216 METALS. 



rite. Much of the tin is also obtained from loose stones : (called skodesj 

 and courses of such stones or tin debris are called streams, whence the 

 name stream tin. 



The ore taken from the mines is first pounded or stamped in a stamp- 

 ing mill, and then washed by running water, which carries off to a great 

 extent the lighter impurities and leaves the heavy ore behind, with still 

 some of the gangue. It is next roasted in a reverberatory furnace, to 

 expel any arsenic or sulphur derived from the presence of other ores, and 

 then again washed. After being thus purified as far as possible, the ore 

 is usually mixed with pit-coal and a little lime, and strongly heated in 

 either a reverberatory furnace or what is called a blowing furnace. A 

 state of fusion is kept up for about eight hours. The metal is then 

 drawn off into iron vessels. As it contains still some slag or earthy 

 matters, it is remelted at a lower temperature, which does not fuse the 

 impurities, and kept agitated for a while by wet charcoal or carbonized 

 wood ; it is then skimmed and run into blocks, weighing from 275 

 to 325 pounds each. The tin thus made from the ore derived from the 

 mines, is called block tin, and is less pure than that from the stream ore ; 

 the latter was formerly called grain tin, though now this is a genera 1 

 term applied to the purest kinds of tin in commerce. 



In an assay of tin ore, after pulverizing, washing, roasting, and weigh- 

 ing, the ore should be mixed with lampblack or charcoal, and heated 

 quickly in a covered crucible to a white heat. On removing the crucible 

 from the fire, a button of tin will be found in it. If the ore is not pure, 

 carbonate of soda or borax may be added to the lampblack. The result 

 is good if the tin obtained is malleable and not brittle. The tin may be 

 farther purified by fusing it in a ladle, and pouring it into another ves- 

 sel whenever the cooling has hardened the alloys, or just before the tin 

 itself begins to harden ; it will flow out, leaving the impurities behind. 



The best tin ores afford 65 to 70 per cent, of tin in the large way. 



The annual production of tin in different countries, is as follows 



Great Britain, 140,000 cwt. 



Banca and Malacca, - - - 100,000 " 



Saxony, 3,500 « 



Austria, 380 " 



Sweden, 750 " 



Tin is used in castings, and also for coating other metals, especially 

 iron and copper. Copper vessels thus coated were in use among the 

 Romans, thongh not common. Pliny says that the tinned articles could 

 scarcely be distinguished from silver, and his use of the words incoquere 

 and incoctilia, seems to imply, as a writer states, that the process was 

 the same as for the iron vares of the present day, by immersing the 

 vessels in melted tin. Tne sheets of iron for tinning are cleaned with 

 acid, heated, and then cold-rolled ; again subjected to dilute acid, and 

 afterwards scoured with sand in pure water : then two or three hundred 



What are the steps in the process of reduction 1 Describe the mode 

 f assaying tin ore. What is the yield of Great Britain in tin ? What 

 he whole amount from the tin mines of the world? How is iron 

 inned 1 



