A technical dictionary of printmaking, André Béguin.
1. THE MOULD OR CAST.
Various materials may be used in this first step such as wax, lead, 'tenaplate', or thermoplastic sheets such as celluloid or vinyl sheets. The mould must be made under pressure. Wax is the most used substance and is particularly sought after on account of its low price but, despite its high flexibility, it causes a certain amount of problems in moulding. Furthermore, when it is removed the hollows between letters must be raised by means of a plaster layer. To make the mould conduct electricity the wax and the mould box must first be metallized with lead. Lead casts are usually only used in photo-engraving and for the colour printing done with this process. The mould is made with a great deal of pressure and because of this there is a certain danger of damaging zinc halftone plates which are quite expensive. Tenaplate is a plastic material covered with graphite loaded wax. This product has, however, been replaced as of the last few years by thermoplastic sheets, the reason being that these sheets present several advantages:
cleanliness, lightness, and solidity. Furthermore, when using such sheets the mould survives the removal of the shell and therefore several electrotypes can be made using only one mould. Such sheets are more expensive than wax but are cheaper than lead ones. However, the result is not quite as good as when using lead, especially if one needs to register for colour printing.
Celluloid, on the other hand, is hard to use and because of this has been replaced with vinyl plastics which are easier to mould and to cover with graphite. In any case the moulds made with these substances must be done rapidly and when they are hot. A mould must be an exact copy of the original plate. Above all, moulds must not be deformed or torn when they are taken off the plate. Metallization is, at times, done before making the mould by pulverizing graphite both into the material to be used and into the mould box. Sometimes metallization is done after the mould has been taken in which case it is done with silver which conducts electricity as much as 4,000 times more than graphite. This kind of pulverization is done with a spray-gun in a special work room.
Electrolysis is carried out in electroplating vats made of wood lined with lead, gutta-percha, rubber, or vinyl chlorate. The vats contain anywhere between 400 and 1,000 liters (about 100 to 250 gallons) of liquid, are rectangular, and have a copper bar running along each top side of the vats. These copper bars are attached one to the positive pole and the other to the negative pole of a source of electricity. The anodes and cathodes are attached to these copper bars. The mould is then fixed to a cathode. The copper bars, the attachments, and all of the conductors must be kept in good repair and be spotlessly clean in order to ensure the best possible conductivity.
The anode and the cathode (the latter holding the mould) are immersed in an electrolitic bath which, if copper is to be deposited (as is often the case), is made with a solution of copper sulfate to which some sulfuric acid has been added. The function of the acid is to increase the conductivity of the bath, to keep the liquid clean, and to facilitate the depositing of metal onto the mould during electrolysis. The bath must be stirred and heated.
Electrolysis can be done rapidly (anywhere between one and eight hours) or it can be done slowly (10 to 14 hours), in which case it can be done overnight. When the current is switched on (it must be a continous current between 1 and 6 volts) the salts contained in the electrolytic bath begin to deposite in the mould. Success is attained only if the depositing is homogenous and even.
Various types of depositing can be done as a variety of metals can be used such as copper, chrome, nickel, or even steel (in case of steel the depositing is done with iron). A similar technique is used today to etch copper plates in an ecologically safe way. It is known as Electro-Etch
3. READYING THE ELECTROTYPE SHELL.
The metal layer deposited on the mould is called a shell and is 2/10 to 4/10 of a mm in thickness (8/1000 to 16/1000 of an inch). Sometimes the thickness is greater and in exceptional cases the shell may have a thickness of as much as 1 mm (39/1000 of an inch). The thickness is in function of the amount of time given to the process of electrolysis. Once the desired thickness is achieved the shell must be detached from the mould. This is not always an easy matter. If the mould is made of wax it can be removed with boiling water. Tenaplate, on the other hand, is removed by a rapid scalding. Plastic moulds are removed by pulling them off, and lead moulds must be cut away along the edges.
Shells are fragile and as they are not rigid they are sometimes damaged when being removed from the mould. In any case, shells must be backed to make them hard enough to print with. Backing is usually done by pouring a molten alloy made of lead (97%) and antimony (3%) onto the back of the shell.
The shell can have further defects, even after backing, since the printing surface is often not perfectly flat. Because of this one must do some slabbing which is a manual operation carried out with a hammer, a matting punch* , and a burnisher*. Cravers* , abrasives* , a brush, a steel ruler, callipers* , and a magnifying qlass* can also be useful in accomplishing the required work. In fact, slabbing is a very delicate job. The shell is put on a perfectly flat steel bed and is worked so that its surface becomes parallel to the steel surface. The electrotype must be reduced, scraped, and hit into shape. At the end of the job the shell is cut, calibrated and finally raised to the proper printing height with a wood, lead, or steel block. [steelfacing , typography].