The Use of Ground Material to Produce Pottery
Around 1720 Thomas Astbury of Shelton, Stoke-on-Trent introduced calcined and ground flint into the glaze and body of pottery to increase its strength and whiteness. This was originally done by pounding the material in mortars and passing it through fine hair sieves. This ‘dry’ process was a major health hazard to the workers and they contracted silicosis which reduced their life span to a few years. In 1726 Thomas Benson of Newcastle, Staffordshire patented a wet grinding process for flint and this, with improvements introduced by James Brindley in about 1757, was adopted by the potters as a safer process and specialist mills were established. In 1748 Thomas Frye at the Bow Porcelain factory in East London used 50% calcined and ground bone with 25% flint and 25% kaolin clay to produce a product similar to porcelain. It was named English Bone China. Between 1789 and 1793 Josiah Spode developed the process at his Stoke manufactory.
The Grinding Process
Material to be ground was hoisted through a hatchway in the floor from the Gear Room below. The slack chain hoist was driven from the extended vertical shaft of the end pan. The material was tipped into one of ten pans and water added. The pan floors are composed of chert blocks with the gaps filled with ‘pitcher’ (broken biscuit ware). Power from the floor below rotates sweep arms which push large chert blocks or ‘runners’ around the pan which tumbles the material and grinds it to a suspension of fine particles. The larger diameter pans contain runners of up to 1 ton (1.02 tonnes) in weight and a hand crane (see at the far end of the room) was used to lift these into the pan. After about 8 hours for flint, less for bone, the material could be run out of the pan to the floor below for further processing.
Pan to Ball Milling
For the production of fine pottery, especially bone china, it is very important to have high quality and consistent raw materials. Today instruments are available to measure material properties such as the viscosity and particle size within the suspension but it is believed that in Victorian times it was done by feeling the material between the fingers, lips and using the tongue. In the late 1800s the process of ball milling was developed in Germany for other industries. In this process the material with water and a hard grinding media, such as flints (not calcined) or ceramic balls, are introduced into a sealed cylinder which is then rotated at speed. This is a more efficient and cleaner process but some potters preferred using material produced by the pan process. The first ball mill to be used in Stoke-on-Trent, at Goodwin’s mill in Hanley, is displayed by the visitor centre.