The production of porcelain for electrical insulation — IV

The three previous articles have been devoted to giving a brief description of the theoretical points involved in the development of porcelain. The following articles will briefly describe the actual manufacturing methods and the application of the theoretical points. Recent developments in plant management and control have been applied to porcelain manufacture in the same manner as they have to other industries, and the old rule of thumb and trade secret methods are fast disappearing. Ample storage room for raw materials is necessary for a uniformly high grade product can only be made when there is ample time to test the raw materials prior to using them, and to permit of the rejection of inferior materials when necessary without causing any delays in production. Accurate methods of weighing the various raw materials to form the final raw body are necessary. Care must be used in determining the per cent of moisture in each raw material so as to compound the body on a dry weight basis. The old method of preparing the body by mixing or blunging, with the idea of merely suspending the raw materials in water without regard for grain size is to be avoided. Many potters are now grinding the materials in pebble mills and working to definite grain sizes. The ground body is passed over a magnetic separator to remove magnetic iron, then passed through a fine mesh (120 meshes to the lineal inch) lawn and filter pressed to eliminate excess water, and thus put the ingredients in a workable condition (approximately 22 per cent water). The materials for porcelain can also be stored easily while in this condition. By storage the plasticity is not only increased, but time is available in which to run actual tests on the materials before releasing them for production. The moisture content in filter press cakes is not uniform throughout the cross-section of the cakes. A greater degree of homogeneity is produced by pugging the body after filter pressing. A pug mill develops uniformit- to a high degree when properly handled. Several methods of shaping the body into final form require different methods of preparation. Each of these will be described in later issues.