Has this ever been tried? Carving a nest out of it won't be easy since you'll need access to a kiln and the product will be heavy, but I imagine it would hold humidity wonderfully and I'm guessing relatively mold resistant.
Or what about using terracotta pieces as humidifiers? For example, using a piece of unglazed tile instead of hydrostone beneath the substrate. Would be easier and cheaper than having to buy and mix hydrostone or grout and can be removed or replaced.
Looks like some Italians have considered it.
That btw, looks like an interesting forum. I should've taken Italian...
My mind is just wandering now, what about pumice? I've seen it mentioned as a substitute for Ytong, but what about using it as a humidifying substrate/subbase?