SICA
A synthetic grit that reduces a glaze from the inside · Oxidises within the melt, taking oxygen from the metal oxides and giving off gas · A tiny amount for colour; several parts in a hundred for craters · It greys a glaze on its own, with no colourant present
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Silicon carbide is a synthetic grit that reduces a glaze from the inside. A very small amount brings copper reds out of an electric kiln; more of it blows the surface open into craters.
Silicon carbide neither melts nor burns away in a glaze. It oxidises, and once the melt closes over it the only oxygen within reach is what the glass and the metal oxides are holding — so it takes that, reducing them, and gives off carbon monoxide as it goes. The gas is where craters come from and the borrowed oxygen is where the red comes from, and both happen down inside the glaze while the surface stays open to the kiln and re-oxidises. How much goes in decides which effect dominates: a part in two or three hundred pulls a copper red out of an electric kiln, while several parts in a hundred swells the surface open into craters.
