Black iron slip test result.

As planned, I electric-fired several different slips under two types of glaze to compare the results. Both the iron slips were a 50/50 mix of the oxide and a white slip. The black magnetite slip does indeed give off oxygen at cone 10 (in oxidation), perhaps slightly more than the red iron oxide! It certainly produces different kinds of bubbles in the shino..more of a honeycomb than open pinholes. The key difference is that the black magnetite slip stays a much darker shade of brown, and looks blacker under the shino glaze (top half ).

The black iron produces a very rich, dark brown under a transparent ash glaze (bottom half)

The red iron oxide (synthetic) produced a less dark brown, which looks reddish purple under the shino. The bubbles in the glaze seem to be smaller, although this is also affected by glaze thickness. It's worth noting that where the glaze is very thick near the top of the jar, there's rather alot of pin-holing going on over the bare clay too.

Interesting that the Masons black stain has come out a very similar shade of brown to the red iron oxide under the transparent glaze, but looks darker under the shino, very similar to the magnetite slip:

slip with 20% Masons black stain

The Masons stain has also bubbled a fair bit, though less than the magnetite. To my eye, an equal mix of black and red iron slips looks more like the red iron than the black:

Finally a couple of pieces I decorated using black and red slips:

Wine cup (magnetite slip outside, red iron oxide slip inside)

Sake cup (magnetite slip)

So what have I learned from all this? Magnetite slip seems to give a richer, darker colour under transparent ash glaze, but is harder to decorate with due to the larger particle size. To minimise bubbling and pinholing with either material under shino, the secret is very thin glaze application. (I'm also wondering if a heavily crawled shino may help in allowing trapped gases to escape around the glaze) Of course I've yet to try genuine black iron oxide, FeO .. but I don't know where I can buy it. Does anyone know of a good source in the UK?