PREPARATION OF CLAYS:
Artisan producers purchase clay in lump, powder or plastic form. Wet clays are wedged, meaning the lumps are banged together to eliminate air and to even the texture, or they’re kneaded, which consistently brings all of the clay to the surface to be compressed and assure all air is eliminated.
Mass-producers of dinnerware purchase clays in a dry form. Each factory develops their own formulae but generally, all coarse materials are first ground very fine in a ball mill and blended with water in a blunger to control plasticity, grain size and moisture.
The result is SLIP, a clay and water suspension used for molding hollow shapes. Slip is passed over electromagnets to remove particles of iron and other impurities.
ARTISAN SHAPING OF CLAY:
PINCHED: PLASTIC CLAY is pinched into a shape.
COILED: PLASTIC CLAY is rolled into coils and joined to create a shape.
SLAB: PLASTIC CLAY is rolled into slabs of uniform thickness, cut into shapes that are ‘stuck’ together using a glue-like slip.
MOLDED: LIQUID SLIP is poured into a plaster-of-paris mold. Water in the clay is slowly absorbed by the plaster. When a clay wall of specified thickness develops, excess slip is poured out. The remaining clay wall shrinks from the mold as it dries.
THROWN: A BALL OF CLAY is thrown and centered on a revolving potter’s wheel. Centrifugal force and water assist the hands of the potter who simultaneously applies pressure on the outside and inside of the whirling clay to control shape and height…..and a pot is born. Throwing is traced to the Middle and Far East as early as 3,000 B.C.
Uniform thickness is critical for every shaping technique in order to assure successful firing. Uneven clay bodies crack or explode when fired. Artisan dinnerware is relatively uncommon although mass-production factories employ artisan techniques for some of their production.
MASS-PRODUCTION: FROM CLAY TO GREENWARE
MOLDED HOLLOW SHAPES:
Plaster-of-Paris molds are integral for mass-production of dinnerware.
Every factory I visited had a MOLD SHOP where the non-mechanical method of forming hollow shapes using plaster-of-paris molds is called slip-casting.
Models of a finished shape are first made in plasticine, wax, or plaster by skilled artisans who must consider the shrinkage of clay that occurs in both the green stage and bisque firing. A negative mold is made from the model and in a sequence of positive and negative castings, plaster working molds are made for mass-production.
Liquid slip (a prepared suspension of clay and water) is poured into a mold. When a sufficiently thick wall of clay clings to the mold, excess slip is poured out. As clay dries, it shrinks, is removed and is now called GREENWARE.
The dried clay shape is inspected and perfected to meet rigid quality standards. e.g. excess clay can be trimmed from edges—which are smoothed against a wet sponge before firing.
In shapes having a relief or embossed design, recesses of the plaster mold become clogged with clay residue. After approximately 20 pourings of slip, molds are replaced in order to maintain uniformity of the relief. This cost is incorporated in the price of the pattern.
OTHER SHAPING TECHNIQUES:
The production of plates requires a more solid form of clay. Liquid slip is pumped into cloth bags for filter-pressing. Water is pressed out and semi-dry cakes of clay are put through a pug machine to be well-mixed and extract air. This firmer and more plastic clay is extruded into ‘giant clay sausages’ called pug. Sliced pug is used for jiggering and jolleying techniques.
Jiggering is a mass-production technique for making plates. Thick slices of pug clay are deposited on spinning upside-down plaster plate molds. A template lowers to stretch the clay to a uniform size and thickness. Making five to six hundred plates a day requires five to six hundred molds a day. Molds must dry thoroughly before they’re used again.
Jolleying is a mass-production technique also for hollow shapes as bowls. Slip is poured inside a revolving plaster-of-paris mold and forced against the interior walls of the mold by a metal template to assume the mold shape and determine the proper thickness.
Handles, spouts, lids, and other components are individually cast in molds and ‘pasted’ by hand on the greenware body of each piece. Liquid slip performs as paste/glue. To the eye, these components must be attached without visible joining.
QUALITY CANNOT BE ‘ADDED-ON’
Pat Breen: EYEWITNESS TO QUALITY
My next post is: FIRING CERAMIC DINNERWARE