THE STUDIO NOTEBOOK · ONE CARD PER SPECIMEN
Clay shrinkage test card
Measure the same marks, every time.
Make a uniform, labeled test bar from your clay. Add two clear marks while wet (100 mm apart is convenient, not required); record the actual spacing immediately. Measure between the same mark edges, not the bar ends, using the same unit and method. Record the fully bone-dry length and each fired length only after cooling to room temperature. Repeat with several bars to check consistency.
Use your established, clay-appropriate firing process and kiln safety guidance; this card prescribes no firing schedule. Record the process actually used. A cone records heatwork, not a fixed temperature equivalent.
01 / Identify the specimen
02 / Record marked lengths
All four lengths use the same unit. W is the wet marked spacing, not the overall bar length. Leave optional or unmeasured stages blank.
03 / Record firing conditions
04 / Calculate linear shrinkage
Use measured lengths and a positive, nonzero starting length. Results are percentages, not millimeters.
Drying · wet → dry
(W − D) ÷ W × 100 = ________ %
Firing · dry → final
(D − F) ÷ D × 100 = ________ %
Total · wet → final
(W − F) ÷ W × 100 = ________ %
Optional · dry → bisque
(D − B) ÷ D × 100 = ________ %
Optional · bisque → final
(B − F) ÷ B × 100 = ________ %
Do not simply add stage percentages. Their starting lengths differ. Calculate total directly from W and F. Dry → final already includes both firing stages; do not add the optional bisque stages to it. Example only: W = 100, D = 94, F = 88 gives 6% drying, about 6.383% firing, and 12% total—not 12.383%.
05 / Observations & defects
Equal lengths mean 0% shrinkage. A negative result indicates measured expansion: check units and mark edges, and investigate bloating or distortion rather than treating it as normal shrinkage.