I have received some surprise when I mention that my floor sand melts during firings. It really does, but it will only become glassy where it is thin enough to spread out on something harder, like the bricks I pointed out in my previous post. Where the sand is thick, only the top layer will melt.
Here is an example from the sixth firing, one of the most beautiful firings and the most disastrous due to multi-level shelf collapsing.
This piece was just back of the firebox — a band of black koge can be seen in the front of the image. The rest of the images from the sixth firing are posted on this site, but the location is pretty buried — I should add them to the photogallery.
Anyway, Furutani’s kiln design has no trouble melting sand. In fact, it is beginning to dawn on me that I will have to work harder to keep firing temperatures down a bit, as I believe I’ve been overfiring to some degree.