A technical dictionary of printmaking, André Béguin.
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Copper
Copper is the most used metal in engraving. The reason is that it has excellent resistance to mechanical stress, can be engraved easily, is sensitive to etching, and is easily worked when heated. Furthermore copper is quite tacky* to ink and is easily wiped clean. All of these qualities make it indispensable to doing quality work. Its shyniness when perfectly polished and lustred has no equal. However, copper tarnishes rapidly when touched by the artist's hands. Because of this it is best to use a hand rest when working. Oxydation can, however, be cut down by rubbing the plate with an oily rag. However, the plate should not be left in this state after the work has been done (at least not for a long time) since the oil will bite the plate. When the plate has been completed and is put away it should be protected by a paper wrapping and, above all, by a layer of varnish.
The copper used in engraving is red copper. "In intaglio engraving as well as in etching red copper is the best. The yellow one which used to be called brass is usually too soft and often not very precise" (A.Bosse ). Yellow copper, or brass, is actually an alloy containing zinc and it is the zinc that makes it soft. It is, however, also true that there are other copper alloys which are sometimes used for engraving [* metals] because copper is easily assimilated to other metals.
Before being worked on, all copper plates must planed and polished. In the past plates were hammered by specialized workers so as to increase their density and in order to guarantee the homogenousness. The copper bought at present is laminated under great pressure and, at times, one regrets that it is no longer hammered. The hard coppers sought after by photo-engravers (who want them in order to minimize the wear, see photo-engraving) are harder to etch and also harder to engrave but, on the other hand, do allow for more precise lines. If the copper is badly laminated and the surface is even just a little harder than the inside what happens is that the biting process will start off very slowly and then suddenly go too fast.
Copper plates can be polished so as to obtain a brilliant or a mat finish. However, copper plates must be scoured before being used especially if ink* drawings, or graining*, or ground laying, etc. are to be done. In the past such thorough cleaning was done using copper water which was a solution of oxalic acid, wine spirits, and turpentine [* attacking agents].
Usually copper can be bought in the form of prepared plates cut to a certain amount of standard sizes. If a large amount of plates are needed one can buy copper in larger sheets and then have these cut to the required sizes. As might be expected the price of copper plates and sheets varies in function of word prices. Plates may be sent to a "polisher" if one so wishes but this will hardly be necessary if one has been able to choose the plates carefully.
Plates used for engraving are usually one millimeter thick (0.039 inches) in which case the copper will weigh 0.9 grams/cm2 (about .21 onces/in2). If the plates are to be large ones they may be thicker but in such a case they must be bevelled*. The thickness of the plate must never exceed 3 millimeters (0.12 inches) as otherwise it will damage the paper.
Some other metals and in particular zinc can be covered with copper using an electroplating process [* steelfacing]. The word copper seems to be derived from the latin cyprium aes meaning Cyprian metal.
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