Shrinkage compounds, it does not add
Clay shrinks twice. It shrinks as water leaves between the plastic state and bone dry, and it shrinks again in the kiln as the body vitrifies. The second shrinkage acts on a piece that has already been reduced by the first, which means the two multiply rather than sum. A body quoted at 6 percent drying and 6 percent firing loses 11.64 percent overall, not 12: multiply 0.94 by 0.94 to get 0.8836, and the piece finishes at 88.36 percent of its wet size.
On a 10 inch platter the difference between 11.64 and 12 percent is under four hundredths of an inch and nobody cares. It starts to matter when tolerances are tight, when the numbers are larger, or when the error is compounding across a set. A 14 percent body at 14 percent drying and 14 percent firing loses 26.04 percent rather than 28 — a two percent error on a piece that has to fit something else.
Working backwards uses the same factor in reverse. To finish at a target dimension, divide by the combined factor rather than multiplying by anything. A piece that must fire to 8 inches through a 0.8836 factor has to be 9.054 inches wet, and the instinct to add 11.64 percent to 8 gives 8.93, which is short. Adding a percentage back is not the inverse of taking one away.
The number belongs to your clay and your kiln
Published shrinkage figures are honest and they are also averages. The same body fires differently at cone 5 and cone 6, throws differently wet and stiff, and behaves differently as a slab than as a thrown wall. Reclaim tends to shrink more than fresh clay from the bag because it has been worked harder and is often wetter.
| What changes | Effect on shrinkage |
|---|---|
| Firing a cone hotter | More, as the body vitrifies further |
| Working the clay wetter | More drying shrinkage — there is more water to leave |
| Heavily grogged bodies | Less, since the grog is already fired and does not shrink |
| Porcelain against stoneware | Usually more, and it moves more in the kiln as well |
| Reclaim against fresh clay | Often slightly more |
The way to settle it is a test bar, which is why this page has a mode for reading one. Roll a flat bar of the body, scribe two lines exactly 100 mm apart while it is still plastic, dry it, fire it in a normal load at your normal schedule, and measure between the lines again. Working in 100 mm makes the arithmetic trivial: a bar measuring 88 mm has shrunk 12 percent. Measure the bone dry length as well and you get the drying and firing figures separately, which is useful when you need to know a greenware dimension for a fitted part.
Shrink rulers, and where they mislead
A shrink ruler is a ruler whose inches are stretched by the shrinkage percentage, so that measuring wet clay with it gives you the fired dimension directly. They are genuinely useful and they encode one specific percentage. Using a 12 percent ruler on a body that shrinks 9 percent produces work that is consistently too small, and the error is invisible until something has to fit. If you use one, own the ruler that matches the body you actually use, and re-check it whenever you change body or firing temperature.
Where shrinkage becomes a fit problem
Uniform shrinkage is not a problem, since everything gets smaller together and proportions hold. Fit is where it bites. A lid and a jar shrink at the same rate only if they are the same body, dried at the same rate, and fired in the same part of the kiln — and kilns are not uniform. Pieces near elements run hotter and vitrify further. A lid fired on a shelf and a jar fired next to a wall can end up half a percent apart, which on a four inch opening is enough to bind.
The practical answers are to make fitted parts from the same batch of clay at the same time, to dry them together and preferably assembled, and to fire them touching or at least adjacent. Where a fit must be exact, make the parts slightly loose green and grind the fired result rather than relying on prediction. Grinding fired ware makes silica dust, so it is a wet job.
Questions people ask
Why is 6% drying plus 6% firing not 12%?
Because the firing shrinkage applies to the already dried piece, not to the original wet one. Take 6 percent off a 10 inch wet dimension and you have 9.4. Take a further 6 percent off 9.4 and you have 8.836, which is 11.64 percent below where you started rather than 12. The pattern is the same as compound interest running in reverse: multiply the surviving fractions, 0.94 times 0.94, rather than adding the losses. The gap grows with the percentages, so on a body shrinking 14 percent at each stage the difference is 28 against a true 26.04, and on a fitted part that is a real error.
How do I measure my own shrinkage rate?
Roll a flat bar of the clay body, roughly six inches long and a centimetre thick, and scribe two fine lines exactly 100 mm apart while it is still plastic. Let it dry to bone dry, measure between the lines and note it, then fire it in a normal load at the schedule you actually use. Measure again. Because the starting length was 100, the fired reading is the surviving percentage directly: an 88 mm reading means 12 percent total shrinkage. Make several bars and fire them in different parts of the kiln, since the spread between them tells you more about your kiln than any single number does. Repeat when you change body, supplier or firing temperature.
Does shrinkage change with firing temperature?
Yes, and it is one of the larger variables. A body fired to a higher cone vitrifies further, meaning more of the material melts and consolidates, so it shrinks more and becomes less porous. The same stoneware taken from cone 5 to cone 6 will typically shrink measurably more, and a body fired well past its rated range will shrink dramatically, slump and lose its shape. Manufacturers publish shrinkage per cone for this reason. If you fire the same body to different temperatures for different work, you need a shrinkage figure for each, not one figure for the body.
What wet size gives me a specific fired size?
Divide the target by the combined shrink factor, which is the drying factor multiplied by the firing factor. For a body at 6 and 6 percent the factor is 0.94 x 0.94 = 0.8836, so an 8 inch fired dimension needs 8 / 0.8836 = 9.054 inches wet. The mistake to avoid is adding the shrinkage percentage back onto the target, which gives 8.93 and leaves the piece a tenth of an inch small. Percentages are not symmetric: taking 11.64 percent off a number and adding 11.64 percent to it do not undo each other. Use the reverse mode here and it does the division for you.
Why did my lid not fit the jar after firing?
Almost always because the two parts did not experience identical conditions. Kilns are not uniform, and a piece near an element or in a hotter zone vitrifies further and shrinks more than one in a cooler corner, easily by half a percent. Drying rate matters too: a lid dries faster than the jar it sits on because it is thinner, and clay dried unevenly shrinks unevenly. Different clay batches, or a mix of reclaim and fresh, will do it as well. The practical fixes are to make fitted parts together from one batch, dry them assembled with the lid in place, and fire them adjacent to each other. Where exactness is essential, make the fit slightly loose and grind the fired piece wet.