Tuesday, April 12, 2016

April 10th Hint of the Week

Firing Clay at the Right Temperature (from Big Ceramics Store)

It is important for functional ware that clay reach maturity. Maturity 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 ∆10 that is theoretically the temperature at which it becomes “vitrified”, or becomes almost glass like. The particles become densely packed, all the water is driven out, and at this point that clay is waterproof. Fire a ∆10 at ∆6 and it is not vitrified and you may have other problems.

Many people think that glaze provides waterproofing for clay…If relying on a glaze for waterproofness, you need a glaze with very good properties for waterproofness, and one which also 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, it may not be food safe, or microwave safe or it could not be durable.

So this is why stoneware is much more durable, because the waterproofness comes from the clay itself. If you fire a ∆ 10 clay to ∆ 6, 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). Over fire a clay, and first it becomes brittle, then it starts to melt. So you want a clay that is made for the temperature you will be firing at. That is why we don’t put ∆6 clay in a kiln firing to ∆10.

Many manufacturers will give a firing range for a clay, for example cone 4-6. A small range is probably justifiable because of variations in the materials, and the inability to always achieve an exact cone level through a kiln due to hot or cold spots. However, some manufacturers take this to the extreme, marking a clay as cone 4-10. This is hogwash. There is a single, specific temperature at which that clay will become vitrified. It simply cannot happen over a large range. In this case I would bet it is closer to ∆ 10 than ∆ 4, because they wouldn’t want to risk their clay melting in your kiln.

How do you determine whether a clay is matured? Do a porosity test Take a piece of clay and fire it to temperature. Weigh it. Put it in boiling water for 5 minutes, and leave in the water as it cools. When cold, remove the piece and dry it off with a sponge. Weigh it again. The difference in weight divided by the original weight time 100 is the % porosity. An ideal clay would have 1-3% porosity. (Zero porosity, in addition to being practically impossible is also not as strong because it is more brittle.) If a clay is around 4-5% porosity, it is “waterproof” enough to be good for functional ware. In the above example of the clay with the large firing range, the cone 4-10 clay fired at cone 10 may have a 3% absorption, and at cone 4 may have a 13% absorption. So while technically it can be fired over that large range, it will not have the same properties when fired over that range. A ∆ 6 clay fired at ∆ 4 might be 6% absorption compared with 4% absorption, and either may be acceptable. Note that for a piece to be “microwave safe” the porosity should be very low. Otherwise, water which has leaked into the piece can quickly expand and cause it piece to crack.

Also, for oven ware use, the clay used should have low thermal expansion. This means that it doesn’t expand much as it gets warmer. If a clay expands a lot as it gets warmer, it will tend to expand more where it is hotter (closer to the elements) and less where it is colder (further from the elements), which creates stresses and can cause cracking. Some manufacturers list thermal expansion, or at least tell which clay bodies are good for oven ware because they have low thermal expansion.

We have had guild members fire cone 10 clay in cone 6 without any visible signs. However, we have also seen blistering and bubbles in the glazes, and the pottery not standup to or deteriorate more after microwaving and/or dishwashing.

A few other thoughts. According to Richard Zakin’s Electric Kiln Book, a similar temperature in reduction will mature clay slightly more than in oxidation. Also, manufacturers typically rate the clays on the high side, to avoid their clay destroying your kiln if your temperature is off slightly. So, for example, I have never had any problems firing ∆ 5 clay at ∆ 6 in oxidation. But always test! I haven’t tried them all! And there can be variation from batch to batch, and kiln to kiln. So in a way it’s nice to have at least a small margin for error.

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