High-manganese glazes

The latest firing was an opportunity to fire a batch of pieces with glazes high in manganese dioxide. I have to say, I am reluctant to fire with manganese as there are health hazards to consider..it does volatize at high temperatures and the fumes can be toxic if inhaled. But I figure if I keep everything well ventilated and only fire with it very occasionally, the risks will be minimised..

Anyway, I was pleased with most of the work which came out this time. The manganese glaze is a very simple recipe (23% china clay & 77% manganese dioxide) which should, in theory, create a kind of grey-black, pewter-metallic finish at 1240 to 1260 degrees C, in an oxidised atmosphere. Given that I only fired to cone 6 with a 15 minute soak, I was surprised that the stoneware pieces came out very shiny..the glaze had even begun to run, even at this relatively low temperature (low for stoneware, at least).

This sculptural piece shows how glossy the glaze had become:

Sculpture, approx 5 inches long

A much more matt finish was achieved where I had brushed this glaze over an existing layer of black slip:

Sculpture, approx 4 inches tall

My electric kiln tends to fire very hot and often the temperature appears to have gone higher than the bent cones would suggest. Perhaps the rate of temperature increase is too high..it's difficult to tell, as I don't have a pyrometer with a digital read out.

Well, I think (or am guessing) this glaze works better over a black slip because the slip layer is melting slightly, allowing the glaze to fuse with the red clay beneath, which also contains extra red iron oxide, cobalt oxide and yet more manganese dioxide. Where there is no slip layer, it seems that the glaze simply melts over the stoneware body and begins to run. If there is a next time with this glaze, I'll fire a cone lower to cone 5 and see if that gives a less glossy finish.

This next piece was glazed with two types of manganese glaze, one of which has some iron ochre in it, giving a slightly reddish tinge:

Lidded vessel, approx 5 inches long

We're having a mini heat wave here at the moment (okay, 27 degrees C is boiling hot by our standards! ;) ..it's amazingly sunny for the time of year so the raw pots are drying out in hours rather than days. Yesterday, I was out most of the afternoon and came back to find several cups and bowls I threw a day or so before were already rock hard..too dry and crusty to trim the bases. Well, I don't mind..it's wonderful to see a bit of the summer which never really arrived earlier in the year. Outside, there is that lovely warm, damp smell of summer evenings..long may it continue!