Even with the most careful grinding, chalk cannot be transformed into such a fine powder that is directly fit for all purposes; and the only way to obtain the requisite fineness is by levigation. Owing to the large quantities that are usually handled in this process, the milky liquid coming from the mill is mostly run into large brick tanks, where it is left to settle until all the chalk has deposited and the supernatant water is perfectly clear. Tapping-off being usually impracticable, the water is generally drawn off by careful syphoning, so as not to disturb the fine sludge at the bottom of the tank. The deposit in the settling-tanks is shovelled into wooden boxes, perforated at the sides to enable the water to drain away, the chalk being prevented from escaping by lining the boxes with linen cloths. The pulp soon loses its liquid character and shrinks considerably, the boxes being then filled up with more sludge, and so on until the contents have ceased to shrink. When the mass is so far dry that it will no longer run when lifted, the boxes are covered with boards and inverted, discharging the contents on to the boards, on which the mass is left to become quite dry. Filter-presses are also used. Large prismatic masses of chalk never dry so uniformly as to prevent the formation of cracks, and if the chalk is to be sold in this form the cracks are plastered up with thick pulp; this operation, however, being superfluous when the chalk is to be sold as powder. In order to obtain a more compact product and accelerate the drying of the moulded lumps, some makers use presses, in which the fairly dry chalk is subjected to progressive heavy pressure. Owing to the fineness of the component particles of chalk, they adhere so firmly together, without any bind, that a fair amount of force is necessary to break down a piece of perfectly dry levigated chalk. Sometimes, however, chalk exhibits the unpleasant property of losing its cohesion almost completely when dry, and in such cases it can only be shaped into prisms with great trouble. This peculiarity is specially accentuated when the chalk contains magnesia; and in order to mould chalk of this kind into blocks, a binding agent, such as ordinary glue, must be added to the water used in grinding, care being taken not to use too much, or the chalk will become too hard, when dry, for certain purposes, _e. g._ as drawing or writing chalk. For some purposes, chalk is sold in powder form, and very high purity is not then essential, an admixture of magnesia or clay being harmless. Gilders, for instance, use large quantities of chalk for priming picture frames, and stir the chalk up with a certain amount of bind (mostly size), to give the particles the desired cohesion.