Numerous systems have been devised; attempts have been made to use the heat escaping from the kilns by bringing it into the rooms by special arrangements, and by using the draught of the chimney, and, later on, of a ventilator. But the handling of the bricks was a rather serious obstacle, besides the difficulty of bringing the hot air into the chambers, and the inconveniences caused to the working of the kiln.

Some have thought of using stoves warmed by a special furnace, and actively ventilated; and, to lessen the labour required, the products have been placed upon waggons and run into heated galleries, called tunnel drying-rooms. These are much used in the United States, where the manufacturing conditions are somewhat different from our own. The principle of these drying-tunnels is as follows: -

The products to be dried are introduced into a closed gallery, and automatically pushed forward, while at the same time a current of air is produced in the opposite direction; in this way the bricks meet more and more dry air as they advance, and finally arrive at the end of the gallery perfectly dry. In order to carry out this principle, the gallery contains a line of rails on which waggons with shelves, as in Fig. 186, run.

At the entrance of the gallery is a room with doors; through one door the products coming from the machines enter, the door is closed, and another leading into the tunnel is opened (Fig. 178). This tunnel is provided with a slow ventilation either by a ventilator or by draught from the factory chimney; but generally the latter is insufficient, and a ventilator is required. At the other end of the tunnel is an opening communicating with the outer air. In summer this air suffices for the drying, but in winter it is mixed with hot air given out by the cooling of baked products, or by any other economical method. The dry air from outside only acts upon objects containing very little moisture; it has no bad effects upon them, and completes their desiccation. As the air advances through the tunnel it becomes more and more loaded with the moisture receives from the bricks, and when it reaches the waggons last introduced, it is almost saturated : it therefore only takes from the fresh products the little moisture required to completely saturate it before being expelled. The quantity of air and its temperature must, for this reason, be carefully regulated as it enters.

When one waggon goes in, another comes out. Thus the action is continuous, and a few days are sufficient, at all times of the year, to effect drying. Besides, it will be seen from these arrangements that the bricks, when once placed on the waggon, are not handled again until put in the kilns. We have just stated the advantages of this system of drying; we must now mention its inconveniences.

Fig. 178. Tunnel Drying-room (Lacroix).

Of these, the most serious is, in our opinion, that it only applies to one kind of product. It will be understood, in fact, that in the kilns it is necessary to mix products in order to utilise space and also to satisfy requirements of consumers. But manufacture cannot exactly follow these different phases; with ordinary drying-rooms there is no inconvenience in this, since each kind of article has its own drying-place, whence it is taken as required to the kiln. But with drying galleries, it is no longer the same; in fact, the products must follow one another in the order in which they are to be placed in the kiln, otherwise they will have to be handled again, and the benefit of economy in labour disappears, or else stock will have to be increased and a place reserved for loaded waggons which are not at once kilned. Another inconvenience, but one more easily remedied, consists in the behaviour of clays while drying. There are some which cannot bear the draught without splitting; others resist the current of air well, but crack under the influence of hot air. It is evident that these difficulties may be overcome by great precautions and a well-regulated current, but it must not be forgotten that, to attain results economically, the drying must be done in two days in order to reduce the stock of waggons. But all clays cannot bear such rapid desiccation, hence only experience can guide us.

Fig. 179. Closed Galleried Drying-rooms (Chambers).

To summarise, galleried drying-places are of great use in case of extensive manufacture of one single article with clay which bears rapid desiccation. That is why these drying-places are much used in the United States, where enormous quantities of the same article are made in one factory. Moreover, by the arrangement adopted (Fig. 179), different products may be placed in the different galleries; but then the cost of installation is very high, and such an expense cannot be thought of for an output which does not exceed several millions, like the majority of French factories whose mean production is two to three millions and maximum about fifteen millions of one single article.