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OK Greg, Carol and Eric, what is or isn't it that makes handmade ceramics microwave safe or unsafe. My favorite potter says not to microwave her pieces. I took a clay & zeolite mineralogy seminar in grad school but don't remember a whole lot except for the ion/OH structure of the clay minerals being important for industrial uses.Getting technical would be fun to hear.

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the problem arises if the clay body is not properly vitrified (melted). the clay body will absorb water, and then when you microwave it the body can get super hot, and even break due to steam trying to escape.
If properly fired, porcelain and stoneware should be fine, but low fire earthenware is not vitreous, and so should never go in a microwave.

The problem with dishwashers is the alkalinity of dish washing solution, which can eat away at certain glazes over time.

Oops, I put my wife's favorite cup in the dishwasher. Thanks for info.

Gregg, all clay bodies have there own vitrification temperature.  So, if you fire a clay body, even a low fire clay body to it's peak temperature, there should not be a problem either microwaving or dishwashing. I've made both dinnerware sets and soup bowls, mugs, etc. for a restaurant in which I fired a cone 06-04 claybody to cone 1-2 and I've been complemented on how long lasting they were compared to the usual restaurant industrial products. In fact, every time I see him, the person who owned the now defunct restaurant tells me he still uses those bowls. I made those bowls 30 years ago. I made some extra ones and I still use them with no problems. So, the answer is: it depends on how high the pieces were fired relative to the melting temp of the clay body and the glaze.  Some colors need to be fired in a certain way, at a certain temperature and that may not be compatible with microwaving and dishwashing in a dishwasher.

As long as we are on the subject, I recently got involved with some soda firing and I am now hooked. Unfortunately, it isn't my kiln so having a booth full of great functional pots probably won't happen. However, from time to time, I may have some.

Well Barry, if it can/is fired to cone 2, is it a low fire body?  ;-)

  As you know many low fire bodies that rely on high amounts of iron as a flux will bloat and slump before reaching vitrification if you try to fire them higher.

Funny you should mention this, Gregg. For the soda firing, my friend gave me some iron baring clay that he thought was cone 9. Half of it was cone 9 and half of it was a low fire clay body.  So, half the pieces looked like Dali melted watches, melted mugs in this case. Most of the rest of the pieces were bloated. This was only the second time firing this downdraft kiln, so, we are still working out the kinks. I remember thinking that the shutters on the burners were too closed off, but, since it wasn't my kiln, I didn't say anything. There was too much backdraft for a clay body with iron in it, meaning there was too much gas to air in the mix resulting in bloatware.  The pieces without any iron in the clay came out perfect.

For those of you who have no idea what I'm talking about, clay bodies with iron, or red clay, have a lot of impurities in them. At lower temperatures, meaning below 1800 degrees, all the gaseous impurities like sulfur and carbon must be burned out of the clay. This is done by firing in an oxidation state. More air to gas. You can tell because the flame is blue and there is no flame coming out of the ports. If you fire with too much gas to air, the glaze begins to melt and traps the gases in the clay. It's called bloating and it looks like hives. Another way to burn out the impurities is to bisque fire at a higher than normal temperature or bisque fire longer than 8 hours. The trend, now, is to bisque for 10 hours. With the new computer programmed kilns, this can be easily controlled. Of course, my equipment is all old, so, I have to manually control my firing.

Pretty interesting stuff. Curious if you guys are concerned about the actual clay mineralogy of your starting material. Are there different mixtures of mineralologies? What about naturally occurring mineralologies i.e. Casas Grandes and some the Anazazi pots reproduced in commercial starting materials?


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