The soot collects on the walls of the chambers in flocks, which finally become so heavy that they fall off and collect on the floor. The entry to the soot chambers should be through a single iron door, tightly closed whilst the chambers are in use, and cemented round. If this door does not fit air-tight the combustion cannot be exactly regulated by the damper on the chimney. The soot is removed from the chambers by a workman, who sweeps off with a soft brush into a sheet-iron vessel the soot adhering to the walls and lying on the ground. It is extremely important that soot and nothing else should be collected; the brush used for loosening the soot should be so soft that it does not rub off mortar from the brickwork; the workman’s shoes should be provided with felt soles, so that no particles are loosened from the floor of the chamber and mixed with the soot. The admixture of the smallest quantity of sand would be extremely harmful in the ensuing grinding of the soot; the mills would be damaged. The flues for collecting the soot in well arranged works are very similar, but the apparatus used for burning the materials which produce the soot varies greatly according to the nature of the material. Some quantity of the soot black used in the arts is still made by burning pine roots and chips, but for the finer qualities American rosin is largely employed. Earth-wax or ozokerite and the hydrocarbons obtained from petroleum and in the distillation of shale are materials frequently used in making soot blacks of very good quality. For the finest qualities, such as are used for fine printing inks, copper-plate inks and black lacquers, soot obtained by burning fish oils or cheap fatty oils is most commonly used. The great differences in the physical nature of these materials demand the use of different apparatus for their combustion. The space allotted to soot blacks in this work would be far exceeded if the construction of all the forms of apparatus were described; the most important only will be given. [Illustration: FIG. 30.]