As is always the case with regard to pottery, these articles had been manufactured and used in England long before we decided to adopt them. All visitors to England are struck with the way in which our neighbours have profited by these applications of pottery to the hygiene and health of dwellings.
As before, we shall study these different products according to their forms and uses.
For a long time these pipes were made exclusively in England, and the products which the firm of Doulton sent all over the world from that country were so perfect that it seemed impossible to imitate them. The fact is, that besides physical appearance, shape, and glaze, another quality is indispensable for pipes - elasticity; without it the pipes are brittle, and will crack in the ground merely in consequence of the vibrations of traffic or other causes.
It is only in the last ten years that this manufacture has been introduced into France. In spite of technical and economic difficulties, which it has surmounted, the industry has developed to such an extent that the consumption of English products has been considerably reduced.
The principal factories in France are at Pouilly-sur-Sa6ne (Jacob et Cie.), at Boulogne (Societe' des produits ceamiques), at Rambervilliers, at Breteuil-sur-Iton (Rousseau et Cie.), etc.
Others of less importance are scattered through the North of France. Belgium and Germany also make many pipes.
They are made exactly like pipes of ordinary clay, but from special pastes which fire into stoneware. The proportions of the different clays used in these pastes depend upon the clays available; it is different in each factory, since the materials employed are variable.
The paste usually consists of a refractory clay mixed with felspar or pegmatite, and sometimes, with a view to economy, other commoner clays. Verifiable clays are also used mixed with antiplastic substances (pulverised fragments of pipes or saggers).
The more refractory the clay is, the less likely is the pipe to lose shape in firing; but then there is no vitrification, and, as in the case of English pipes, the pieces retain a certain degree of porosity. This slight porosity has no effect on the quality of the products.
These special shapes are very numerous (Figs. 883 to 903); in addition to single (Fig. 884), double (885, 886), and treble (Fig. 887) branched pipes, conical (Fig. 888) pipes are also made for connecting two conduits of different diameter, also opercular pipes (Fig. 890), the upper parts of which are removable so as to allow of cleaning the interior. The drainage pipes (Fig. 893) and drains (Fig. 902) for carrying away flood-gate water do not present any difficulties. The siphons (Figs. 897 to 901) are moulded in two parts which are afterwards welded together.
The firing of stoneware pipes takes place at a high temperature in round reverberatory kilns (Fig. 206), or semi-continuous kilns, or in continuous kilns with several firing chambers (Fig. 231).
The semi-continuous kilns are of rectangular shape and are composed of a series of compartments occupying the whole breadth of the kiln and separated by walls. Communication is made by means of openings in the floors of the chambers. The furnaces, four in number, are placed in the angles of each compartment; work is carried on inside the kiln, through two doors or gaps situated opposite to one another.
Figs. 883 to 903. Stoneware Pipes of Different Shapes.
A "camp" of bricks is laid upon the floor of the kiln, and upon this the pipes are placed vertically, some inside others. The socket of the large pipes is protected by a cover which surrounds it and serves, so to speak, as a sagger. The space between the pipes should be large enough to allow the gases to circulate freely, thus ensuring a uniform glaze when salting takes place.
The fire is lighted at one end of the kiln, and the gases pass through several full compartments before reaching the chimney. The firing is effected at a high temperature and ends with the salting. This operation, which was used for the first time at the end of the 17th century by the English brothers Flers, consists of throwing sea-salt into the kiln when the maximum temperature of firing is reached. This temperature varies from 1300° to 1500° C. according to the nature of the pastes; upon contact with the silica of the pottery the salt (NaCl) splits up into chlorine and sodium, which latter combines with the silica, forming a complex fusible silicate; this is the glaze. The freed chlorine decomposes the water-vapour present in the products of, combustion, and escapes into the air in the form of hydrochloric acid (HC1).
This explanation is in accordance with our knowledge of chemical reactions, but it is not sufficient to account for the facts observed by Salvetat, namely -
1. That dull stonewares, not salt-glazed, contain less silica (10 per cent, at least) than the glazed stonewares; 2. that sea-salt glazing requires that there should be an excess of silica in the paste; 3. that the glazing of the stonewares scarcely increases at all the quantity of alkali which they naturally contain.