When a not apply when the tin exceeds 67 per cent. more fusible solder is required, the metal bismuth is added in addition, and sometimes cadmium. Metallic tin is sometimes used alone, as in soldering fine utensils of tin plate. Lead is also soldered to lead by simply melting the edges by means of a blowpipe flame, as in the case of lead sheets for sulphuric acid chambers. This is termed autogenous soldering. Soft solders are termed common, medium, or best, according to the amount of tin, those containing most lead being the cheapest. Fine or best solder is largely used for Britannia metal, best tin - plate, brass, and other metal articles. The commoner varieties are used by plumbers. An alloy of 1 part tin to 2 parts lead is termed "plumbers' sealed solder," and stamped by the "Plumbers' Company." The following table gives the proportions employed for different kinds of soft solder : The quality of the solder is roughly judged by the appearance of the surface when cast into a mould and allowed to cool. With excess of lead, the surface shows a uniformly grayish-white colour. With excess of tin, the surface is bright with dull grayish-white spots; in fact, the appearance approximates to that of lead or tin according to the amount of lead or tin present. § 99. Alloys of Tin, Lead, and Zinc.-The presence of lead imparts more body and resistance to the alloys of tin and zinc than when tin and zinc alone are used, but the triple alloys clog the file as much as the latter. The fractures are more decided than tin-zinc alloys. When the three metals are present in about equal proportions, the alloys are malleable, although not very ductile, and may be economically employed in some cases. 1 Various alloys are given in the following table : No. 2 is as hard and brittle as zinc, although more resisting. No. 3 resembles hard lead, leaves a mark on paper, appears to be uniform in composition, and has a leaden colour. Both these alloys are better adapted for castings than either of the three metals taken separately. Nos. 1, 3, and 7 appear to withstand friction very well. Nos. 2, 4, and 5 will do for work requiring more resistance than pure zinc. No. 6 will answer for small castings requiring a certain malleability. It is serviceable for ornaments, and will bear engraving and chasing. For these uses Nos. 2, 4, and 5 would be too brittle; and Nos. 1, 2, and 7 too soft and yielding. All these alloys of tin, zinc, and lead have little lustre when polished, and become readily tarnished by exposure to air. Some of them may be used as type metal. Most of them may be easily rolled. It is preferable to melt the zinc at the lowest possible temperature, to add the tin, and then the lead. The metals should be covered with charcoal, or with resin, and a little 1 Guettier, Guide Pratique des Alliages, 1865. borax added, in order to prevent oxidation. For white alloys, the best proportions are within the following limits: : The proportion of zinc is increased if toughness and hardness are desired. More tin increases the malleability, whiteness, and lustre. But the proportion of lead should not much exceed the amount indicated above. $100. Alloys of Tin and Antimony. These metals when united together form the base of what is termed Britannia metal, many varieties of which consist of tin hardened by antimony. Such alloys are as white as tin, but harder and less malleable. The brittleness increases as the proportion of antimony is greater. Guettier states that the specific gravity of tin-antimony alloys is below that which would be calculated from the specific gravity of each metal taken singly. This indicates that expansion takes place by the union of these metals. An alloy of 80 parts tin and 20 of antimony is sufficiently malleable to be hammered and rolled in the cold. It is by keeping near these proportions that the best alloys of tin and antimony, for making pots and engravers' plates, are obtained. A white metal desertspoon analysed by the author was found to contain The following table will show the proportions of different tin-antimony alloys : According to Chaudet, 10 parts tin to 1 part antimony form a perfectly ductile alloy. Alloys of tin and antimony are made by fusing the two metals together. Also they may be made by reducing sulphide of antimony in contact with tin. § 101. Britannia Metal.-Reference has just been made to some alloys called by this name, consisting of tin and antimony, but more generally other metals in small quantities are added to the mixture, such as copper, zinc, lead, bismuth, etc. Britannia metal has a white colour, with a bluish tint ; it takes a high polish, is hard, malleable, and ductile in proportion to the amount of tin and copper present. The latter metal, however, must always be limited in quantity, as it tends to impart a yellowish tint, and diminish the fusibility, for which reasons the quantity of copper used is always very small compared with the tin and antimony. Good alloys show a fine-grained jagged fracture. If the alloy exhibits a crystalline fracture, it either contains too much antimony, or requires to be remelted in order to promote more intimate union of the constituents. Iron and zinc appear to be very objectionable, as they considerably increase the hardness and brittleness. If much zinc is used to make a cheap alloy, the antimony must be in much smaller amount than is usual with better alloys. Lead is advantageous in cast work, making the alloy more fusible, but it impairs the colour and lustre, and the alloy tarnishes more readily in air than in alloys in which lead is absent. Arsenic, even in small quantity, induces brittleness, and should be avoided as much as possible. Alloys containing metals other than tin and antimony are less brilliant in lustre than when these two metals alone are employed. The following table shows the composition of a few varieties of Britannia metal : |