An alloy of 11 parts tin to 1 part zinc, beaten out into leaf, forms spurious silver-leaf. According to Rudberg ZnSn ̧ (1 part zinc to 10 parts tin) solidifies completely at 204° C., but all the other alloys separate on cooling from a state of fusion into two portions, the one consisting of ZnSn, not solidifying till cooled to 204° C., while the remainder, consisting of an alloy containing a larger proportion of one or the other metal, solidifies at a higher temperature: thus :- Variable point Sn12Zn Sn6Zn Sn4Zn SngZn SnZn SnZn 210 A metal, now largely employed for buttons, is prepared by rolling a thin sheet of tin on each side of a thick sheet of zinc. The metals become firmly welded together, and the pressed work has the advantage of a coating of tin. § 96. Tin and Lead. These metals are easily melted, and unite together in all proportions, forming a series of valuable alloys. Lead leaves a dark mark when drawn across paper, and when only a limited quantity of tin is alloyed with it, this property is still retained; but if a certain limit be exceeded, the alloy no longer has the property of producing the dark mark; hence it is possible to roughly estimate the quantity of the lead present by this test, taken in conjunction with the behaviour of the metal under the hammer, file, and chisel. Tin 90 parts, and lead 10 parts, does not streak paper. Tin 75, lead 25, gives a very faint mark. Between these limits no mark can be observed when the alloys are drawn across paper. All lead-tin alloys containing less than 75 per cent of tin have the power of marking paper. Alloys of lead and tin shrink, or settle less on cooling than either of the metals taken singly; they are not so fluid when melted, and the castings have not the same sharpness. The effect of lead on tin is to increase its malleability and ductility, but to diminish its tenacity and toughness. In the alloy, tin 90, lead 10, tin preserves the crackling noise, but in a less degree than in pure tin. On the contrary, 1 per cent of zinc in tin is sufficient to destroy the crackling noise when the metal is bent. The following table contains the results of Kuffer's experiments with respect to the specific gravities and melting points of lead-tin alloys. The author has added the percentage compositions. Sn, Pb 46.73 Sn, Pb 36 90 Sn Pb 30.49 Sn, Pb 25.85 Sn, Pb 53.27 8.7518 8.7454 63.10 8.3983 8.2914 0.0069 69 51 8.1516 8.1730 0.0096 189° 74.15 8.0372 8.0279 0.0093 17.04 82.96 7.9526 7.9210 0.0116 0.0064 196° 186° 194° * Sn=118. Pb 207. "Alloys of lead and tin are distinguished by the facility with which they ignite and burn. The alloy of 4 or 5 parts lead and 1 part tin burns like charcoal at a red heat, the combustion continuing like that of an inferior peat, with the formation of cauliflower excrescences. The action appears to be due to the affinity which exists between the two oxides." 1 § 97. Pewter is essentially an alloy of lead and tin, to which small quantities of other metals are sometimes added. 1 Watt's Dict. of Chem. p. 534. Common pewter consists of tin 80 parts and lead 20 parts. Holtzapfel states that some pewters are made nearly as common as that of equal parts of the metals; when cast they are black, shining, and soft; when turned they are dark and bluish. Other pewters only contain or of lead; these when cast are white, without gloss, and hard. Such are pronounced very good metals, and but little darker than tin. The French legislature sanctions the employment of 18 per cent lead with 82 per cent tin, as quite harmless in vessels for wine and vinegar. The finest pewter, called tin and temper, consists chiefly of tin, with only a little lead and copper, which make it hard and somewhat sonorous, but the metal becomes brown in colour when the copper exceeds a certain quantity. The copper with twice its weight of tin is melted, and from lb. to 7 lbs. of this alloy termed temper are added to a block of tin, weighing from 360 to 390 lbs. Zinc in small quantity is added to the molten alloy, and the mixture well stirred. The operator considers that the zinc removes impurities, bringing them to the surface as dross, and also that the burning of the zinc during casting lessens the oxidation of the pewter. Unalloyed tin is now being largely used in place of pewter, and it is not only whiter in colour, but for domestic purposes it is certainly safer, on account of the poisonous nature of lead compounds, although, as before stated, lead when thoroughly alloyed with tin may be present to the extent of 18 per cent without injury. An alloy of 3 parts lead and 5 parts tin is used for tinning certain articles of copper. Alloys of lead and tin have a bright lustrous appearance, and are used for the manufacture of stage jewellery. The so-called Fahlum brilliants are made of an alloy of 39.6 parts lead and 60-4 parts tin. The molten alloy is poured into moulds faceted in the same manner as diamonds. Toys, such as tin-soldiers, and many other articles are made of alloys of lead and tin. For tempering various articles of steel, where it is important to have a definite and uniform temperature, Messrs. Parkes and Martin have proposed the following alloys : The same authorities have determined the melting points of lead-tin alloys; their results are embodied in the following Composition. Melting point. Composition. Melting point. According to Tilden,' the most fusible alloy of lead and tin consists of very nearly 2 parts tin to 1 part lead, or 3.3 atoms of tin to 1 atom of lead. The melting points of this and other alloys are graphically represented by the following curve, Fig. 31: § 98. Soft solders usually consist of lead and tin in various proportions, and to a certain extent the fusibility of the alloy increases with the content of tin; but this does 1 Chem. Philosophy, p. 15. S |