Post 23 Clay Memory

I've looked at lots of pictures of pots and at my pots and and other potter's pots and everywhere I see the evidence of that intractable phenemon of clay memory. It behaves just like any other plastic material - to get it to where you want it you have to take it past an elastic limit so it comes back part of the way to where you want it. Just like bending a piece of wire which is why using wire to shape bonsai branches can be problematic too. You have to bend the wire, and the branch, past where you want it to finish up, putting the branch at risk in the bending.

In shaping a piece of clay into the form you want, depending on how wet (elastic) it is at the time, every change in position will leave residual stresses in the material which will only be relieved if and when the material is soft enough to move again. The time for that is up near the clay's final curing temperature and that's the time when it might move, to yield to the stresses and set up in a position you may not have planned.

The coaster below is a great example. After working it was dead flat, also at bone dry and dead flat after bisque only to move in the final glaze firing. It was fired sitting flat on a shelf so the corner actually lifted during firing.

As you can see the corners from left to top are ok but that right hand edge has lifted. It has to have happened as I lifted it off the table from working it, put a small bend in it and then pushed it flat again as I set it down to dry, but in doing so locking in a memory about being just that little bit higher than the other corners. Arguably I should have taken it beyond flat and then back to flat before setting it to dry.

This stuff is much more metallic in its behavour than you would imagine and perhaps thats not a bad mindset to adopt. How to apply that learning to potmaking. Well there's something there about drying and positional changes as the clay dries. Dry it slowly so there are no moisture differential stresses locked in and any shape adjustments need to be made early and to go beyond and come back; perhaps. I've got a couple of pots drying right now and have them well wrapped. The first of these was made over two weeks ago and I'm going to give it more time yet. The experiments go on!