Microwave safety is a concern in the production of function ceramic ware.
The most obvious problem almost everyone has seen is the failure of gold decorated bone china in the oven. This is because conducting metals arc and spark. Notwithstanding this, any non-porcelain has a certain percentage of iron in the clay. Iron red stonewares, for example, can contain 4-5% iron. However this metal is not in the metallic form, it exists as part of the silica crystal matrix of the ceramic. Notwithstanding this, bodies of higher iron content do heat up so common sense is needed. Of course body materials containing particles of pure metal (for specking) would be bad. Glazes also are an issue. Those of of high iron oxide content (or other metal oxide) will clearly be an issue (e.g. tenmokus).
However ware exposed to micro waves can also super-heat pieces for another reason. Ceramic bodies often fire with porosity. Earthenwares, for example, can have 10% or more, so they can soak up a lot of water. But porous bodies that can only absorb water into the matrix if part of the surface is not glaze covered (e.g. the foot ring). If the glaze is crazed water can also enter through the crack lines. When pieces are in contact with water for a period of time they can become completely water logged, and the glaze cover can keep them waterlogged for an extended period. Obviously this water is going to turn into steam in a microwave and that will certainly super-heat the piece. It may even exert enough pressure to fracture it. Industrially, 1% porosity is considered the margin for microwave safety. Lots of stoneware made by potters is higher, 2% or even 3% is common. In practice, these pieces service well, not just because they are more dense than earthenwares, but also because glazes fit better and water finds little opportunity to soak in.
A common-sense issue is cross-section of the ware. If it is uneven (very thin in some places and thick in others) then the warming food in the container will heat it unevenly resulting in possible cracking. This is not the fault of the clay.
To test microwave safety, just put a little water in a piece and heat it in a microwave for 30 seconds. If the piece feels a lot hotter than the water then there is a problem.
The cone 6 glaze is well developed, it is not crazed. But the clay underneath is not developed, not vitreous. This crack happened when the mug was bumped (because of poor strength). It is barely visible. When the mug is filled with water, this happens. How fast? This picture was taken about 5 seconds later. If this was crazing, and this piece was in actual use, the clay would gradually become completely water logged. Then one day someone would put it in the microwave! Boom.
Fired on a porcelain in a gas kiln.
Notice the water has wicked up to about 1 cm from the rim (the piece sat in water overnight). The glaze fits so there are no cracks for the water to seep through. However, being fired at cone 04, the body is quite porous. The piece has a unglazed base. Notice the water even travelled up the handle. Less exposed bare clay on the base would improve the situation somewhat, however it would be much better to choose a body that vitrifies sufficiently dense so that it does not absorb water (or fire to a higher temperature). There is a not-so-obvious issue here also: Although this piece did not explode in the microwave, it got incredibly hot. Amazingly, through all of this, the glaze has not crazed. It is G3879.
Is Your Fired Ware Safe?
Glazed ware can be a safety hazard to end users because it may leach metals into food and drink, it could harbor bacteria and it could flake of in knife-edged pieces.
There is an increasing awareness of the food safety of glazes among potters. Be skeptical of claims of food safety from potters who cannot explain or demonstrate why.
A term used in ceramic to express the degree to which an item is safe and stands up to everyday use. Functionality embodies strength, hardness, resistance to acid attack and thermal shock, etc.
ASTM method for microwave safety