TIN 



1007 



parts to fall to the bottom. That froth, composed almost wholly of the oxides of tin 

 and foreign metals, is successively skimmed off, and thrown back into the furnace. 

 When it is judged that the tin has boiled long enough, the green wood is lifted out, 

 and the bath is allowed to settle. It separates into different zones, the upper being 

 the purest ; those of the middle are charged with a little of the foreign metals ; and 

 the lower are much contaminated with them. When the tin begins to cool, and when 

 a more complete separation of its different qualities cannot be looked for, it is lifted 

 out in ladles, and poured into cast-iron moulds. It is obvious that the order in which 

 the successive blocks are obtained is that of their purity ; those formed from the 

 bottom of the basin being usually so impure that they must be subjected anew to the 

 refining process, as if they had been directly smelted from the ore. 



The refining operation takes five or six hours ; namely, an hour to fill the basin, 

 three hours to boil the tin with the green wood, and from one to two hours for the 

 subsidence. 



Sometimes a simpler operation, called tossing, is substituted for the above artificial 

 ebullition. To effect it a workman lifts some tin in a ladle, and lets it fall back into 

 the boiler from a considerable height, so as to agitate the whole mass. He continues 

 this manipulation for a certain time ; after which, he skims with care the surface of 

 the bath. The tin is afterwards poured into moulds, unless it be still impure. In 

 this case the separation of the metals is completed by keeping the tin in a fused state 

 in the boiler for a certain period, without agitation ; whereby the upper portion of the 

 bath (at least one-half) is pure enough for the market. 



The moulds into which the tin-blocks are cast are usually made of granite. Their 

 capacity is such, that each block shall weigh a little more than 3 cwts. This metal is 

 called ' block tin.' The law requires them to be stamped or coined by public officers, 

 before being exposed to sale. The purest block tin is called ' refined tin.' 



The treatment just detailed gives rise to two stanniferous residuums, which have to 

 be smelted again. These are 



1 . The scoriae B and c, which contain some granulated particles of tin. 



2. The dross found on the bottom of the reverberatory furnace, after re-melting the 

 tin to refine it. 



The scoriae c are smelted without any preparation ; but those marked B are stamped 

 in the mill, and washed, to concentrate the tin-grains ; and from this rich mixture, 

 called prillion, smelted by itself a tin is procured of very inferior quality. This may 

 be readily imagined, since the metal which forms these granulations is what, being 

 less fusible than the pure tin, solidified quickly, and could not flow off into the metallic 

 bath. 



Whenever all the tin-blocks have thoroughly undergone the process of liquation, 

 the fire is increased, to melt the less fusible residuary alloy of tin with iron and some 

 other metals, and this is run out into a small basin, totally distinct from the refining- 

 basin. After this alloy has reposed for some time, the upper portion is lifted out into 

 block-moulds, as impure tin, which needs to be refined anew. On the bottom and 

 sides of the basin there is deposited a white, brittle alloy, with a crystalline fracture, 

 which contains so great a proportion of foreign metals that little use can be made of 

 it. About 3i tons of coal are consumed in producing 2 of tin. 



Summary of Prodttce of Tin in Cornwall and Devon for each of the ten years 

 ending 1873. 



