Iron is usually tinned before, but copper always after it has been formed into utensils. The object to be attained by the tinning of copper is to prevent the vessels made of that metal from being corroded, and to preserve the food prepared in them from being mixed with any particles of that poisonous substance called verdigris, which is formed by such corrosion. In the tinning of copper vessels, their interior surface is first scraped very clean with an iron instrument, and then rubbed over with sal-ammoniac ([207]), for the purpose of more completely cleansing them, and also of preventing the formation of verdigris from the copper during the operation. The vessel is then heated, and a little pitch is thrown into it. While quite hot, a piece of tin is applied to the copper, and this, instantly uniting with it, soon clothes the whole surface with tin.
This metal, when amalgamated with mercury, is used for the silvering of looking-glasses ([228]). When tin is melted in an open vessel, its surface is soon found to be covered with a grey powder, which is an oxide ([21]) of the metal, and is generally called dross. If the heat be continued, the colour of this powder becomes yellow. In this state it is known by the name of tin-putty, and is employed in polishing glass, steel, and other hard substances. When the heat is very violent, the metal takes fire, and is converted into a fine white oxide, which is used to render glass opaque, for the forming of enamel. Oxide of tin is also an important article to dyers. It is employed by them, in large quantities, to give brightness to such colours as are used in forming scarlets and other reds: and to precipitate the colouring matter of other dyes.
Tin is an essential ingredient in bell-metal, bronze, pewter, and various other compounds. It may be combined with lead, in any proportion, by fusion; and this alloy is harder, and possesses much more tenacity than tin. The hardest alloy is a composition of three parts of tin and one of lead. The presence of the tin destroys, in a great measure, the noxious qualities of the lead. It is sometimes customary to tin copper vessels with this mixture, and it has been ascertained that such vessels are in no respects injurious.
There are three kinds of pewter in common use. These are called plate, trifle, and ley pewter. The first, which is made into plates and dishes, is formed of tin, with a small proportion of lead and antimony ([245]). The second, or trifle pewter, which is made in somewhat different proportions, is used for the quart and pint pots of the publicans: and the ley pewter, which is formed of three parts of tin and one of lead, is manufactured into wine and spirit measures.
Tin may be beaten into leaves or plates that are much thinner than paper. But, when it is thus worked, several leaves must be joined together. They then support each other, and yield to the hammer without tearing. These leaves are used for the silvering of glass globes, and the plating of other metals. Those that are used for the silvering of looking-glasses are much thicker. The article called tin-foil is an alloy, consisting generally of two parts of tin and one of lead; and capable of being beaten to less than the thousandth part of an inch in thickness.
239. LEAD is a heavy metal, of pale and livid grey colour when broken, not sonorous when pure, very flexible, and so soft that it may be marked with the nail. It stains paper or the fingers of a bluish colour, and is about eleven times heavier than water.
The most common state in which lead is found is in combination with sulphur and a small portion of silver. This ore is known by the name of galena, and is frequently in the form of blackish cubical crystals. Lead is also found in union with arsenic ([242]) and many acids.
Great Britain possesses the most important lead-mines in the world; and those that are best known are in the counties of Flint and Derby. The latter are supposed to have been worked even in the time of the Romans.
Lead mines are entered sometimes by perpendicular shafts, and sometimes (when in the sides of hills) by levels. In some of the Derbyshire mines, where the depth of the veins will admit of it, the men work, at different heights, of from four to six feet above each other, along what are called stoops; the uppermost men being two or three yards before those next in succession, and thus forming a kind of steps. The implements used are picks, hammers, and strong iron wedges; and the rocks are also frequently loosened by means of gunpowder.
When the ore is brought out of the mine, it is sorted and washed, to free it from dirt and rubbish. After this it is spread on a board; the best pieces are picked out and separated; and those containing ore mixed with spar ([194]) or other substances, are placed separate, to be broken, and again picked. After the ore, by pickings and washing, has been sufficiently cleansed from extraneous matters, it is roasted in a kind of kiln to free it from the sulphur that is combined with it. The next process is to mix it with a certain quantity of coke, charcoal, or peat, and submit it to the smelting furnace. In this furnace there are tap-holes, which, when the lead is melted, are opened, and the metal, in a fluid state, runs into a large iron pan. The dross which floats on its surface is now skimmed off; and the metal is taken out by ladles, and poured into cast iron moulds, with round ends. The lead thus formed, is ready for use, and has the name of pig lead. According to their size, the pieces that are thus cast have the appellation of pigs, and half-pigs.