The salt is either introduced direct through the furnaces or by orifices in the roof; about a kilogramme and a half of salt is required for each cubic metre of stacked products.
This quantity is introduced gradually, for the volatilisation of the salt, combined with the entrance of the cold air, considerably lowers the interior temperature of the kiln, and it must regain its maximum before salt is again introduced.
The walls of the kiln also become covered with glaze, and repeated operations at last completely disintegrate them; their durability may be increased by constructing them of materials of basic nature (alumina, limestone, magnesia). After salting, all the orifices of the kiln are closed, it is allowed to cool slowly, and then emptied.
The advantages of stoneware pipes over other pipes of cast-iron, cement, etc., are that they are unaffected by damp, completely impervious and smooth in surface, and proof against the acids and other corrosive matter contained in the sluice-waters.
Pipes of moderate thickness and with collar-joint will not support great pressure, and are therefore only used for drainage and evacuation of water. For conduits which have to bear considerable pressure, such as those for collection of spring-waters, thick pipes with socket-joint are used.
The first, which are much more extensively used than the second, are usually laid in trenches, with a minimum slope of one in fifty if there is no flush; it may be reduced in case of there being fairly frequent flushes, but it is always prudent to give the greatest slope permitted by the configuration of the ground.
The joints are made with cement which takes with moderate rapidity and is mixed with sand; cements should be avoided which swell and would burst the collars. Well-made joints are an essential condition of good canalisation.
On the other hand, it is not sufficient that the joints should be water-tight and well made, they must also be easily removed when, in consequence of rupture or any other cause, a pipe has to be replaced. These contradictory conditions are difficult to fulfil, and the problem of finding a good joint for stoneware conduits is always being studied. We give as a suggestion(Fig. 905) the plan of a system of drains formed of stoneware pipes.
Fig. 905. Plan of a System of Stoneware Drain-pipes.
Description. R. Inspection opeuing. R d C. Automatic flush reservoirs. S. Covering siphons. Note. - The arrows indicate the direction of flow.