Iridescence can also be formed by re-heating crystal glassware within a chamber in which salts of tin, barium, aluminium, and strontium are volatilised. This method produces a superficial iridescence which is not quite so permanent as the previous process. =Glass Silvering.= The silvering of mirrors is carried out by taking a thoroughly cleaned plate of polished glass and floating one surface in a solution of silver nitrate, to which a reducing agent is added. The silver is thereby precipitated or deposited in a thin lustrous film upon the glass, which causes reflection by the rays of light striking against the silvered background. After silvering, the back of the plate is coated with a protecting paint or varnish, which dries and preserves the silver deposit and gives it permanency. In the manufacture of fancy ornaments, such as birds, hat pins, and small animals, various coloured glass cane and tube are worked together by the operator melting and welding the respective colours together before a blow-pipe flame, the tails of the birds being formed by sealing in a fan of spun glass into the body of the bird, which has been blown out and formed from a piece of tube. Some very curious ornaments are formed in this way. Glass buttons, pearl, and bead ornaments are formed by working cane and tube of various coloured compositions before the blow-pipe, sticking and shaping the various forms on to wire. Mosaic glass decoration is used in jewellery in a mural or tessellated form. In this method small cubical or other shaped cuttings of various coloured opaque glass are inlaid in mastic cements or pastes to form the design, the face being afterwards ground and polished smooth, and mounted or set within the ornament. Larger cuttings may be inlaid in cement for pavement or mural decoration. ------------------------------------------------------------------------ CHAPTER XVI ENGLISH AND FOREIGN METHODS OF GLASS MANUFACTURE COMPARED The continental methods of glassmaking differ so much from the English methods that a few remarks giving comparisons will be of interest. It is noticeable that chemical and engineering science is more thoroughly applied in the manufacture of glassware abroad. Their method of specialising wherever possible, and the introduction of mechanical and automatic machines have done much toward increasing their production and efficiency. The flourishing and extensive state of glassmaking abroad is shown by the size and extent of the glass works, some of which work as many as forty or fifty furnaces and employ 3,000 to 5,000 hands. Gas-fired regenerative or recuperative furnaces are more generally used, which permit higher temperatures, cheaper metal, and greater economy in fuel and labour.