This is what he says about slips (or engobes) which contain 10% iron oxide:
"There is a fundamental problem with any all-purpose base slip recipe, arising whenever making black and brown slips, which might contain 10% iron oxide. Red iron oxide is refractory (melt-resistant), but any reduction firing and all high firing will convert it to black iron oxide, a very powerful flux. A slip base formulated from a standard porcelain body might contain 25% potash feldspar, and the addition of 10% iron oxide will produce a serious flux overload, often resulting in boiling, blistering, and crawling at high temperatures. To address this problem, we use two versions of the base slip recipe. One contains 20% potash feldspar, and is intended for all colors except black and brown. The second version, intended for black and brown slips, contains 12% potash feldspar in order to compensate for the addition of the iron oxide."
So, as well as decomposing above 1232 degrees C and letting go of oxygen molecules, red iron oxide also converts to black iron oxide, which has ten times the fluxing power of r.i.o.!
I haven't used porcelain slips (or engobes) but I was using a red clay as the base for my slip, which may contain around 5% or more r.i.o. and then adding an extra 4% r.i.o., so the slip may have become overloaded with it. Both manganese and cobalt also act as fluxes, thereby adding to the problems. As a next step I plan to make a dark slip (probably more dark brown than black) using a stoneware clay plus 6% black iron oxide, 4% manganese dioxide and 2% cobalt oxide. I'm hoping that the use of black iron oxide (FeO) will avoid the release of oxygen under the glaze, and the lower concentration of iron will reduce the likelihood of the slip layer blistering.
The other interesting point made is that thin applications of slip can be made opaque by adding zircon, something I'd only used previously to make pure white glazes.
Here's one of my bottle vases from the last firing, with poured black-slip decoration..this one came out with no faults:
|Bottle vase, approx. 3.5 inches tall.|
Oxidation firing, cone 9.