How is it done? Is there a special additive?

Well, it's all in the clay.

Clay has got a particular composition.

Different clays vitrify at different temperatures.

Vitrification means that the clay dies, is dead.

So when you're going to submit something to heat again, like put it in the oven, It needs to be a particular clay composition.

And so the clay that you use for that is clay that will vitrify a particular temperature and that you will fire that below that temperature.

It's a back filling or something, isn't