The method of burning together, although it only admits of limited application, is in many cases of great importance, as when successfully performed the works assume the condition of greatest strength, from all parts being alike. There is no dissimilarity between the several parts as when ordinary solders are used, which are open to an objection, that the solders expand and contract by heat either more or less than the metals, to which they are attached. There is another objection of far greater moment: the solders oxidize either more or less freely than the metals, and upon which circumstances hinge some galvanic or electrical phenomena; and thence the soldered joints constitute galvanic circuits, which in some cases, cause the more oxidizable of the two metals to waste with the greater rapidity, especially when heat, moisture, or acids are present.
In chemical works this is a most serious inconvenience, and therefore leaden vessels and chambers for sulphuric acid must not be soldered with tin solder, the tin being so much more freely dissolved than the lead. Such works were formerly burned together by pouring red-hot lead on the joint, and fusing the parts into one mass, by means of a red-hot soldering-iron, as noticed at page 394; this is troublesome and tedious, and it is now replaced by the autogenous soldering, to be explained.
Pewter is sometimes burned together at the external angles of works, simply that no difference of colour may exist; the one edge is allowed to stand a little above the other, as in fig. 234, page 391, a strip of the same pewter is laid in the angle, and the whole are melted together, with a large copper-bit, fig. 311, page 445, heated almost to redness; the superfluous metal is then filed off, leaving a well-defined angle without any visible joint.
Brass is likewise burned together; for instance the rims of large mural circles for observnatiories that arc five, six, or seven feet diameter, are sometimes cast in six or more segments, and attached by burning. The ends of the segments are filed clean, two pieces are fixed vertically in a sand mould in their relative positions, a shallow space is left around the joint, and the entire charge of a crucible, say thirty or forty pounds of the melted brass a little hotter than usual, is then poured on the joint to heat it to the melting point. The metal overflows the shallow chamber or hole, and runs into a pit prepared for it in the sand; but the last quantity of metal that remains solidifies with the ends of the segments, and forms a joint almost or quite as perfect as the general substance of the metal; the process is repeated for every joint of the circle.*
Cast-iron is likewise united by burning, as will be explained by the following example: to add a flange to an iron pipe, a sand mould is made from a wood model of the required pipe, but the gusset or chamfered band between the flange and tube, is made rather fuller than usual, to afford a little extra base for the flange. The mould is furnished with an ingate, entering exactly on the horizontal parting of the mould, at the edge of the flange, and with a waste head or runner proceeding upwards from the top of the flange, and leading over the edge of the flask to a hollow or pit sunk in the sand of the floor.
* The compensation balance of the chronometer and superior watches, is an interesting example of natural soldering. The balance is a small fly-wheel made of one piece of steel, covered with a hoop of brass; the rim consisting of the two metals, is divided at the two extremities of the one diametrical arm of the balance, so that the increase of temperature which weakens the balance-spring, contracts in a proportionate degree the diameter of the balance, leaving the spring less resistance to overcome. This occurs from the brass expanding much more by heat than steel, and it therefore curls the semicircular arcs inwards, an action that will be immediately understood if we conceive the compound bar of brass and steel to be straight, as the heat would render the brass side longer and convex, and in the balance it renders it more curved.
In the compensation balance, the two metals are thus united; the disk of steel when turned and pierced with a central hole, is fixed by a little screw-bolt and nut at the bottom of a small crucible with a central elevation, smaller than the disk; the brass is now melted and the whole allowed to cool. The crucible is broken, the excess of brass is turned off in the lathe, the arms are made with the file as usual, the rim is tapped to receive the compensation screws or weights, and lastly the hoop is divided in two places, at opposite ends of its diametrical arm.
A little black lead is generally introduced between the steel and the crucible; and other but less exact modes of combining the metals are also employed.
The end of the pipe is filed quite clean at the place of junction, and a shallow nick is filed at the inner edge to assist in keying on the flange; lastly the pipe is plugged with sand and laid in the mould. After the mould is closed, about six or eight times as much hot metal as the flange requires, is poured through the mould; this heats the pipe to the temperature of the fluid iron, so that on cooling, the flange is attached sufficiently firm to bear the ordinary pressure of screw-bolts, steam, etc.*
The method of burning is occasionally employed in most of the metals and alloys, in making small additions to old castings, and also in repairing trifling holes and defects in new ones; it is only successful, however, when the pieces are filed quite clean, and abundance of fluid metal is employed, in order to impart sufficient heat to make a natural soldering. A process which is also, although differently accomplished, in plating copper with silver, (page 282,) as the two metals are raised to a heat just short of the melting point of the silver, and the metals then unite without solder by partial alloying.