For the old Sèvres soft porcelain, the frit was crushed, cleared of salts, and ground in water. The paste was then mixed with the other ingredients, as previously given in the table.
[Illustration: Fig. 21.--Vertical Pug-Mill, in use at Union Porcelain Works, Greenpoint.]
To prepare clay for making earthen-ware or stone-ware, machines are now generally used. That for the coarser kind of wares, such as bricks or common stone-ware jars, is a pug-mill (Figs. 21 and 22). The clay, having been brought by water to a certain workable consistency, is put into the mill. This is simply a cylindrical box, with blades projecting from the inside, and having in the centre a shaft also armed with blades. By the revolving of the shaft the clay is worked into a perfect pulp, and in that condition issues from a hole in the lower end of the mill. Should any hard substance have resisted the knives, it is removed by hand.
[Illustration: Fig. 22.--Horizontal Pug-Mill, in use at Union Porcelain Works, Greenpoint.]
For the finer kinds of earthen-ware, into the composition of which pipe-clay, kaolin, quartz, and felspar enter, the ingredients are mixed in a "blunger." This machine is not unlike a steam butter churn, there being a shaft passing from end to end, in exactly the same way, and armed with similar paddles. Water is added to the ingredients, and, as the blunger turns, these are all thoroughly mixed into a "slip," which is drawn off at the bottom. It is then strained and finally passed through a pug-mill, and is ready for use.
Piccolpasso, or the Cavaliere Cipriano Piccolpasso Durantino, who wrote in Italy, in 1548, gives very minute information regarding the processes of the potters of his time and country. The clay was either washed down by rivers or taken from pits. In the former case it was taken from the river-bed when the water was low, and was placed in holes in the ground, either after or without being dried in the sun. The object of keeping it was to allow all impurities to pass off. Where there were no rivers, a series of pits was dug in any convenient hollow, and connected by a channel. The earth was washed down by the rain into these pits, and purified by the passage from one to another. In some cases it was found necessary to place the earth on sieves exposed to the rain, through which the finer particles were washed into receivers placed below. Instead of using a pug-mill, the Italian potters put the earth upon a table, where it was beaten with an iron instrument, and thoroughly kneaded and cleaned by hand.