"Common salt is nearly always present in minute quantity in clays; but when these are taken from the sea-shore, or without or beneath the sea-washes, or from localities in and about the salt formations (Trias), they frequently, though in all other respects excellent clays, are unfit for burning into good bricks.

"Chloride of sodium 2 is not only a powerful flux when mixed even in very small proportion with clays, but it possesses the property of being volatilised by the heat of the brick kilns, and in that condition it carries with it in a volatile state various metallic compounds, as those of iron, which exist in all clays, and act as fluxes. The result is that bricks made of such clays warp, twist, and agglutinate together upon the surfaces long before they have been exposed to a sufficient and sufficiently prolonged heat to burn them to the core into good hard brick. Place bricks can be made of such clay, but nothing more, and these are nearly always bad, because never after free from hygro-metric moisture." 1

Oxide of Iron in clay influences the colour of the bricks to be produced (see p. 89). The tint resulting after burning depends upon the proportion of iron in the clay and the temperature to which it has been raised.

When in the presence of silica and alumina whose proportions are nearly equal, iron renders them fusible.