For this reason metallic netting has been largely adopted. The best material has proved to be a heavily galvanized iron-wire netting having no less than 15 to 25 per cent. of its weight of zinc. It can be strengthened by longitudinal and transverse wires or ribs. It must be examined by the microscope to see that it is perfectly free from holes or cracks, and should last at least two years in constant use.
The temperature of the drying-room requires careful regulation, and should never be allowed to rise above 68° to 77° F., as otherwise the glue would soften and run through the meshes of the net, or adhere so firmly to the twine as to require the nets to be put in hot water for its separation. Dryness of air is of far greater importance in the drying process than a high temperature. To promote this dryness of air and prevent the aqueous vapor from condensing, evaporating, and again condensing upon the cold walls of the room, they are wainscoted. Thus protected by a bad conductor, they acquire a higher temperature, and the aqueous vapor, instead of being precipitated upon them, is carried off by the air-currents.
As the cakes placed in the immediate neighborhood of the steam pipes and near the floor where the dry air enters, dry quickest, the nets containing them are shifted after some time to a higher part of the drying-room and their former places filled with cakes still wet. When the cakes are dry, they are finally desiccated in a room at a higher temperature, which serves to harden and improve them.
In modern times drying-rooms have been almost entirely abandoned and in this country long drying galleries are used, sometimes 250 feet in length and 6 to 8 feet square, with traveling platforms on rails carrying the sheets of glue on stout galvanized netting. Wood is found to be a better material for the galleries than stone or brick.
Figs. 26 to 28 show an apparatus for drying glue which is the invention of W. A. Hoeveller.
Fig. 26 is a plan section, and Fig. 27 a side elevation in section, of this improved drying-alley. Fig. 28 is an end view in section.
The form and arrangement are as follows:—
A B represent the two parts of the alley, separated by the partition C, which is shorter than the alley, so as to leave a communicating space at both ends.
At the front of section A, is located a blower, D, actuated by a steam-engine or other motor, E, also located within the walls of the alley. The whole current from blower D, is directed through section A of the alley, whence it turns into section B, and comes back through it, to be again drawn into and forced out of the blower into section A. By this means the contained air of the alley is set in continuous motion through the two sections successively, and as the structure is made as air-tight as practicable in such cases, the air remains unchanged until the doors F, or either of them, are opened to discharge the vitiated air and let in the fresh.
In sections A and B, is placed the railway a a, to admit of the convenient movement of the contents in process of drying, which are generally set on cars or buggies.