture of gum Arabic, water, grape sugar, bichromate of potassium, and a few drops of ammonia, and exposed to light under a glass positive. After exposure the plate is removed to the dark room, and etched with a strong solution of perchloride'of iron. Strong lines are attacked first and etched deeply, the thin and delicate parts afterwards. The process lasts but 5 minutes, and after cleaning it may be printed from at once. To give the plate more durability it is copper-plated and eventually steel-faced. The copper-plating of zinc cannot be done by electrotypes of the sulphate of copper: the free sulphuric acid will attack the zinc. Instead of it sub-cyanide of copper CuCy (Kupfer-cyanur) is used. When cyanide of potassium is added in excess to sulphate of copper, the copper electrotype CuCy is formed: CuSo4 + 2KCy = CuCy + K2S04 + Cy. Sulphite of sodium added previously to the cyanide of potassium changes the cyanide into prussic acid, the sulphite of sodium into sulphate: Cy2 + Na2S03 + H20 = 2HCy + Na2S04. Ammonia added to the solution forms, with the prussic acid, cyanide of ammonium NH4Cy. Two solutions are made, (1) 140 parts sulphate of copper, 840 water; (2) 140-200 cyanide of potassium, 1000 of water, with the addition of sulphite of sodium and ammonia.
The solutions are then mixed. The zinc plate to be copper-faced is placed on the kathode. The current is generated by two zinc-carbon elements.
The electro-negative substance of the electrotype, e. g. the acid of salt, or the chlorine of a chloride, unites chemically with the anode, forming a soluble combination which etches the electrode. Chrom-gelatine paper is exposed under a transparency, inked with roller, and developed; the negative picture is transferred upon a copper-plate which makes the picture metallic-lustrous. The covered parts resist etching. The sulphuric acid liberated by electrotypes combines with the copper; it etches the plate.
In galvanography and stilography the plate is coated with the fatty matter coloured with ochre or lampblack. A drawing is made upon it with the graving tool, the plates are made conductive by graphite, and by placing them in the trough apparatus a relief is made from that on intaglio plate.
Natural objects like parts of plants, or laces and other open fabrics, are pressed in guttapercha or lead, and galvano-plastic matrices made from them. If lace be the object to be reproduced, it is pasted with gum upon a steel plate, a sheet of lead placed upon it, and subjected to a high pressure. Relief or intaglio plates are made this way; the former answering well for typographic printing.
Stereotypes are now substituted by galvano-plastic copper precipitates. The form is impressed in guttapercha, the copper graphited, and, moistened with alcohol, laid in the galvano-plastic apparatus. When the copper is sufficiently thick, the plate is taken up, the reversed side and edges planed off, and backed with ordinary type-metal. Woodcuts similarly treated give galvano-plastic copies from which 70,000 to 80,000 prints can be made.