The article usually called _tin_, or _tin─plate_, and, in Scotland, _white iron_, of which saucepans, boilers, drinking 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 attained 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 over with sal─ammoniac (207), for the purpose of more completely cleansing them, and also of preventing the formation of verdigris from the copper during the operation. The vessel is then heated, and a little pitch is thrown into it. While quite hot, a piece of tin is applied 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 melted in an open vessel, its surface is soon found to be covered with a grey powder, which is an oxide (21) of the metal, and is generally called _dross_. If the heat be continued, the colour of this powder becomes yellow. In this state it is known by the name of _tin─putty_, and is employed in polishing glass, steel, and other hard substances. When the heat is very violent, the metal takes fire, and is converted into a fine white oxide, which is used to render glass opaque, for the forming of enamel. Oxide of tin is also an important article to dyers. It is employed by them, in large quantities, to give brightness to such colours as are used in forming scarlets and other reds: and to precipitate the colouring matter of other dyes.