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!

Post 22 New mould part 2

After a day or so the plaster has a bit more strength and it is ready to flip over. Here it is again with the dam removed, still a little rough around the edges but then what the outside looks like is not so important.

Once inverted it is simply a matter of removing the armature which is easy with the thin layer of clay between it and the mould.

With the armature removed you can see the moulded clay which then just peels away from the paster. With a smaller pot its probably just as easy to shape a full block of clay as use an armature but at this size (430mm length) this is a more comfortable way to go.

The plaster is still very wet and quite soft. Best now to leave it entirely alone for a few weeks to really dry out. It can't be used until then anyway. Once it's dry it is more easily touched up with the odd scrape or sanding. I'm really looking forward to turning a pot out of this mould but will just have to occupy myself with other challenges for now.

Post 21 New Oval Pot Mould

I promised a few shots of the development of my next mould that I have been wanting to make.

The model for this pot is the one in the following shot.

It is a great ficus and the pot is beautifully suited to the trunk mass and inertia of the tree. I'd like to lay claim to it but unfortumately it's certainly not one I've got in my backyard. The pot has great proportions and visual appeal with just enough relief decoration to not distract from the tree.

So the place to start working this one up is to make what I would call an armiture, in sculptural terms. I first cut out the upper and lower oval shapes from a thin sheet of fibre board. These are then mounted either side of a shaped framework representing the body of the pot. I wrapped the framework with masking tape to provide a surface to clad with clay to shape the mould profile. This doesn't need to be too sophisticated - just robust enough to support the clay during shaping.

This is what the sandwich armiture then looks like. As you see it starts to look like the pot upside down.

 Then the armiture is packed out with clay - in this case just a good fine white terracotta clay. Any fine clay will do the job.

 Then a custom scraper, crafted to replicate the side profile of the final pot, is used around the outside surface to remove excess clay and cut down to the right size with a nice smooth surface. This shot shows the start of that process.

This shows what it looks like when the profiling cut is finished and ready to cast.

And if you invert a picture you begin to see what the body of the final pot will look like.

This is now ready to cast so it's time to get the dam in place. Obviously it is very important to make sure the dam is leak proof. There's nothing worse than having plaster squirting out onto the ground and trying to reseal it from the outside. Trust me I speak from experience. Any clay you use as as sealant will be exposed to the moisture in the plaster and water as the plaster dewaters if it does, so care is needed to make sure there is enough in place.

And here it is with the plaster poured into place, sealed nicely this time with no leaks.
Now sit back and reflect on a good job while it firms up.

The dam came come off pretty much after and hour or two but it otherwise it needs to sit for a couple of days undisturbed to firm up even more before removing the armiture.

 

As is usually the case the devil is in the detail and a few carpentry skills and equipment don't go astray as well.