A technical dictionary of printmaking, André Béguin.

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Blackening of metal plates
At times artists try to blacken metal plates so as to increase the visibility of the image being drawn onto the plate. Blackening is only done in the lines composing the image while the relief parts of the plate are left polished [* polishing]. This process may be compared to blackening lines with charcoal in smoking* wood engravings. Putting a patina onto a negative is usually done on photographic plates.

BLACKENING OF ZINC. For the purpose one may use a solution of antimony chloride (15 grams dissolved in tartaric acid and mixed with one liter or 2.2 quarts of water) or in a bath of ferric chloride saturated with copper. The latter solution, however, has a drawback since it bites the plate and thus it can only be used provisionaly. The following solution may also be used:

BLACKENING OF COPPER. The job can be done with a heavily diluted solution of sodium polysulfide. If the blackening is only temporary one may use a 12% solution of sodium sulfide in hydrogen peroxide (10% concentration) mixed with some whitening. For more regular use it is better to use diluted sodium bisulfite or even potassium sulfite.

BLACKENING OF STEEL. Insofar as stainless steel is concerned use a bath of sodium bichromate heated to about 400° Centigrade (750° Fahrenheit). Leave the plate in the bath until it has reached the desired colour then remove it, let it cool down and wash it with hot water. The patina achieved by such means is very resistant
For normal steel dissolve the following ingredients into a half liter of water:

The solution is applied either with a brush or then by immersion. After drying scald the plate 20 minutes. Repeat the operation if necessary.

BLACKENING OF IRON. Immerse the plate for two hours in a boiling solution of 40 grams of phosphoric acid, 5 grams of manganese dioxide and 1800 grams of water.

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