quickly, and is therefore, in some cases, really attended with disadvantage; but this objection may be removed by the following process: Take a pint of nut, or poppy oil, put it into a large earthen vessel; let it boil slowly over a moderate fire; add by degrees two ounces of white lead, and stir it continually, till the whole is dissolved. Prepare a pint of the copal-oil varnish heated in a separate vessel; pour this gradually into the hot oil, and stir them together till all the spirit of turpentine is dissipated; let it stand till cold, when it will be fit for use.
It is obvious that, as this is a compound of the copal-varnish with the least exceptionable of the drying oils, it will partake of the properties of each. Although it imparts less brightness and durability to colours than the varnish, yet it may be used by painters in the same manner as any other drying oil, on account of the greater lustre and permanency which such colours derive from it, than are obtained from the common oil varnishes. Notwithstanding we have mentioned specific quantities of the ingredients, it must be obvious that the relative proportions may be varied, accordingly as it is required to dry faster, or slower. It should also be remarked, that both the ingredients must be hot; because, if either of them be cold, the mixture will become turbid, and a part, nay, often the whole of the copal, be precipitated: but this inconvenience may be avoided, by mixing and boiling them together, in the manner above directed. Yet, as after some time, a spontaneous alteration takes place, which diminishes, and at length destroys, the drying quality of this mixture; it will be advisable to use it fresh, or at least not to employ it, after it has been prepared longer than a month or six weeks.
This varnish is also applied to snuff-boxes, tea-boards, and other articles. It preserves and gives lustre to paintings, and greatly restores the faded colours of old pictures, by filling up the cracks, and rendering the surfaces capable of reflecting light more uniformly.
COPPER, one of the finest imperfect metals, is found in the bowels of the earth, in the following states.
I. Native or pure copper, which possesses the red colour, the malleability, and all the other properties of this metal, and is discovered in various parts of England and Wales, but more particularly in the county of Cornwall. It is formed into threads or branches, and lies in veins of considerable thickness, contained in blackish serpentine stone, mixed with a brownish red, and covered externally with a greenish nephrites.
II. Mineralized by fixed air; of which there are several varieties: 1. Red copper, or hepatic ore of copper, which is known by its dusky colour. It is generally mixed with native copper and mountain green. 2. Earthy copper, or mountain green, which is mostly found in a loose friable state, and frequently blended with calcareous earth, iron, and sometimes with arsenic.
III. Mineralized by sulphur, with a small proportion of iron. This is of a deep violet grey, or liver colour, melts with a gentle heat, is ponderous, flexible, and yields to the knife. When broken, it appears of a bright golden colour, and is the richest of all the
