Clay Memory and Movement: Warps, Shrinkage, and Splits
Clay memory:
Two flat ornaments, one of which has warped significantly
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First is to take into account that clay has a ‘memory’. I’m sure there’s a very logical, and probably very technical, explanation for this, but I don’t know it. I do, however, have oodles of experience in seeing its effect first-hand, usually accompanied by the dulcet tones of me swearing. It’s definitely something to try and avoid, particularly with large flat pieces.
Kneeding the clay does seem to ‘reset’ the memory to an extent, I hypothesise – with all the power of Science that I possess – that mixing up the molecule do-dahs makes them balance out, rather than having a focused area where they all ‘remember’ to point in one direction. |
This can be fairly difficult to avoid, as during the rolling process it’s normal to flip the piece over or move it around. The best way around it I’ve found is to lay a secondary paper-covered board on top of the piece whenever I want to flip it and switch boards, transferring it to the secondary one and peeling the original piece of paper from the bottom (which is now the top). This gives the clay enough support so that it doesn’t usually imprint with any pesky bends.
Warping:
A fired piece that has warped due to uneven drying
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Unfortunately, clay memory isn’t the only cause of annoyingly uneven pieces, warpage during both the drying and firing processes can also be a big culprit.
Clay shrinks as it dries – I’ll talk about that in a second – and also when it heats up. There’s a surprising amount of movement involved in the whole process and every time it moves it has the potential to go in a direction we don’t want it to. With drying this is particularly the case if it’s dried too quickly, it tends to crumple like a wet piece of paper left in the sun. It’s far better to put the piece away from any source of heat so it can dry evenly. If you’re really concerned about the drying, as with very large pieces (particularly if they have detailing on the edge), you can lay a piece of plastic wrap lightly over the top to slow the drying process down. The edges will still dry out the quickest but there will be more time for moisture to work its way from the centre and equalise over the entire piece. I'm also currently trialing laying my pieces on a 5mm sheet of open-cell foam to ensure air can reach the middle sections underneath. So far the results have been promising. |
Shrinkage:
The original, mould and copy, showing the sizes
The copy on top of the original to show shrinkage
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It happens to the best of us...
Wet clay – somewhat obviously – has a certain percentage of water content. This will evaporate as it dries and result in a reduction in size. Just how great a reduction this is depends entirely on your clay, but the clays I’ve worked with have all hovered around the 10% - 13% mark, from wet clay to the finished fully fired piece. That’s a significant amount and really must be taken into account if you’re hoping to make something a specific size – say, like a replacement teapot lid. Warning, maths will be involved. Size is lost every step of the way, from drying to bisque and finally in the gloss. You’ll be able to see a perceptible difference even after just letting a piece dry for half an hour. In fact, I’ve used this to my advantage sometimes when I haven’t been able to get a piece out of a cookie cutter or mould without bending it (and risking it picking up a memory). Leave it for half an hour and it’ll pop straight out without a problem. |
Consider if you want to make something fairly precisely sized like a set of coasters. Let’s say, for the sake of argument and easier maths, that you want your finished coasters to be 10cm wide. So you merrily go ahead and use a 10cm cookie cutter, carve your design and create your ‘original’, the piece that you’ll take the mould off to make an identical set. So far, so good.
You dry it out and pop it in the kiln without measuring, ‘cause you did that back when you used the amazing state-of-the-art technology of the cookie cutter that assured you it would be 10cm. No problemo. The bisqued piece comes out and you take a mould from it, which itself is dried and fired up to bisque. You’re now ready to make your finished pieces from the moulds: six wonderfully identical perfectly sized 10cm coasters.
It may start to gnaw at the edge of your consciousness that they don’t look quite as large as you originally made them, but you push the thought to one side. You measured, after all, so there’s no way it could possibly go wrong. You make the coasters, bisque them, and then send them through the gloss with your best glaze.
They come out looking amazing, but as you try them out for the first time you realise that at 7.9cm wide they’re only suitable for Hobbits.
A 22% shrinkage put in perspective
The important part with the above example is that you have to take into account two lots of partial shrinking (the original to bisque, then the mould to bisque) as well as one amount of full shrinkage with the final piece. For my clay, I lose 12% in size from the original size I began with through to the size of the piece that comes out of the bisqued mould. Then from there I lose a further 10% getting the final piece through both bisque and gloss firing.
That’s a loss of just above 20% of the size of the original carving, which can make a huge difference to your finished product.
Splits:
A split in the base of a 'Speak Friend'
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Splits and cracks are the nasty result of shrinking problems, either while drying the greenware or firing in the kiln. With such a large reduction in size it’s important that the piece dries as evenly as possible (also in terms of warpage, as seen above). If you get one side drying quicker than another it will be shrinking at a faster rate, which pulls at the integral structure and puts it under pressure. If the pressure is too great it will pull itself apart and a crack will appear.
Many times this will be a small hairline crack at the bone dry stage, almost invisible, and you may be tempted to put it through the firings. There’s a decent chance of it surviving the bisque, if your bisque temperatures aren’t too high, but it’s almost certain to open up to a gaping split in the gloss firing. |