Fig. 13.
The mode of working with this apparatus is very simple. Water being admitted into the pan through L, the glue-stock is introduced and the mass brought to boiling by admitting steam. The finished glue-liquor is from time to time drawn off through the pipe F into the settling vessel.
It is generally preferred not to concentrate the glue-liquor in the pans to such a degree as required to obtain a jelly, which after cooling, can be immediately moulded, experience having shown that less concentrated liquors can be more readily and better clarified, and yield a lighter and more transparent glue.
Mr. Thomas Lambert gives the following process of cooking: The skins are taken to the glue-boiler, which is an open vessel, 8 feet in diameter at the top and 7 feet deep, and provided with a perforated false bottom, through the center of which passes a two-inch pipe, one end dipping below a layer of water at the bottom, the other projecting about half the height of the boiler, this part being covered with a perforated hood to spray the liquor through the mass. The skins are placed on the false bottom and the added water at the bottom of the boiler is brought to the boil by means of a steam pipe. The steam not being able to escape quickly through the dense mass of glue-stock above, exerts a pressure on the water, forces it through the pipe, to be sprayed through the mass, and ultimately works its way to the bottom of the boiler to be forced up again. This continual circulation of the hot liquor rapidly dissolves the gelatinous matter, and when a strength of 18 per cent. dry glue is reached, the first run is made to the evaporating pan, the liquor passing through a filter of fine shavings, to remove any suspended matter. Fresh water is added to the boiler, and the boiling renewed. Three extractions are usually made, the last being used for size.
In order to avoid annoyance to the neighborhood from foul odors, Terne’s glue-boiler shown in Fig. 14 may be recommended. The lead-lined iron boiler A, with manholes B and C, on top and side, is furnished with a false perforated bottom upon which the glue-stock is placed. Underneath the false bottom lies the coil E with valve-box e. The boiler is filled through the upper manhole with glue-stock and water admitted, steam being at the same time introduced in the coil and in order to quickly heat the water, direct steam is also admitted to the boiler through the pipe F and cock G. When the water is boiling the cocks G and F are closed, the coil furnishing now sufficient heat. During boiling some steam is allowed to escape through the partly-opened cock L, all badly-smelling gases being thereby carried to the fire-box of a boiler where they are burned. When boiling is finished, the glue liquor remains for a short time in the boiler to allow the melted fat to separate on the surface, the cocks K1 and K5 and serving for drawing off the fat. The insoluble residues of the glue-stock remain upon the false bottom and are taken out through the manhole C.
Fig. 14.
2. CLARIFYING THE GLUE-LIQUOR.
The clearness of glue, i. e., its freedom from undissolved substances, is by no means a criterion of its value as an agglutinant, since pulverulent inorganic substances (white lead) are frequently intentionally introduced into some varieties, for instance into Russian glue, without injury to their adhesive power. But as a turbid appearance may also be an indication of unsoundness and decomposition, the manufacturer endeavors by all means to obtain a clear product.