Clays by Type

Clay is a complicated thing. Seems easy enough, after all it’s just dirt, right? Nope.

Of course it comes in different colors, and different textures (rough and groggy, or smooth), and these factors can influence the look of your piece and the way your glaze reacts. Especially at higher temperatures (cone 8 and above) the materials in the clay, such as iron, start to interact with the glazes and this is often used to good effect. Some clays (such as porcelain, or porcelain-like stonewares) are are made from materials with smaller particles and are more plastic (can be pulled and manipulated in wet form without cracking); the tradeoff is that these tend to shrink more than others.

It is important for functional ware that clay reach maturity, which happens at a certain firing temperature. Commercial clay’s are actually a mixture of raw clays and other materials which are combined to create good working properties, and fire to a certain temperature. If a clay is rated cone 5, that is theoretically the temperature at which it becomes “vitrified”, or undergoes a chemical change and becomes almost glass like. The particles become densely packed, all the water is driven out, and at this point the clay is waterproof.

Many people think that glaze provides waterproofing for clay. In fact, with low fire clay it is about your only option since low fire clays never fully vitrify. But it’s effectiveness is limited. If relying on a glaze for waterproofness, you need a glaze with very good properties for waterproofness, and one which exactly fits the clay body so they expand and contract at the same rate. If not, the glaze will crack (craze) and even if it is microscopic crazing you cannot see, your piece will no longer be waterproof. If you think you have a piece with a good glaze fit, place it alternately in the freezer, and in boiling water. Or another test is to pour boiling water into a frozen cup. If the glaze doesn’t craze, it is probably ok.

So this is why stoneware is much more durable, because the waterproofness comes from the clay itself. If you fire a cone 10 clay to cone 5, it will be fine for sculptural or decorative work, but it will not hold up to daily use, microwaves, dishwashers, etc. because it has not matured (it is still too porous). Overfire a clay, and first it becomes brittle, then it starts to melt.

Shipping: Clay unfortunately is heavy, so especially if it's shipping far, the costs can add up. If you purchase large quantities, we will do a truck ship quote for you. We have especially good rates for most of the western states. If you purchase greater than 200 pounds of clay, but not a whole pallet, heavyweight discounts kick in and shipping goes down, usually 10-30%. This does not happen in the shopping cart, so you have to ask and we can calculated it for you. Also, for smaller quantities such as a box or two, we can ship clay bu USPS for $20a box (each 50#.) Our shopping cart can't do that either, so instead of ordering the clay, just type in the comments of the order which ones you want via USPS. Thanks

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