Black iron oxide: not necessarily what it says on the tin

I was reading Nigel Wood's fascinating book, Chinese Glazes, yesterday and came across a section on the technology of northern Cizhou wares. Particularly interesting to me as they made glazed wares coated with white slip and decorated with black-slip painting (produced from the late 11th century AD). Chemical analysis of the glaze has shown that the main component of the black slip was magnetite, the magnetic form of iron oxide, Fe3O4. It is not clear from the text whether magnetite was still present as Fe3O4 in the slip after firing (to 1240 - 1290 degrees C), or whether it had been wholly or partially transformed into one of the other molecular forms of iron oxide.

Apparently, magnetite is quite difficult to grind into a fine powder, so one wonders if the 11th century Cizhou potters had the technology to pulverise the raw mineral, or whether they collected it as relatively fine-grained deposits found in river banks and beaches, as this artist, Zachary Kator has done.

Many of the Cizhou pots were also made using sgraffito techniques, the pot being coated with successive layers of white and black slip, then carving and scratching the design through the black layer. It's interesting to speculate whether magnetite was used because of it's larger particle size compared to other forms of iron oxide. This kind of decoration would be very difficult to do with fine red iron oxide as it would almost certainly stain the white slip layer with pigment.

Song Dynasty Meiping, Cizhou Ware, Stoneware

For the past year or so, I've been experimenting with various formulations of black slip, and had begun using black iron oxide (what I thought was FeO) on the basis that this could reduce the amount of oxygen given off by the slip at temperatures above 1232 degrees C. Above that temperature, normal red iron oxide Fe2O3, begins to decompose to FeO, giving off oxygen atoms as it does so. I've found that this can cause thicker shino glazes to bubble and blister quite severely when fired in oxidation at cone 10, so I thought using black iron oxide might alleviate the problem.

Many years ago I bought a tub of "iron spangles", the name given to a very coarse form of magnetite that looks a bit like silvery iron filings..this is usually used to create a speckled effect in clays and glazes. However, I just read a post in Clayart from 2000 by David Hewitt (1925 to 2006) where he showed that most black iron oxide supplied in the UK is in fact a more finely ground form of magnetite, Fe3O4, not Fe0 ! This is quite significant, as magnetite is said to decompose to red iron oxide under gentle heating ..which in theory renders pointless my attempts to use black instead of red iron oxide!

But the thing I find intriguing is that magnetite in its finely ground state seems to give a more intense black colour than red iron oxide at stoneware temperature. And yet magnetite is supposed to turn back into r.i.o. (Fe2O3) at only red heat. According to David Hewitt, Fe3O4 heated to 1000 deg C in oxidation gives 1.5Fe2O3 .

In Nigel Wood's text, it is argued that "the status of the magnetic iron oxide used on Cizhou pottery, would have been preserved by the generally oxidising-to-neutral firings". So this begs the question, does some or all of the magnetite finally decompose to FeO in the same way that r.i.o does above 1232 degrees C? If it does, then it should be giving off just as much oxygen as red iron oxide at high stoneware temperatures. It seems that red and black iron oxides do behave very differently when heated, as shown in this podcast about iron oxide use in foundries. It suggests that red iron oxide gives up oxygen more readily than the naturally occurring black oxide.

I also remember reading somewhere that the Japanese use a type of brown iron oxide for under-glaze painting on Oribe style pottery. I'm wondering if this might be a mixture of black magnetite mixed with some red or yellow iron oxide..perhaps an unprocessed mineral which has been ground to a fine powder.

Well, I'm not a glaze chemist, so I intend to carry out a fairly unscientific test of two clay slips made with equal proportions of synthetic r.i.o and black iron oxide (magnetite). I will visually compare the depth and intensity of the fired colours, and the amount of oxygen given off simply by comparing the amount of bubbles produced in the glaze.

I know this is probably not very interesting for most people, but thanks for reading..and please do leave a comment if you can shed any more light on the subject.