copper ores, affording from 80 to 90 per cent. of copper, 10 or 12 of sulphur, and a small proportion of iron.
IV. Mineralized by sulphur, with a large proportion of iron, or azure copper ore; it varies from the preceding sort only in the quantity of iron it contains, which sometimes amounts to 50 per cent. It yields 50 or 60 pounds of copper per cwt. the rest being sulphur.
The principal parts of Great Britain, which afford copper, are the counties of Cardigan, Chester, Cornwall, Cumberland, Derby, Devon, Northumberland, Lancaster, Salop, Somerset, Stafford, York, Warwick, Westmoreland; in the Islands of Mann and Anglesey; and also in Scotland.
Copper is less difficult to be purified than iron; and, when exposed to the air, calcines, being converted into a green rust or calx, which is soluble in water, and imparts an astringent taste, as well as pernicious qualities.
When taken into the human body, copper acts as a violent emetic, and is generally considered as poisonous: and, though it has occasionally been prescribed by physicians, it is always an unsafe and hazardous remedy. Hence, the greatest precaution is necessary in using this metal, of which so many kitchen utensils are manufactured. Beside the most scrupulous attention to cleanliness, it is extremely improper to leave any liquid to cool in a copper vessel; for this metal is more easily decomposed by liquids, when cold, than in a heated state.
In order to prevent the deleterious effects of copper, the vessels made of it are usually covered with tin, on the inside. It is nevertheless justly complained, that the tinning of copper vessels is not sufficient to defend them from the action of the air, moisture, and saline substances; because, even when strongly coated, they are liable to rust. This may be remedied by a thicker covering of tin; and a manufacture of this kind was established a few years since at Edinburgh; in which the following method is adopted: The surface of the copper is made very rough, by means of a machine contrived for that purpose; then a thick coat of tin is laid on, and the copper hammered smooth as before. To prevent the tin from being melted, and the surface of the copper from being left uncovered, in consequence of a degree of heat superior to that of boiling water, the tin is alloyed with iron, silver, or platina, in order to diminish its fusibility, and render it capable of being applied in thicker layers on the copper.
A patent was also granted, in August 1770, to Mr. Maurice Crawford, of Edinburgh, for his new method of tinning copper, which would last ten times longer than that by any former process. This patent is now expired; we shall therefore communicate the following particulars: The copper must be wrought in the common way, till it is ready for the first pickling, which should be performed in the usual mode. It is next frozen on the inside, on rough stakes, or by any other method of freezing, which opens the pores of the copper, and causes the tinning to penetrate. It is then to be pickled a second time, and scoured clean on both sides, when it should be coated with sal ammoniac and grain tin; after which the copper
