When I first fired it, I got pitch-black breaking rust over edges. It was very lovely on specially designed pieces like this incense burner
It works for teapots, too, but markedly better on non-Limoges porcelains for some reason.
At higher temperature, 1280 Celsius, it displays a fascinating change in surface - breaking rust where thin, and forming oilspots where thick, running and rolling....
This is the type of glaze with which I love to work. Though you can have your share of mishaps until you learn the right thickness to apply and the right way to fire, when it does works it feels like a blessing.
The recipe is widely known and as follows:
Gerstley Borate 12.5
Please send me photos if you try this and get interesting results!