The enamelling of bricks and tiles, like glazing, is intended to render them impervious and to produce a decorative effect by means of colouring. Enamelled bricks are really faiences, and as in the case of these latter, the enamel and the paste must be in harmony to avoid cracking; but besides this, the adhesion between the two bodies must be such that the bricks may be able to resist frost, damp, the different degrees of dilatation of summer and winter, etc. - in a word, all variations of environment.

It will be understood that these conditions are not satisfied in common - pottery, and no means have yet been found of applying to ordinary bricks an enamel which fulfils the conditions of solidity mentioned above.

It was in consequence of accidents which have happened to enamelled bricks used in the construction of subterranean arched passages, that attention was called to their powers of resistance to inclemencies of the weather.

For instance, under certain conditions of damp this resistance is not so great as that of ordinary bricks, and in these special applications it will be well to make arrangements for putting enamelled bricks in the best possible state to resist inclemencies.

In order to avoid such inconveniences, manufacturers have still further improved their products; for instance, Muller applies enamel to bricks made of stoneware firing clay.

Loebnitz uses faience paste moulded by hand. The enamel adheres firmly to such paste, becomes a part of the piece, and thus satisfies the required conditions of solidity.

Engineers have gone a step farther, and have had recourse to porcelain bricks. An important use of these products was made in constructing the tunnels of the extension of the Sceaux line to Luxembourg. Time will show the value of these impervious products under such conditions; in any case, they cause a very heavy expense.

Without going as far as this, it is evident that the examples left behind them by the ancients prove that enamelled products successfully resist the lapse of time when they are well made and used with certain precautions.

The moulding of bricks which are to be enamelled is similar to that of ordinary bricks; nevertheless hand-moulding seems to be preferable, perhaps on account of the greater porosity of the resulting products.

The enamel holds less well to machine-expressed bricks, or even to those simply stamped - stoneware bricks excepted.

Transparent enamels can only be used directly in certain cases, for, bricks being always coloured, a contrast of tints is produced. With orange or red pastes, we can obtain brown with an enamel containing about 5 per cent, of oxide of manganese, and black if oxides of iron and cobalt are also added; all other colours require a dip.

Yellowish pastes will, with manganese and iron enamels, give red or brown colourings, with those of copper green, and with those of cobalt blue. But white and light colourings can only be obtained with dips or opaque enamels, unless we work upon white pastes like those of fine stoneware or felspar faience, which form exceptions.

The composition of some enamels is given below -

Blue.

Brown.

Yellow.

Black.

Green.

Alquifoux .

81

• • •

78

60

78

63

77

...

76

63

Minium

• • •

66

3

• • •

3

..

58

• •

• . •

White sand.

12

24

10

• •

11

23

8

• •

12

23

White clay .

6

9

6

• •

8

9

6

• • •

6

9

Vegetable mould.

• • •

• •

• • •

33

• •

...

32

• •

• •

Sulphate of iron .

• •

• • •

..

5

• • •

5

• •

5

• • •

..

,, copper

• • •

• •

• • •

• •

• • •

..

3

3

6

5

Oxide of manganese .

• •

• •

3

2

• •

• • •

3

2

• •

• •

,, iron

• •

• •

...

• •

• • •

• •

3

..•

.. •

..

„ cobalt .

0.3

0.3

• • •

• •

• •

• • •

• •

0.04

• • •

• • •