noun: any of various minerals consisting of hydrous silicates of aluminum or potassium etc. that crystallize in forms that allow perfect cleavage into very thin leaves; used as dielectrics because of their resistance to electricity

Ramsay
A very important thing in the making of a violin, after a good form, a right balancing of part against part, and all of wood in skillful condition, is the violin varnish. Composition:-

Linseed oil boiled,... ... ... 1 part.
Isinglass, ... ... ... ... 1/2 part.
Turpentine, ... ... ... ... Quantum suf.

Give two coats with this, then rub down with fine sandpaper. Then, best copal varnish, one coat. Finish then with boiled linseed oil, thickened with sifted 'rotten stone.' This gives a fine, smooth, and dull surface. Ramsay's violins are of surpassing tone; and he considered the varnish an important element in violin-making. [Scientific Basis and Build of Music, page 85]