Polymetaal

A technical dictionary of printmaking, André Béguin.

The scouríng ol metal plates is an essential step in a variety of engraving techniques. ln fact, in order to apply a grc›und*, to prepare a dust ground (in aquatint), to etch or to use the sugar lift or ink lift processes, the plate must be perfectly clean. The scouring of a plate is more than a simple removal of all the traces of grease since it must also ensure the complete removal of oxydized areas and carbonate deposits. The degreasing process is thus considered to be just a part of the scouring process.

SCOURING COPPER PLATES.
ln the past Copper plates were rubbed with bread crumbs that had been worked into a ball or then with a very soft white "chalk" which was applied with a cloth. One can also use vinegar to which is added some whitíng or turpentine uil end tin powder (emery powder is not as fine and as a consequence may scratch the plate). The scouríng is done by rubbing the plate with a cloth. To clean slight spots one can use dry whiting. If a wet paste is used to clean the plate make sure to use a cloth that is not fluffy or, better still, replace it with tissue paper or a cellulose dabber. lf the scouring has to be rather agressive use a sodium or potassiun \ye of which 10 cl are mixed into one liter of water. lf you need a yet stronger solution make a 50% solution (half and half).If the scouring is difficult you may scrape the plate with a vegetable fiber brush and a paste made with whiting and sodium or potassium lye. When all is done wash the plate with clear running water and make sure that the water runs over the plate in a uniform sheet otherwise the plate will remain greasy.
For the most delicate procedures, such as the sensitization of copper plates in grained photogravure*, the scouring must be very accurate. The sodium or potassium solution must be very strong (50% solution) and must be mixed with an extremely fine chalk powder. The paste is rubbed on the plate with a ball of tissue paper. The plate is then well rinced with water without touching the plate surface with one's hand or finger. The last step in scouring these plates is to pour over them a nitric acid solution which is made with 5 cm3 of acid and 250 cm3 of water (equal to about 20g of 40°B acid per liter). One can also use a solution that is made with 90 cm3 of rough salt and the same amount of acetic acid in one liter of water. The scouring done with potassium cyanate, which is used in some workshops, is extremely dangerous and hardly to be recommended.
In the past some artists used copper water, a solution that contained oxalic acid, wine spirits, and turpentine oil.
Metal polish products are usually made with ammonia solutions and contain oleine (about 300 g), ammonia (about 100 g), diatomine (about 700 g) and some mineral spirits (about 1500 g ).

SCOURING OF ZINC PLATES. Light scouring procedures may be carried out with turpentine oil and whiting. Heavier scouring is done by washing the plate with a nitric acid solution (5 cm3} of 26°B nitric acid in one liter of water) or with a hydrochloric acid solution (ten times more water, by weight, than acid). The plate is then well rinced and dried with a fine, dry sponge or then with a clean sheet of smooth blotting paper.

[* abrasives, attacking agents, polishing].