This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
The process of uniting two or more pieces of metal by partial fusion is called "burning." This operation differs from the ordinary soldering, in the fact that the uniting or intermediate metal is the same as those to be joined, and generally no flux is used, but the metals are simply brought almost to the fusing-point and united. The process of burning is, in many cases, of great importance; when the operation is successfully performed, the work is stronger than when soldered, for all parts of it are alike, and will expand and contract evenly when heated, while solders often expand and contract more or less than the metals which they unite, and this uneven contraction and expansion of the metal and solder often tears the joint apart; another objection to soldering is that the solders oxidize either more or less freely than the metals, and weaken the joints, as is the case if leaden vessels or chambers for sulphuric acid are soldered with tin, the tin, being so much more freely dissolved by the acid than the lead, soon weakens or opens the joints.
Fine work in pewter is generally burned together at the corners or sharp angles, where it cannot be soldered from the inside; this is done that there may be no difference of colour in the external surface of the work. In this operation, a piece or strip of the same pewter is laid on the parts to be united, and the whole is melted together with a large soldering-iron or copper bit, heated almost to redness; the superfluous metal is then dressed off, and leaves the surfaces thoroughly united, without any visible joint. In burning together pewter or any of the very fusible metals, great care is required to avoid melting and spoiling the work.
Castings of brass are often united by burning. In this operation, the ends of the 2 pieces to be united are filed or scraped, so as to remove the outside surface or scale; they are then embedded in a sand mould in their proper position, and a shallow or open space is left around the joint or ends of the castings; 30 or 40 lb. of melted brass are then poured on to the joint, and the surplus metal is allowed to escape through a flow-gate. In this way 2 castings may be united so that they are as solid as if they had been cast in one piece. This process is resorted to by all brassfounders in making large and light castings, such as wheels, large circular rims, etc.; when too large to be run in one piece, they are usually cast in segments and united by burning together.
Cast iron is often united by burning together, or, more properly, burning on, for in this case one of the metals added or united is in the fluid state. When about to burn on to a piece of casting, the part to be united to is scraped or filed perfectly clean, and is then embedded in sand, and a mould of the desired shape is formed around the casting; the metal is then poured into the mould, and allowed to escape through a flow-gate until the surface of the casting is melted, and the metals unite, the same as in burning together brass castings. In this way, small pieces that have broken oft large castings are burned on, and cylinders that have had part of the flanges torn off by blowing out the heads are repaired by burning on a new flange or the part that has been torn off. In burning on to cast iron there are several very important points that must be observed in order to make it a success. The ingate, as well as the flow-gate, should be made of a good size, so that the molten metal may be flowed through them rapidly if necessary. The molten iron used should be the hottest that can be procured, and in pouring it into the gate it should be let in rapidly at first, and allowed to run out freely at the flow-gate, so as to prevent its being chilled upon the surface of the casting.
After the casting has been heated in this way, the metal should be poured and flowed through the gates slowly, so as to give the solid metal a chance to melt and unite with the fluid metal. After the surface of the metal has been melted, the pouring should be urged, so as to unite the metals more thoroughly; the operation should be continued for some time, so that the casting may be more thoroughly heated, and not be so liable to crack from uneven expansion and shrinkage.
The process of burning together or mending is often resorted to by stove-plate moulders for stopping small holes in the plates; this is done by laying the plate on the sand, with the sand firmly tucked under the part to be mended; a little sand is also put on top of the plate, around the part to be mended, so as to prevent the iron spreading over the plate; the molten iron is poured on the part to be mended, until the edges are fused, and the surplus metal is then scraped off with the trowel or a clamp iron while in the molten state.
The simplest method of burning is that adopted in the manufacture of leaden tubs, tanks, and other vessels, the success of the operation depending more upon the quantity and state of the materials than upon the skill of the workman. Thus if a round or square tank is required, a piece of the sheet lead sufficient in size to form the sides and ends of the tank, or the hoop, if a round one, is bent into shape, the overlapping ends being secured by a few touches of solder or a few nails, driven from the inside, so as to keep the overlapping edges perfectly close. On the outside of the joint a piece of stout brown paper is pasted, so as to cover the whole of the joint. The hoop or parts to be joined, are then turned downwards on to the casting floor, and moulding sand of good quality is packed over the joint to about 5 or 6 in. in depth, a piece of wood about 3/4 in. thick being placed over the junction of the edges, while the sand is being rammed together. This wood is to form the runner or channel for the molten metal, and must be slightly longer than the joint to be made, so that it can be drawn out lengthways.
The sand being tolerably firm, cut down to the wood, with a trowel, forming a sort of V-shaped groove along nearly the whole length of the intended joint, leaving a few inches of the wood buried at one end, which is also to be completely stopped. When the wood is drawn out, which is the next operation, the other end of the "runner" is to be stopped up to a greater or lesser height, according to the thickness of the metal; about 1 in. is usually sufficient. It will be understood that we have here, as it were, a broad-mouthed ditch in the sand, stopped at one end, and with a "bar" 1 in. deep at the other; and at the bottom are the overlapping edges of the lead that is to be joined. A quantity of lead is then melted in a furnace, and brought to a heat sufficient to melt the 2 edges in the metal to be joined. Everything being in readiness, a small quantity of rosin is dusted along the intended joint at the bottom of the runner, and a bay is formed to catch the overflow of metal. The latter is then poured in steadily but quickly, giving it as much fall as possible, and keeping up the supply till by means of a trying stick it is known that the cold metal of the edges has been melted.
The overflow end is then stopped up, and more metal is poured in, the molten lead being kept ready to fill up as shrinkage shows itself. When set, the sand is removed, and the "runner," or the remains of the metal poured on the joint, is cut off with a chisel and mallet; the surface is finished off with a scratch-brush or wire-card. The paper that was pasted over the outside will have fallen off, and will be seen to have left a smooth surface, in which no trace of a join is visible. The secret of success lies in having a good bed of sand, plenty of hot metal, and careful attention to the shrinkage. The bottom of the tub or tank is put in by a similar process. The hoop or sides, when the tank is not too deep, being completely sunk in a hole in the casting-shop, is filled up with sand inside and out. The sand is then removed from the inside to a depth equal to the thickness required in the bottom of the tank, and smoothed over well with the trowel. The sand outside the tank must be rammed hard, and a bay left all round to take the overflow. As before, rosin is sprinkled over the edge of the metal, and the melting-furnace is brought close to the work.
When the metal is as hot as possible, 2 or more men take a ladleful and pour along the edge, and when the latter is melted, the molten metal is poured in until it is up to and running over the level of the outside sand all round. The dross is then skimmed off and the metal is left to cool, as it shrinks equally all over and requires no further attention. It is obvious that instead of making the bottom by pouring on molten metal, a piece of the required size can be cut out of thinner sheet lead, and placed on the top of the inside sand; but the majority of experienced workmen prefer the first-mentioned method of burning in a bottom. If the article is of considerable size, however, it is necessary to have more than one workman, as the metal must be poured on as quickly as possible.