FIRING CERAMIC DINNERWARE
Clay, hardened by heat– is called ceramic.
Generally, the purer and finer the clay, the higher the firing temperature—which explains the strength and durability of some dinnerware that looks fragile.
Once fired, the composition of clay cannot be reversed. Depending upon the clay and the decoration, ceramic dinnerware may go through several firings at different stages of production.
Because kilns require intense heat, dinnerware factories were usually located near forests until wood heat was replaced by coal, and much later gas, then electricity.
1st FIRING: CHANGING GREENWARE to BISQUE:
Clay shapes— dried but not yet fired, are greenware. Firebrick separates stacked and loaded greenware on a moving framework which passes slowly through a kiln for a first firing to the bisque stage. Bisque is the name of porous clay shapes that are fired —but not glazed.
In the bisque stage, foot rims and edges of all shapes can be ground and polished to eliminate roughness prior to being glazed—or they are tumbled with polishing pellets for extra smoothness. Each piece is examined before a transparent glaze is applied and fired to the ‘glost’ stage.
2nd FIRING: CHANGING BISQUE to GLOST:
After greenware is dipped or sprayed with a transparent glaze, the shapes are dried and stacked on refractory firebrick to separate each piece as they move slowly through controlled zones of heating and final cooling temperatures, usually within a tunnel kiln.
BISQUE shapes, now GLOST , await further glazes and decoration.
Tunnel kilns illustrate how capital and real estate intensive is the dinnerware industry. I visited several factories having tracks in the floor to guide loads of ceramic shapes in various stages of production—including their journey through a tunnel kiln.
Uniform thickness of clay is critical to assure successful firing; uneven clay bodies crack or explode when fired. Firing temperatures must be lower than the deforming or slumping temperature for each variety of clay and they vary greatly for different clays and glazes. Different colors and applied ornamentation may require firing at different temperatures and wares may be fired several times before completion. Glazes seal the porosity of clay and protect decals or painted designs from wearing away.
Additional firings depend upon the glazes, patterns and decorations required for each pattern. Labor costs increase every time a ceramic body is handled for decoration or firing.
Pat Breen: EYEWITNESS TO QUALITY
My next post is: GLAZING CERAMIC DINNERWARE