Monday, 16 September 2013

Post 121 Pot drying

Drying the base of pots and at the same time keeping them flat and in the right position can be quite a challenge if you are after a great result.

It has long been my practice when I make the pot feet to make up small pieces of clay of the same thickness to use as supports for the floor of the pot while drying. I use a small single piece of newspaper to separate the pot clay from the support clay so they don't stick together. I then take the same supports through bisque firing and then again final firing.
There is no movement in the clay during bisque firing becaues that's really just elevated drying, so I don't put them under the pot, but definitiely do so for the final glaze firing as the clay is likely to sag if not supported during vitrification.

So that's what I've been doing for a while. I have also dried the pots standing on their feet rather than their rims. It's much easier to support the floor that way and if you stand the pot on its rim you then have to accommodate the shrinkage in the wall height in a similar shrinkage in the support. Trying to do all that in clay is a big job. So I dry them right side up and the shrinkage of the supports matches the feet shrinkage and so there is no relative floor deformation in drying. I should say also that there is no point trying to dry a pot sitting on its rim as the final fired rim stability will be influenced more by how the clay is manipulated when wet and firm than when drying.

That's all good but then as in all things perhaps there is something lost and in this case it is that the drying speed of the floor is now less than the walls. Air comes into contact with both sides of the wall whereas it only touches the inside of the floor. Furthermore the inside of the pot is enclosed compared to the outer wall surface which sees some air exchange with the outside world, even with everything covered in plastic. This is compounded by the supports because there is now more clay mass to dry and all that moisture can only be removed by migrating to the inside of the pot. Not surprising that the walls are dry before the floor. This is gong to set up stresses that may return to crack the pot at the wrong time. It would be really good to eliminate this drying stress and I've been turning my mind to it for some time.

An option I considered and rejected was to place the wet pot on a piece of plaster wall board. That stuff is certainly very effective in pulling water out of clay, but how much is enough and how do you regulate the speed so the floor doesn't dry first. You could just end up with the opposite problem.
Then I started to think about how I could dry the pots on a rigid mesh support that was ventilated from underneath. That way, while covered in a sheet of plastic the moisture content of the air under the pot would equilibrate with the air on the outside of the pot and reduce the drying differential between floor and walls. Rack my brain as I did I couldn't come up with a cheap and easy way to do this.

Back to the thinking.
Lucky I've really only got two important things to think about at the moment - growing bonsai and making pots; what a dilemma! You see an engineer can only cope with so much.
So more thinking and enlightenment observation and perhaps just a little chemical engineering was called for.

You see if I could get some air under the pot it would wick the moisture to the outside. Did I say wick? Is there something else that would serve the same purpose, that would be easier than a piece of rigid mesh and static air? I have known for a long time that newspaper very effectively pulls moisture out of clay and not only that but continues to do so quite a distance from the point of contact - it is a great wick! The answer was right there, I'd just been using the newspaper for something else.

So what I've been trying lately is to set the freshly made pots on 6 pieces of newspaper, on top of a plastic sheet, then cover everything in a plastic bag. The newspaper wicks the moisture from the floor to outside the wall space where it will saturate the air in that space and thus slow the drying of the walls, or at least bring the wall and floor drying closer together. I have 6 pots drying this way right now and so far it seems to be very effective, as well as reducing the overall drying time. With the wick pulling the moisture from the floor you don't have to let poor old slow and challenged equilibrium do the same thing over a longer time. Why 6 pieces. Well no I haven't done the mass transfer calcs but it feels about right. Let's hope our bridges aren't designed the same way.

During drying the air under the bag becomes saturated and no more drying will take place until that air is changed out. Every day I do just that by reversing the bags, inside to outside and the inside surface is always wet, showing the air is saturated, doing it's job. It's important that the newspaper wick is also entirely under the cover bag for obvious reasons.

I don't know why I'm telling you all this, giving away my secrets, but it's a great tip and looks like a new SOP for me. It's probably something that most experienced potters discovered long ago and I'm just catching up. Let's see how they go in firing.
You'll note the colour of the pot in the picture too. Yes a new clay, a yellow staneware that fires a sandstone colour. I've switched after recent problems the reformulated red stoneware.

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