This is a patented Nara Seiki ceramic which quickly heats up to 3,450 F/ 1,400 C in a standard microwave. http://www7a.biglobe...i/serami_e.html
Heating in a microwave can result from clay or glaze, or the interface structure between clay and glaze.
Fats, sugars and water heat because they have oppositely charged polar ends. But there's more.
Heating in a microwave can occur in a ceramic or glass which is an electric insulator (a dielctric). Microwaves induce the formation of positive charges on the side nearest the magnetron and negative charges on side furthest from the magnetron. As these charges build-up a current begins to flow through the ceramic or glass. This current then begins to reorient the glass and ceramic molecules in alignment with the microwave field which makes the ceramic or glass even better at absorbing microwave energy.
This is Wikipedia's verion of this explanation of cermaic/glass insulator heating. http://en.wikipedia..../Microwave_oven
"Microwave heating can cause localized thermal runaways in some materials with low thermal conductivity which also have dielectric constants that increase with temperature. An example is glass, which can exhibit thermal runaway in a microwave to the point of melting. Some ceramics can also be melted."
A thermal runaway in this context means the ceramic or glass becomes better able to absorb mircowave energy as it heats up, rather than an exothermic reaction like you get when adding Calcium Chloride to water with glaze to flocculate it, which is why Calcium Chloride is sold as a sidwealk ice melter.
You can test this out yourself with an empty pyrex measuring cup. After a couple of minutes, depending on the oven wattage, it has heated-up just a little bit. But once it becomes hot to the touch it will quickly become hotter than a a kitchen oven - hot enough to crack the pyrex. And it makes a loud bang as it cracks because pyrex is glass which is cooled so quickly that the interior glass structure is under high compression from a smaller tight exterior, making it scratch and chip resistant -- exactly like a good glaze fit on a clay body.
Microwaves heat electrically conductive materials, like a glaze with more than 12% iron oxide, through a different process. Material density involves yet another heating process which may also explain the iron oxide problem and metal oxides in general.