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|
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)|