The third necessary property is commonly known as vitrification but could be better named "densification" because complete vitrification is not attained in ordinary clay wares. This property may be defined as that which causes a clay to yield to the action of a high temperature so that the result is a ware, more or less dense, which is hard, durable and sonorous. With this there must be coupled a certain amount of resistance to heat treatment so that the pottery does not fuse or collapse during the firing. Here also is found the need for adjustment. The clay must yield to the fire but not completely. It must resist but not entirely.
Plasticity is due to the clay base. Not only to its quantity but to its quality also. Some forms of clay in which clay base predominates are not plastic because the clay base itself is coarse grained. Other forms with less clay base present are plastic because this ingredient is fine grained and tough. Pure clay base is also highly resistant to fire and therefore contributes to the refractoriness of the mass.
Porosity is caused by the sand in the clay. Any kind of sand will produce porosity but the effect differs with the condition of the sand. Coarse sand is more effective than fine sand. More sand will, of course, cause greater porosity.
Vitrification or densification is due to the feldspar or fusible sand. This also varies with the condition. A fine-grained feldspar will produce vitrification more easily than the same amount of coarse feldspar.
Certain substances are available for use in pottery mixtures, which possess one or other of the necessary properties in high degree so that they will impart these properties to a mass to which they are added.
Kaolin or china clay is usually fine, white, and refractory. Some kaolins are rather plastic but most of them are "short" in working and rather tender. For the production of a white ware kaolin is indispensable. No other ingredient will afford the pure white color which is sought after in porcelain and china.
Ball clay is very plastic, easily vitrified, but is not white. The color varies from a cream to a gray. The use of a ball clay is therefore limited in white wares because it will spoil the color. For wares in which a light cream color is not objectionable ball clays are valuable and almost indispensable.