One of the potters in our studio turn us to on another potters blog about glazes. He uses a glaze he calls amber over a glossy clear white liner glaze (recipe from Mastering Cone 6 glazes). We mixed this combo up and voila, a new instantly awesome glaze. I will be posting a picture soon. It looks like an oil spot.
Well the purpose of this post is to discuss what is physically happening in the glaze to create this effect. I find it absolutely fascinating. It is as if the amber glaze was bubbling up through the glossy white glaze. And if you think of what is physically occurring in the kiln this is exactly what is happening. His amber glaze has whiting in it which is a calcium carbonate. However it is on ~45% calcium carbonate and they rest of the stuff in the chemical gets burnt out during firing. So essentially what is happening is that when it is firing the clear glaze starts melting while the other stuff underneath with the whiting starts trying to burn itself out and exhales through the glossy white to leave this little pinholes of the chemicals underneath.
When it pools on the bottom of a glaze it leave rivulets of whiting mixed with the amber colored glaze. It looks like a cone 10 oil spot. You can see more of his blog post here:
From his site: