Varnishes are colourless transparent -glazes\ generally plumbiferous, which are applied to bricks and tiles to make them impervious and also to decorate them, either simply by bringing out their colour or by modifying it by placing the varnish over a dip.

Simple varnishing is done with alquifoux or minium, mixed or not with silica; in the first case the varnish borrows the silica it requires from the paste of the pottery. Here are two old formulae: -

Alquifoux .... 80 White sand .... 12 White clay .... 8

Minium..... 67

White sand .... 24 White clay .... 9

The varnish is laid on the products when unfired.

The process, which consists of sprinkling the products when still fresh with powdered fluxes, is not to be recommended on account of the dust which it causes and which is doubly dangerous both as being inert matter and as poison. It is advisable to use the immersion process. Generally only one side of the brick is glazed (end or side); sometimes, for angles in building, both.

The other non-plumbiferous glazes are scarcely ever used on account of their higher price; a saturated solution of sea-salt is, however, recommended, in which the products are dipped when quite dry; they are then again dried and fired.

This silico-alkaline glaze requires pottery rich in silica to obtain good adhesion.

The preceding glazes allow the colour of the products to be seen. If it is desired to hide this colour, the glaze must be placed over a dip; this renders the manufacture a more delicate operation, as has been explained in the general remarks on dips, for we must obtain a similar dilatation of the paste, dip, and glaze, if we are to get perfect products.

The glaze, when placed over a dip, is much more brilliant than when placed direct upon the paste, but it is more likely to crack. In fact, glazed bricks and tiles are rarely free from this fault Manufacturers do not take the trouble, as they ought, to accommodate the glaze to the paste of which the pieces are made.

It is true that, in the case of tiles, the disadvantages which would result from the water staying in the cracks of the glaze are avoided by giving a great slope to the roofs; this adds to the architectural effect, but necessitates a larger quantity of products which are in themselves already dear; therefore the consumption of tiles which are glazed over dips is necessarily limited.