Fig. 24.
The size of the drying-room should be proportional to the daily production. Constructions are fitted up with the requisite frames for the reception of the glue cakes, and are heated by steam pipes arranged along the walls. In the floor in the immediate neighborhood of the steam pipes are openings, which can be opened and shut at pleasure, for the admission of fresh dry air. The latter on entering the room is heated, and after passing over the frames and absorbing water from the glue cakes, escapes through openings in the ceiling to a space above it from which it is withdrawn by means of ventilators in the roof. A constant change of air must be kept up. The quick drying of the glue is of the utmost importance, as otherwise the jelly putrefies either entirely or partially, and the glue acquires a turbid and mean appearance. Too much heat causes the cakes to bend and crack. The cakes are laid upon widemeshed nets of twine stretched in frames 6½ to 8 feet long and 3¼ feet wide. Fig. 24 represents the form of nets commonly used. The nets are placed upon frames, such as shown in Fig. 25, arranged around the drying-room in the neighborhood of the steam pipes and air flues. As the cakes have to be occasionally turned upside down upon the nets, the latter must be placed at convenient distances, one above the other in the frames.
Fig. 25.
The use of twine netting has been found to be attended with many disadvantages, the principal ones of which are given by S. Rideal as follows:
1. “Being freely handled in the making, the netting is almost always impregnated with dangerous organisms which penetrate the moist glue cakes, and cause moulding or putrefaction. When this occurs, it is usually attributed to a state of the atmosphere, but if the cakes are examined, the alteration will generally be found to originate along the lines made by the netting. The fault could be cured by sterilizing the net for an hour at 212° to 248° F. in a hot oven, but besides the expense, the fibre is thereby weakened. Moreover, the spores of a few bacteria, such as Bacillus subtilis, which is widely distributed and has the power of liquefying gelatine, will bear a heat of 248° F. for over an hour, and still be capable of growing.
2. “However smooth the fibre, the glue will stick in places, leaving small remains, which being hygroscopic, become ’sour,’ and set up the objectionable bacterial changes in the subsequent batches.
3. “Owing to sagging, rotting, scouring, or wearing into holes, the life of cotton or hemp netting is so short that the constant renewal is a considerable item. A whole batch is frequently spoilt by the fault of a net. In some works, heaps of old netting are found, which become very putrid in the rain and sun, and give rise to mysterious bacterial inroads in the factory. In others they are regularly burnt under the boilers.
4. “The considerable overlap or selvedge required for securing the edges of the net involves a waste of the area, and also some difficulty in refixing.”