When the hissing noise caused by escaping steam indicates that all the fluid has been removed from the boiler, the cock _d_ of the branch-pipe is closed, and through a rose fitted above the boiler, water is allowed to flow upon the latter. By this cooling the greater portion of the steam in the boiler is condensed and water may be admitted through _W_.
The operation of glue-boiling is then commenced anew by admitting steam into the steam coil, and continued until the cartilaginous mass has been reduced to about one-third of its original bulk, when the apparatus is opened, fresh material introduced, and the whole operation repeated.
In order to be able to replace the filter without having to empty the entire apparatus, it has been given the shape shown in Fig. 42. The upper portion of the discharge-pipe _A_ is connected with the lower portion by the box-screw _H_. In the latter is inserted a short cylinder, _C_, with a perforated bottom upon which is placed the filter-cloth; the latter is kept in position by the ring _R_.
For every apparatus two of the above-described filters will be required. If, notwithstanding a full steam pressure, the glue solution runs off sluggishly, it is indicative of the pores of the filter being choked up. The screw-box _H_ is then removed, the filter taken out and replaced by another one.
The glue solution discharged from the apparatus is in most cases sufficiently clear to allow of its being immediately evaporated. However, for the production of a particularly fine quality of glue, it is advisable to clarify the solution by settling. As the liquor has to be kept warm to allow of the solid particles to settle, W. Friedberg recommends the use of the apparatus shown in Fig. 43. It consists of an iron cylinder with a diameter equal to one-third of its height. The front of the cylinder is furnished with a number of cocks placed at equal distances from each other, and also with a pipe in the slightly conical bottom. It is surrounded by a wooden jacket, the intermediate space being filled with a bad conductor of heat. By this arrangement the liquor is kept warm and in a liquid state for several hours, giving ample time for the solid bodies held in suspension to settle on the bottom. The condition of the liquor is from time to time tested by allowing a small quantity of it to run into a glass from the lowest cock in front. If the sample is perfectly clear, the liquor may be drawn off. If, however, after several hours’ standing only the upper portions of the liquor are clear, while the lower ones are still turbid, further clarification by this means is impossible. The upper portions of the liquor are then used for finer qualities of glue and the lower ones for inferior grades.