Clay Shrinkage Calculator

In short Clay shrinks twice — once as the water leaves on drying, and again in the kiln as the body vitrifies — and the two multiply rather than add. Measure it on a test bar: roll a strip, mark exactly 100 mm on it while it is still wet, and measure that mark again bone dry and again after firing. A bar reading 100, 94 and 88 lost 6% on drying and another 6.38% of what was left in the kiln, and the true total is 12% rather than the 12.38% you get by adding them, because the second shrinkage acts on an already smaller bar. THE VOLUME IS THE PART NOBODY IS READY FOR. Linear shrinkage cubes, because all three dimensions shrink at once. At 12% linear a pot loses 31.9% of its VOLUME. A mug thrown to hold 400 ml holds 273 ml when it comes out of the glaze firing, and that single fact explains almost every "it looked so much bigger on the wheel". If a piece has to hold a specific amount, work in volume and not in height. And the throwing size is the other direction of the same arithmetic, with the same trap in it. If a body shrinks 12%, you do not make the pot 12% bigger — you divide by what survives. For a finished 10 cm rim you throw 11.36 cm, which is 13.64% bigger, and the gap between those two numbers widens as the shrinkage does. Two honest limits. Typical totals run about 5 to 8% for earthenware, 10 to 13% for stoneware and 12 to 15% for porcelain, but those are bands rather than figures: shrinkage depends on the body, on how wet you threw it, and on what cone you fire to — the same clay taken two cones higher shrinks measurably more. Your supplier’s data sheet beats a band, and your own test bar beats the data sheet. It takes five minutes and it is the only way to know your clay.