These are the fronts and backs of dust-pressed bars. After final drying the width at each line is carefully recorded. They are fired horizontally in a furnace able to reproduce linear thermal gradients along the length of the bar. Thermocouples monitor the temperatures along the bar, so the temperature reached at each line is known. After firing the widths are re-measured, this produces a graph of fired shrinkage vs. temperature. Clays can be visually inspected side-by-side and differences or changes in maturity are immediately obvious.
Orton Gradient Furnaces
A term used in the ceramics industry to signify the degree of vitrification in a fired clay. Mature clays are dense and strong, immature ones porous and weak.
During drying, clay particles draw together and shrinkage occurs. During firing the matrix densifies and shrinkage continues. More vitreous bodies shrink more.