Yes that is right, now you mention it, so it might also be made by a process which needs to be cleaned up afterwards. You end up with those blackened clinker stuff, which I note has been used to fill up numerous clay quarries around where I am and houses built on top,. I do hope the stuff is inert.
The thing I always recall about it was the smell, it was actually quite nice, but of course as it often never quite dried, you could easily get it on your clothing and it was the devils job to get it off again, and that is over and above any toxicity of long term exposure. Brian