While this is the thorough way, I’m happy sticking with the simplest way, at least with this clay. If you really want to go the full hog, then you can make multiple test tiles, all equal size and thickness, and do multiple firings across the cone spectrum, graph their shrinkage and pinpoint the optimal temperature/cone for firing. HOWEVER, I don’t really mind, and it’s easy to bung any wild clay pieces into my usual firings, without having to fire them separately. In other words, it may be slightly overfired. If you look at my test tiles, you can see that the stoneware tile (right) is only slightly smaller than the bisqued tile (left), this tells me that perhaps my clay has reached it maximum density and has started to expand again. You may even see the clay bloating in places, before melting. Beyond that, the clay starts to become less dense and actually increases in size. This is the stage where you have found the top temperature/cone to fire your wild clay to. The clay shrinks until it reaches its peak density, maximum shrinkage and vitrification. This is because the clay particles draw together to form a glass, also known as vitrification. 1) that with increasing temperature, the clay becomes more dense and as a result, it shrinks. You can see in the example graph below (Fig. Without going into the real nitty gritty of it here, you can simply look at the shrinkage of your fired clay (ok, let’s get slightly nitty gritty, because the scientist in me can’t resist a graph). How do I know if the clay has matured or is overfired?Īpart from the obvious, which is that your clay has melted into an offensive puddle in your kiln, there are other ways to see whether your clay has overfired as well as seeing if it has matured. The small cracks and ‘imperfections’ are a result of the presence of little stones and roots that burnt out but I was happy with this rustic, earthy look. The firing was successful! the clay bisque-fired to a beautiful terracotta orange and stoneware-fired to a rich, chocolate brown. If you want to get into the science behind plasticity, you can read more about it here. Roll a piece of your found clay into a sausage and see you can bend it into a rainbow without it breaking, If it holds its shape then you know that your clay is probably plastic enough for making things, without it crumbling like short crust pastry (although don’t quote me on that, I’ve never made short crust pastry, only eaten it). The start of my wild story, starts with lovely, sticky, slick and soft clay and here’s what I did to test its suitability for pottery… The plasticity test Lastly, why not back up your findings (or start with) with a local geology map, a quick Google search can probably tell you what you’re standing on. If the area of dirt in question is dry, then look for signs of desiccation cracks, crazy paving style cracks on the surface are often a sign of clay. Obviously be carefully if it’s an area of high dog-walking traffic - you know what I mean. I go around smushing mud between my fingers to see if it matches these criteria. If the ‘mud’ is wet then it could be clay if it’s sticky, slick and soft. Well there are a couple of tell-tale signs. I suppose the first question really is, how do I even know that the mud or dirt I’m looking at is clay? But how do I know if I can make pottery with wild clay? But discovering the clay in the stream, shaped into rounded lumps, just seems that bit more magical. Of course, I could (and have) dug this same clay out of the ground, outside of my studio, or down the lane, where the tractor has cut the banks of the road with its massive tyres. The stream cuts through the clay, pieces of clay fall out of the banks, are cleaned up by the running water and they are ripe for the picking! Easily recognisable as golden nuggets. I live in Devon, on a clay soil and there is a little stream, or brook at the bottom of the valley. Wild clay collected from in Alphin’s brook, Devon, down the valley from my studio.
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