e glaze melts to a transparent glassy coating upon the
surface of the vessel. In some cases fusible mixtures of quite
different composition from that used in fashioning the vessel
may be used as a glaze. Oxides of lead, zinc, and barium are
often used in this way.

When less carefully selected materials are used, or quite thick
vessels are made, various grades of stoneware are produced. The
inferior grades are glazed by throwing a quantity of common
salt into the kiln towards the end of the first firing. In the
form of vapor the salt attacks the surface of the baked ware
and forms an easily fusible sodium silicate upon it, which
constitutes a glaze.

Vitrified bricks, made from clay or ground shale, are burned
until the materials begin to fuse superficially, forming their
own glaze. Other forms of brick and tile are not glazed at all,
but are left porous. The red color of ordinary brick and
earthenware is due to an oxide of iron formed in the burning
process.

The decorations upon china are sometimes painted upon the baked
ware and then glazed over, and sometimes painted upon the glaze
and burned in by a third firing. Care must be taken to use such
pigments as are not affected by a high heat and do not react
chemically with the constituents of the baked ware or the
glaze.

EXERCISES

1. What metals and compounds studied are prepared by electrolysis?

2. Write the equation for the reaction between aluminium and
hydrochloric acid; between aluminium and sulphuric acid (in two steps).

3. What hydroxides other than aluminium hydroxide have both acid and
basic properties?

4. Write equations showing the methods used for preparing aluminium
hydroxide and sulphate.

5. Write the general formula of an alum, representing an atom of an
alkali metal by X and an atom of a trivalent metal by Y.

6. What is meant by the term polysilicic acid, as used in the discussion
of aluminium silicates?

7. Compare the properties of the hydroxides of the different groups of
metals so far studied.

8. In what respects does aluminium oxide differ from calcium oxide in
properties?

9. Supposing bauxite to be 90% aluminium hydroxide, what weight of it is
necessary for the preparation of 100 kg. of aluminium?

CHAPTER XXVII

THE IRON FAMILY

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