The _shrinkage_ which all clays undergo on drying and when heated is another important characteristic. It is due to the fact that as water is removed the solid particles approach closer to each other, the volume of the whole mass being thereby reduced. In a wet piece of clay each particle is surrounded by a film of water, the thickness of which depends on the nature of the clay. As this water evaporates from the surface of the clay its place is taken by water from the interior which rises to the surface by capillary attraction. So long as there is any water between the particles of clay there will be shrinkage when this water is removed, but a stage is eventually reached when the particles of clay are in contact with each other and no more shrinkage can occur. That this cessation of shrinkage may take place before all the water has been removed from the clay is easily understood when it is remembered that whilst the clay particles may be in contact, yet there are still places (pores) where the contact is incomplete, and in these pores water may be retained. The amount of shrinkage clays undergo on drying depends partly on the proportion of water added to them and partly on the sizes of the different particles of clay, sand, etc. present. An average reduction in volume of 12 to 38 per cent. may be regarded as normal, but coarse loams may shrink only 1 per cent. and very finely ground, highly plastic ball clays may shrink as much as 50 per cent., though this is unusual. As all coagulated colloids, which have absorbed water, shrink on drying, this behaviour of clay appears to confirm the view as to its partially colloidal nature held by some investigators. When a piece of dry clay is heated sufficiently a further shrinkage (technically known as _kiln shrinkage_) occurs. This begins somewhat below a red heat and increases in rough proportion to the temperature and the duration of the heating. Prolonged heating at a lower temperature will effect the same amount of shrinkage as a short exposure to a higher temperature, but though the greater part of the shrinkage occurs in a comparatively short time, continued heating will be accompanied by a further reduction in volume. This is due to the fact that clays have no definite melting point, but undergo partial fusion at all temperatures above 950° C. or, in some cases, at even lower ones. As a portion of the material fuses, it fills up the pores in the mass and attacks the unfused material, this process being continued until either the heating is stopped or the whole material is reduced to a viscous slag.