Let's find out. I'm betting that it's not able to be in or near a fire, because it's concrete, after all, and that's not what it's designed to do.
Concrete has some great properties, like being stable, able to hold or keep out water in many cases, and it makes great planters. Add in some other ingredients, like peat moss and perlite, and things become a little murkier.
The peat moss is actually combustible - have you ever heard of peat bog fires? They can burn for years underground. So this is something that would give me pause if I wanted to use hypertufa over an open flame.
You've piqued my interest and so this is the experiment. I've got some hypertufa pots that didn't make the cut, or fell apart - the perfect candidates for experimenting with!
Get that bonfire burning with the scrap wood that I store in the woodshed, and let's find out.
I'm ecstatic to say that I was wrong!
There is absolutely no change in the texture, the strength or the size of the hypertufa that I experimented with. Here's a picture of the two pieces of broken pot - one left as is, the other put through the incineration process.
The one on the right is the one that I burned. The only difference is that the moss growth is gone, but other than that, it's exactly the same as when it went in.
One thing I would hesitate to do with a hot piece of hypertufa pot is put it into cold water. As it was, these pots were damp from being outside, which didn't seem to bother it a bit.
Other things to try; dry hypertufa pots, or dunking it into a bucket of water - all in the interests of science.