There are two distinct kinds of sheet lead, cast, and milled or rolled. The first-mentioned is the original kind, and as it is preferred we shall first describe it as usually practised by the plumbers. A large cast-iron cauldron is built over a furnace, enclosed in solid masonry, at one end of the casting-shop, and near to the mould or casting-table. This table is generally of the form of a parallelogram, about six feet wide, and twenty feet long, substantially made of wood, and bound together at the corners and other parts with iron. The face of the table is surrounded by a raised border about three inches thick, and five inches in depth; the legs and framing are of course strong and firmly jointed, to prevent any yielding or trembling during the casting. The top of the table is of boards, laid very even, and this is covered by a stratum of fine sand laid very smooth and even; at the end of the table, nearest to the cauldron in which the lead is melted, is adapted a box, equal in length to the width of the table; at the bottom of the box is a long horizontal slit, through which the metal flows out uniformly over the breadth of the table; this box is mounted upon rollers, which run on the rim of the table as a railway, and is set in motion by a rope and pulley.
When the metal in the cauldron is sufficiently heated to retain its fluidity throughout the spreading of the sheet, the requisite quantity of it is ladled into the casting-box, and the dross taken off its surface by means of a perforated scummer. As soon as the box has dispersed its contents upon the table, a man levels the surface with a striker, which takes off the impurities also, before it cools; as soon as it has set, the edges are taken off in a straight line, and when sufficiently cool it is rolled up and removed away to make room and prepare for the succeeding castings, which are conducted in a similar way.
A method different from the foregoing is practised in some places. Instead of a casting-box travelling over an horizontal surface, the table is a little inclined, and an iron vessel at the upper end of the table next to the cauldron is tilted so as to pour out the fluid which flows to the other end, during which operation a workman levels the surface with a striker or straight edge, which reduces the mass to a uniform thickness. Cast sheet-lead, made by these processes, does not possess that very uniform thickness, nor that smoothness of surface which distinguishes milled-lead, or such as has been laminated between large powerful rollers, actuated by a steam engine or other suitable prime mover. The method by which this is done on the large scale is as follows: - A cauldron, capable of melting ten or more tons of metal at a time, is substantially erected over a common furnace; when the lead is at that temperature above the melting point, which will prevent its congelation before it has flowed to the remotest part of the mould, the vessel is tapped by the pulling out of a plug; this plug is attached to a bent extremity of a lever of the first class, the other arm of which is loaded with a weight, that acts as a compressing force to keep the plug in the tapping hole; a rope attached to the end of the loaded arm of the lever, and passing over a pulley, being pulled by a workman, the plug is thereby easily withdrawn; and upon the workman letting go the rope, the weight upon the lever forces the plug into the hole again. (Owing to the pressure of the superincumbent portion of the metal in the cauldron above the tapping-hole, the lead is spirted with considerable force around the plug at the moment of its entering or leaving the tapping-hole, which renders it dangerous to persons standing within the distance of a few yards; and as this dangerous effect might easily be prevented, we wonder that it is not done; such as applying a lateral screen to the tapping-hole, or the plug, and making the plug, as a tap-hole, cylindrical, instead of conical.) The metal is discharged into a very large square cast-iron pan, laid perfectly level, and capable of holding a plate of lead about an inch and a half thick, and weighing about five tons; when cold, the cast-iron pan or mould is hooked at the corners to chains, in the manner of a scale-board, and by the assistance of a large jib extending over it, and a powerful crane, is raised from its seat and swung round upon a table upon a level with the laminating rolls.
On this table, the plate is now divided into five, six, or more narrow plates, the numbers and dimensions of these depending upon the size and weight of the sheets to be made from them. The division of the plate is effected by very rude means; one man, holding an ash-rod, applies a cold chisel at the end of it to the chalk division line scribed on the plate, whilst another workman, with a sort of sledge-hammer, made of agreat lump of lead, at the end of a long handle, swings it round vigorously, and gives the chisel such heavy thumps as to send it through the thick plate of lead at each blow.
The laminating rollers are cast-iron cylinders, usually about eighteen inches in diameter, and about six feet long, turned and ground to a very true and smooth surface; the lower roller turns in fixed bearings, but the upper in adjustable bearings, which are acted upon by screws for regulating the distances between the rollers. The power is communicated to the lower roller through the medium of a reversing motion, which causes the rollers to change the directions of their respective rotations, according as the sheet of lead may be on one side or the other of them; on either of which it is supported upon a species of table, from twenty tothirty feet long, the surfaces of which are composed of a seriesof wooden bearing rollers. The plate of lead being introduced between the cylinders, is griped by them, and forced through by their revolution: the plate is thus extended by a reduction of its thickness, and is received upon the bearing rollers on the surface of the table; the workmen on each side of the machine now give the regulating screws a turn, by which the laminating rollers are brought nearer together; then the motion of these rollers is reversed, and the sheet of lead traverses back through them to the opposite side, where it is received on the bearing rollers of that table, considerably extended; the rollers are again adjusted nearer together, and the motion of them is again reversed for the next rolling through; the operation being thus repeated until the plate is brought to the required thickness.
When this is done, the rough edges are cut off to a straight line, and the sheet rolled up off the table on to a truck adapted to the work, and wheeled away. Whilst this is being completed, another plate of lead is passing through the laminating rollers; and whilst all the plates of lead divided from the great cast-plate before-mentioned are being laminated in the manner of the first described, the casting department of the establishment is engaged in preparing to cast, or in casting another great plate, which is subsequently divided and placed in readiness for the continuation of the laminating operation.
The very thin sheet-lead, with which the tea-chests from China are lined, is made, according to common report, in the following manner: - A man sits upon a floor with a large flat stone before him, and another movable one at his side on a stand; his fellow-workman stands beside him with a vessel full of melted lead, and having poured out a certain quantity on the large flat stone upon the floor, the other immediately lifts the movable stone, and dashing it on the fluid lead, presses it out into a flat and very thin plate; the stone and lead are then quickly removed, and the operation renewed, which is repeated in quick succession. The rough edges are afterwards cut off, and the sheets soldered together for use.
The Tinning of Sheet-Lead may be effected in two ways. First, place the sheet of lead upon a hot stove, until it acquires sufficient heat to keep melted tin poured upon it in a fluid state; then throw a little powdered resin over the sheet, and when it has melted, with a greasy rag rub the tin and resin over the sheet of lead until it is completely covered with the tin; after which, wipe off the superfluous matter. Secondly, the tin in the cold state, and in small quantities at a time, may be laid on the plate of lead, carefully heated sufficiently to fuse the tin, (but not more so,) and bythe help of resin and similar manipulation to the first-mentioned plan, the lead may be perfectly coated.