A high silica-content tends towards both hardness and chemical stability, and this can be further increased by the addition of small proportions of boric acid; in larger quantities, however, the latter constituent produces the opposite effect.
It combines directly with ammonia to form the compound SiF 4 2NH,, and is absorbed by dry boric acid and by many metallic oxides.
By the fusion of potassium bichromate with boric acid, and extraction of the melt with water, a residue is left which possesses a fine green colour, and is used as a pigment under the name of Guignet's green.
The soffioni contain a small quantity of boric acid (usually less than o 1%), together with a certain amount of ammoniacal vapours.
The operation is carried on through the entire series, until the liquor in the last basin contains about 2% of boric acid.