TIN. 201 
lion, the arms of that duchy. This is rendered a neces- 
sary operation before the tin can he offered for sale ; ( and 
on stamping, it pays a duty of four shillings per hundred 
weight to the Prince of Wales, as Duke of Cornwall, 
who thence derives a very considerable income. 
The article usually called tin, or tin-plate, and, in 
Scotland, white iron, of which saucepans, boilers, drink- 
ing vessels, and other utensils of domestic economy are 
made, consists only of thin iron plate coated with tin. 
It is thus formed. The iron plates are immersed in 
water rendered slightly acid by spirit of salt (muriatic 
acid, 202) or spirit of vitriol (sulphuric acid, 211): after 
which, to clean them completely, they are scoured quite 
bright. These plates are then each dipped into a vessel 
filled with melted tin, the surface of which is covered 
with suet, pitch, or resin, to prevent the formation of 
dross upon it. The tin not only covers the surface of 
the iron, but completely penetrates it, giving to its 
whole substance a white colour. 
In. a manner similar to this, stirrups, buckles, bridle* 
bits, and other articles, are tinned. 
Iron is usually tinned before, but copper always after 
it has been formed into utensils. The object to be at- 
tained by the tinning of copper is to prevent the vessels 
made of that metal from being corroded, and to preserve 
the food prepared in them from being mixed with any 
particles of that poisonous substance called verdigris, 
which is formed by such corrosion. In the tinning of 
copper vessels, their interior surface is first scraped very 
clean with an iron instrument, and then rubbed oveV 
with sal-ammoniac (207), for the purpose of more com- 
pletely cleansing them, and also of preventing the for- 
mation of verdigris from the copper during the opera- 
tion. The vessel is then heated, and a little pitch is 
thrown into it. While quite hot, a piece of tin is ap- 
plied to the copper, and this, instantly uniting with it, 
soon clothes the whole surface with tin. 
This metal, when amalgamated with mercury, is used 
for the silvering of looking-glasses (228) . When tin is 
K 5 
