The extent to which a given clay will vitrify depends on the amount of fluxing material (metallic compounds, and oxides other than ferric oxide and alumina) it contains, on the smallness of its particles and on the duration and intensity of the heating. Clays containing alkalies and lime compounds vitrify with great rapidity when once the necessary temperature has been reached, so that unless great care is exercised the action will proceed too far and the goods will be warped and twisted or may even form a rough slag. Refractory clays, on the contrary, vitrify more slowly and at much higher temperatures so that accidental overheatings of them are far less common. The difference between the temperature at which sintering or vitrification occurs and that at which the clay melts completely--usually termed the 'vitrification range'--varies with the nature of the clay. In some cases the clay melts as soon as vitrification becomes noticeable, in others the vitrification occurs at a dull red heat, but the material does not lose its shape until after a prolonged heating at the highest temperature of a firebrick kiln or testing furnace. Calcareous clays have the melting and sintering points close together, so that it is almost impossible to produce vitrified and impervious ware from them, as they lose their shape too readily. If, however, the difference between the sintering and fusing temperatures can be enlarged--that is, if the vitrification range can be extended--more impervious ware can be made. The easiest means of extending the vitrification range consists in regulating the proportion of large and small particles. The former increase and the latter diminish the range. Basic compounds and fluxes cause a lowering of the melting-point and a shortening of the vitrification range. The _porosity_ of raw clay is usually of small importance, but the porosity of fired clay or ware is often a serious factor in determining the suitability of certain articles for their intended purposes. In its natural state, clay does not readily absorb much water; on the contrary it becomes pasty and impervious unless it is disturbed and its texture destroyed, when it may be mixed with water to form a paste or, with more water, a thin 'cream' or 'slurry.' When heated moderately, clay forms a porous material and, unless the heating is excessive, it will absorb about one-eighth of its weight of water. Further heating at a higher temperature reduces its porosity--the more easily fused material filling some of the pores--until a stage is reached when the material is completely vitrified and is no longer porous.