The natural colour of lead is a bluish white; when newly melted, or cut, it is very bright; but is soon tarnished on exposure to the atmosphere ; assuming first a dirty gray colour, which afterwards becomes white. It is capable of being hammered into very thin plates, and may be drawn into wire; but its tenacity is very inferior to that of other metals; for a leaden wire the hundred-and-twentieth part of an inch in diameter is only capable of supporting 1.9.•11b. breaking. Lead nicks at the temperature of 61'2° of Fahrenheit's thermometer; and, if a stronger heat be applied, it boils and evaporates. If cooled slowly. it crystal lizes. The change of its external colour is owing to its gradual combination with oxygen, which converts its exterior surface into an oxide : this outward crust, however, preserves the rest of the metal for is long time, as the air can pene trate it but very slowly. Lead is not acted upon immediately by water, though that element greatly facilitates the action of the air upon it ; for it is known, that when lead is exposed to the atmosphere, and kept constantly wet, the process of oxidation takes place much more rapidly than it does under other circumstances : hence the white crust, that is to be observed upon the sides of leaden vessels containing water, just at the place where the surface of the water terminates. For other particulars relative to this metal, see LEAD.
Plumbers purchase lead it) pigs, and reduce it themselves into sheets, or pipes, as they have occasion. Of sheet-leach, they have two kinds, cast and 1nilled ; the former is used for covering flat roofs of buildings, laying of terraces, form ing gutters, lining reservoirs, &c. ; the latter, which is very thin, for covering the hips and ridges of roofs: this last they do not manufileture themselves, but purchase it ready pre pared of the lead merchants, as it comes from the ore and roasting furnaces.
In casting sheet lead, a copper is provided, well fixed in masonry at the upper end of the workshop, near the mould, or casting; table, which consists of strung deal boards, well jointed together, and bound with bars of iron at the ends. The sides of this table, of which the is a parallelogram, varying in size from -1 to 6 feet in width, and from 16 to IS feet and upwards it) length, are guarded by a frame or edg ing of wood, 3 inches thick, and -1 or 5 inches higher than the interior surface, called the shafts; the table is fixed upon firm legs, strongly framed together, about l; or 7 inches lower than the top of the copper ; at the upper end of the mould, nearest the copper, a box, called the pqn, is adapted in its length to the breadth of the table, having at its bottom a long horizontal slit, from which the heated metal is to issue, after it has been poured in from the copper. This box moves upon rollers along the surface of the rim of the table, and is put ht motion by means of ropes and pulleys fixed to beams above. While the metal is melting, the sur face of the mould, or table, is prepared by covering it with a stratum of dry and clean sand, regularly smoothed over with a kind of rake, called a strike, which consists of a board, about 5 inches broad, and rather longer than the inside of the mould, so that its ends, which are notched about two iuches deep, may ride upon the shafts ; this, being passed down the whole length of the table, reduces the sand to a uniform surfiice. When this is done, the pan is brought to the head of the table, close to the copper, its side having been previously guarded by a coat of moistened sand, to pre vent its tiring from the heat of the metal, which is now emptied in with ladles from the copper. These pans, or boxes, it must he observed, are made, as to their contents, equal to the quantity of melted lead requiredto east a whole sheet at one time ; and the slit in the bottom is so adjusted as to let out, during its progress along the table, just as much as will completely cover it, of the thickness and weight per foot required. Everything being thus prepared, the slit is
opened, and the box is moved along the table, dispersing its contents from the top to the bottom, and leaving in its pro gress a sheet of lead of the desired thickness. When cool, the sheet is rolled up and removed from the table, and other sheets are cast, till all the metal in the copper is exhausted. The sheets so formed are rolled up, and weighed, it being by weight that the public is charged for sheet lead.
In some places, instead of having a square box, upon wheels, with a slit in the bottom, the pan consists of a kind of trough, being composed of two planks nailed together at right angles in their length, with two triangular pieces fitted in between them at their ends. The length of this pan, as well as that of the box, is equal to the whole breadth of the mould : it stands with its bottom, which is a sharp edge, on a bench at the head of the table, leaning with one side against it : and on the opposite side is a handle to lift it up by, in order to pour out the liquid metal. Un the side of the pan next the mould, are two iron hooks to hold it to the table, and prevent it fi um slipping, while the metal is pour ing out of it into the mould. The mould, as well as the pan, is spread over, about two inches thick, with sand sifted and moistened, which is rendered perfectly level by moving over it the strike, and smoothing it down with a plane of polished brass, about a quarter of an inch thick, and nine inches square, turned up on all the four edges, and with a handle fitted to the upper, or concave side. Before they begin to east, the strike is made ready, by tacking two pieces of old hat on the notches, or by covering them with leather cases, so as to raise the under side of the strike about the eighth of an inch, or more, above the sand, according to the proposed thickness of the sheet : the face, or under surface of the strike is then smeared with tallow, and laid across the breadth of the mould, with its ends resting on the shafts. The melted lead is then put into the pan with ladles, and, when a suffi cient quantity has been put in, the scum is swept off with a piece of board to the edge of the pan, and is suffered to settle on the coat of sand, to prevent its falling into the mould when the metal is poured out. It generally happens, that the lead, when first taken from the copper, is too hot for casting; it is therefore suffered to cool in the pan, till it begins to stand with a shell or wall on the sand with which the pan is lined. Two men then take the pan by the handle, or one man takes it by means of a bar and chain fixed to a beam in the ceiling, and, turning it down, the metal runs into the mould : another man stands ready with the strike, and as soon as all the metal is poured in, he sweeps it forward, and draws the residue into a trough at the bottom prepared to receive the reltise. The sheet is then rolled up. as before. In this mode of operation, the table inclines in its length about an inch, or an inch and a hall; in the length of 16 or 17 feet, or more, according to the required thickness of the sheets ; the thinner the sheet the greater the declivity, and rice versa ; the lower end of the mould is also left open, to admit of the superfluous metal being thrown off: When it is intended to cast a cistern, the size of the four sides is measured out ; and the dimensions of the front having been taken, long slips of wood, on which the mould ings are carved, are pressed upon the sand, and leave their impression ; and figures of birds, beasts, &c. are likewise stamped in the internal area, by means of leaden moulds: whatever of the sand has been disturbed in doing this, is then made smooth, and the process of casting goes on as for plain sheets ; only, instead of rolling up the lead when cast, it is bent into four sides, so that the two ends may be joined at the hack, where they are soldered together ; and afterwards the bottom is soldered up.