I've been interested if my pots are safe to use in the microwave. I have a few questions I hope someone may be able to help with.

Firstly regarding metal content. I don't use any lustres or metals over glazes. I have only read vague things about 'too much' iron content in clay bodies or glazes causing arcing. Are there any specific percentages to work to regarding metal contents when using microwaves? So far none of my pots seem to be causing a problem in this area.

Secondly regarding absorption. My testing method has been to soak the pots overnight in water, then put them in the microwave for 1 min on high, with another container with water in to protect the microwave. All pots in the test have flat unglazed bottoms.

These pots have been bisqued at ^07 and glaze at ^6

The first pot I tried, the bottom came out quite hot - not too hot to touch, but pretty hot. Not a good sign I thought.

I decided to try another pot. This time the pot was hardly hot at all, but slightly warmer than room temp.

After a while scratching my head, I realised the second pot had been glaze fired twice to ^6 as I hadn't been happy with the initial glazing. I've read that vitrification can continue in subsequent firing so I guess this explains the lower heat due to less water absorption...

I'm currently doing a weighing test to calculate the amount of absorption on the twice fired vs once fired pots. But I obviously don't want to be twice glaze firing everything.

The only info I have on the clay maturity is its firing range is 1120C -1280C

I'm currently throwing with a different clay from the same manufacturer, which states the same firing range, but I guess it could vitrify at a slightly different temp..

So I don't really want to just try firing to ^7 because my glazes have been designed for ^6, and it would cost me more in electric etc. 

I was wondering if the bisque temp would have much affect on final vitrification? Im only using ^07 because there wasn't any ^06 orton bars in stock  when I bought them.  

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Many ceramics are sintered or melted in industrial microwave kilns. So eliminating metal and moisture in pottery is a good place to start to make ceramic ware "transparent to microwaves" but obviously those are not the only factors.

1.) Water, fats and many other molecules in food are somewhat dipolar, having opposite electric charges on either end so they heat quickly from friction as the rapidly changing electrical field created by the microwave vibrates these molecules.

2,) Most glass and ceramic are initially a material called a dielectric. This is an electrical insulator which can accumulate opposing charges on either sides of their surface like a capacitor as the microwave field creates electrical charges.

A dielectric is not initially heated much by microwaves, but if a dielectric heats-up many of these materials begin to lose their dielectric property as the temperature increases - thus becoming more easily heated by microwaves, the hotter they get. Further microwaving of the dielectric creates a "thermal runaway", an increase in temperature, often localized so one area of the dielectric becomes much hotter than other parts.

Microwave Thermal Runaway

An empty glass container generally doesn't heat in a home microwave, but put water in it and the boiling water heats the glass and starts the glass onto its loss of dielectric properties after the last of the water has evaporated, resulting in the glass cracking, even pyrex, due to thermal variation within the glass container which exceeds even pyrex's abilities.  So just like food, ceramics can develop difficult to predict hot spots.

3.) So what you're really trying to make is a dielectric material which retains these microwave transparent properties over the limited duration of microwave exposure needed to heat liquid or food.

Unfortunately this means you'll have to test fired-ware in a microwave to see if it behaves the way you want.

The extent to which a ceramic is heated by a microwave oven, other than dielectric heating, is determined by many factors only partly under your practical control.

a.) particle size

b.) porosity

c.) ionic conductivity

d.) electric conductivity

e.) magnetic coupling

Since particle size and porosity (a and b) change as ceramic densifies in the kiln you will find some greenware which heats-up quickly in a microwave oven doesn't heat much in a microwave once fired.

Fire this ware again or to an even hotter cone and the densified ware with far fewer individual particles might remain cooler in the microwave oven, as with your experience of a twice-fired bowl.

But ware fired hotter or longer might just as easily heat-up very quickly in a microwave, and you'd need an electron microscope to figure out exactly what changed. I wish this were not the case.

Iron and other metals in a clay tend to create heating from factors a, c, d and e.  But some clays high in metal are fairly microwave transparent, mostly depending on the metal particle spacing relative to fractions of the wavelength created by the oven, which is about 12 cm long, and also the size and shape of the bowl which is acting partially as an antenna. 

I've purchased very thin white porcelain items which have heated-up so quickly in a microwave they're much hotter than the food, so white clay is not a guarantee,

Everyone knows a metal foil conducts and collects the induced ionized charges and arcs to the grounded wall of the microwave.  But a surface of sputtered metal (random spots of metal flecks which do not touch each other) heats up very quickly like a toaster element as each metal fleck becomes ionized with electric charges which flow and arc from one spot to the next with the electrical resistance of the material in between creating heat. Sheets of sputtered metal on dielectric plastic are what heats-up a bag of microwave popcorn.

So it will take some trial and error to eliminate problems making microwave-safe ceramics.

There are actually reference books such as this one below which can be used to provide some guidance.

It's a specialized science - something that people actually work on as their PhD thesis in Material Sciences Engineering.

Dielectric and Thermal Properties

Ware bisqued at a lower temperature can allow more glaze to penetrate deeper, depending on the glaze. The clay infiltrated with glaze obviously densifies more than the clay would on its own.

I have sometimes needed greater densification at Cone 6, so I dipped the bisqued ware in a solution of water soluble flux, like lithium carbonate and potassium carbonate, then let it dry before glazing. Using a soluble flux, it doesn't matter what temperature the ware was bisqued at because it penetrates all the way through even at a hard bisque like a Cone 03.

You can sometimes achieve somewhat more densification at Cone 6 with a long hold, or a second firing as you did, but it's far less expensive to dunk the bisque in a soluble flux.

Aww I just wrote a reply and lost it somehow before posting...

Quickly again - Thanks Norm lots to think about! I will try new clay and see what happens before adjusting and bisque/glaze cones.

I weighed the pots that had been soaking again, results were once fired pots porosity around 3%, twice fired pots around 1%. I understand from your post there are many other considerations but is there a rough porosity % people aim for when thinking about microwave usage?

Thanks again for your wise words!

Aim for <1%. If your making functional pots, find a clay that says from the producer, at cone 6 <1%. There are many out there, but if your making things that are going to be used in dishwasher and microwave you want a clay body that is <1% fully vitrified, which if your working at cone 6 is going to be <1% at cone 6. There are several out there that are like that.

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