How a clay matures, and why hotter is not better.

When you fire a clay, you are not just drying it out. The heat changes the clay itself. Up to a point, hotter is better. Past that point it is worse. This note is about where that point is, what is happening on either side of it, and how we can tell.

What maturing means

A raw, dried clay is full of tiny gaps. Pack any powder together and there is always space between the grains, and clay is no different. When you fire it, the heat starts to soften the surfaces of those grains so they fuse together. The gaps shrink. The piece gets denser, harder, and shrinks a little as the spaces close.

A clay is “mature” when those open pores have closed about as far as they are going to. A simple way to measure how far it has got is water absorption. You weigh a fired piece dry, soak it, then weigh it wet. The extra weight is water that soaked into the pores. A high number means lots of open space left inside. A low number means the clay has tightened up and there is little room for water to get in.

So as a clay matures, you expect its water absorption to fall.

What the Park Royal numbers show

We fired the same Park Royal clay at six temperatures, from 700 to 1150 C, and measured water absorption at each. The pattern is clear, at least for this clay in these tests.

• At 700 C the clay is barely started. Water absorption is 17.6 percent. It is still soft and porous.
• At 1000 C it has come a long way. Absorption is down to 5.1 percent.
• At 1050 C it reaches its tightest, densest state. Absorption is 0.7 percent, which is close to fully sealed.

That 0.7 percent is the bottom of the curve. The clay shrinks most at this temperature too, which is what you would expect when the pores have closed as far as they can. For this clay, 1050 C is roughly where it matures.

Why hotter is not better

It would be reasonable to guess that if 1050 C is good, 1100 C must be better. It is not. Above its maturing point the clay starts to over-fire, and the numbers reverse.

• At 1100 C water absorption jumps back up to 24.8 percent.
• At 1150 C it climbs further, to 38.3 percent.

Absorption going up means the clay has gained pore space again, not lost it, which sounds backwards until you see what is happening. Pushed too hot, the clay begins to bloat. Gases trapped inside have nowhere to go and the softening body puffs up around them, opening new bubbles and voids. The piece can swell, distort, blister, or slump. Net shrinkage falls because the bloating works against it.

There is a useful way to picture it. Toast it just right and the bread firms up. Leave it too long and it does not get better, it chars and ruins. A clay has a sweet spot in the same way. The job is to find it and stop there, not to keep pushing the heat up.

What this does and does not tell us

These numbers describe this particular Park Royal clay, fired on this schedule, in this kiln. So far they place its sweet spot near 1050 C, with clear over-firing by 1100 C. That is enough to know not to fire it much hotter than 1050 C in normal use, and it gives a temperature to design recipes and pieces around.

It does not mean every clay behaves this way. Different clays mature at very different temperatures, and the same clay can shift if the firing speed or the time held at peak changes. What we can say is that this clay has a narrow, findable window, and that hotter is not safer.

Open questions

• How wide is the safe window around 1050 C in practice. The bloating at 1100 C is sharp, so there may be less margin than the single 1050 C result suggests.
• Does holding the kiln at peak for longer let the clay mature at a slightly lower temperature, which would save energy.
• Does the 2.5 percent barium carbonate added to make the clay slip-castable change where it matures or how steeply it over-fires. We have not tested that head to head yet.

For the full batch by batch account, from clay dug out of a construction site to a mapped firing range, see the Park Royal clay firing story. The measured numbers, the table, and the charts live on the Park Royal Clay findings page.