This section is from the "Henley's Twentieth Century Formulas Recipes Processes" encyclopedia, by Norman W. Henley and others.
When, instead of brilliancy, a matted appearance is desired for metals, the article is corroded either mechanically or chemically. In the first case it is pierced with fine holes near together, rubbed with emery powder or pumice stone and tamponned. In the other case the corrosion is effected in acid baths thus composed:
Nitric acid of 36° Be., 200 parts, by volume; sulphuric acid of 56° Be., 200 parts, by volume; sea salt, 1 part, by volume; zinc sulphate, 1 to 5 parts, by volume.
With this proportion of acids the articles can remain from 5 to 20 minutes in the mixture cold; the prominence of the matt depends on the length of time of the immersion. The pieces on being taken from the bath have an earthy appearance which is lightened by dipping them quickly in a brightening acid. If left too long the matted appearance is destroyed.
This matt, thus called on account of its soft shade, is rarely employed except for articles of stamped brass, statuettes, or small objects. As much zinc is dissolved in the bath as it will take. The pieces are left in it from 15 to 30 minutes. On coming from the bath they are dull, and to brighten them somewhat they are generally dipped into acids as before described.
Articles of value for which gilding is desired are matted by covering them with a light coating of silver by the battery. It is known that this deposit is always matt, unless the bath contains too large a quantity of potassium cyanide. A brilliant silvering' can be regularly obtained with electric baths only by adding carbon sulphide. Four drachms are put in an emery flask containing a quart of the bath fluid and allowed to rest for 24 hours, at the end of which a blackish precipitate is formed. After decanting, a quart is poured into the electric bath for each quart before every operation of silvering.
The operation of dipping should be carried out only in a place where the escaping fumes of hyponitric acid and chlorine can pass off without molesting the workmen, e. g., under a well-drawing chimney, preferably in a vapor chamber. If, such an arrangement is not present the operator should choose a draughty place and protect himself from the fumes by tying a wet sponge under his nose. The vapors are liable to produce very violent and dangerous inflammations of the respiratory organs, coming on in a surprisingly quick manner after one has felt no previous injurious effect at all.