Saturday, 23 November 2013

Some nice glazes and interesting discoveries

The latest firing of the electric kiln was not entirely successful, but I was very happy with some of the pots which came out. The Nuka glazes in particular worked very well indeed, having been put over a new slip formula which has reduced the fluxing slightly and retained more of the unmelted silica..that is to say they are a bit more white and opaque than before.

Close-up of Nuka glaze

I have to be careful with this glaze though..when applied thick it does like to run down the pot, and in some cases drips off it! In future I need to leave at least half an inch of space at the bottom to allow for this.

I also tried a new glaze combination; a crackle shino over matte calcium glaze applied to a sake cup. It was fired in a sealed saggar with heavy reduction from charcoal, and the result was very pleasing:

The last couple of firings I've used an external pyrometer as well as cones, and I've found this very useful in monitoring the rate of temperature increase. The pyro' seems reasonably accurate, so I can see almost exactly what temperature the pots next to the thermocouple have been fired too. This carved espresso cup with oribe glaze was fired to 1275 degrees C with a half-hour soak. I noticed that the oribe pot placed on the higher, hotter shelf was slightly pinholed, leading me to the conclusion that I may have been slightly over-firing the oribe in the past by going to 1285 or higher.

Alongside these pieces I also fired a number of test glazes, mainly with high alumina slips and matte ash glazes. All the glazes contained copper oxide and this combined with metal oxides in the slips to create really dark colours..a little disappointing as I was hoping to achieve some green hues as well.

Nonetheless, the results were extremely interesting. The matte ash glaze has made the underlying slip crack and crawl in places, which is an effect I'm trying to achieve. It also cracked and peeled in an interesting way over the Valentines black clay..the glaze is very dark brown where thin and and a paler brown where it's pooled in the nooks and crannies:

I groaned a little when pulling this vase from the kiln, thinking it was way too dark, and a blob of Nuka had dropped from a pot from the shelf above and landed squarely in the middle! But having lived with it for a few days, it's really growing on has a strong, individual character.

Finally, I layered two different matte ash glazes (containing black copper oxide) and a dolomite glaze on this actually has five layers of slips and glazes in total! The results don't look terribly inspiring in the photo but the surface tells me a great deal about how the glazes and slips are interacting with each other. The most interesting thing here is the way the copper dolomite glaze (the paler blobs) is quite matte over one glaze, but more glossy and green over another..where thin it's also darker, picking up colours from the base slips:

The more highly fluxed ash glaze has sealed the base slip (the black patches), whereas the high clay matte glaze has made it crack and split like dried mud. The dolomite glaze definitely has potential, I really like the variety of colours and textures it produces.

Thanks for reading!

Friday, 8 November 2013

Cold and damp November

The other day it was Guy Fawkes night, the 5th of November, and the weather brought back strong memories of childhood, when it was often misty and freezing cold and the smoke from hundreds of bonfires would create a thick fog across the town. The combined smells of wood smoke and exploded gunpowder are extraordinarily evocative. Seems amazing now that in those days we kids were allowed to light our own rockets in milk bottles, throw Bangers around and stand so close to the bonfire that your face would hurt.

Not the best weather for making pots in a studio with no central heating, but I'm trying to make the best of it. If the clay is still too wet for throwing large pots, I focus on making small pieces like these bud vases.

I gain a great deal of satisfaction from throwing these rounded forms off the hump. The extra height makes it easier to watch the whole form emerging, and they are often finished when they come off the wheel..that is to say, the bottoms don't need an extra stage of trimming.

I also did a bisque firing yesterday, which warmed things up a little..and these tea caddies are now ready for glazing.

It's not easy getting lids to fit exactly, and sometimes they get damaged in the firing, or they can warp. So I'm building up a stock of spare lids which are of a similar size but with slight variations in may be possible to match them to pots, if there's a problem with the original one.

Success with the dry glaze in the last wood firing (2nd photo down in the last post) has encouraged me to start exploring other combinations of matte slips and glazes, adding calcined alumina and/or alumina hydrate into the mix. The pieces below are just preliminary glaze tests, fired in the electric kiln, and alot more experimentation will be needed to work out exactly how the slips and glazes are behaving and interacting. The slips here had several metal oxides added, and the glaze contained copper oxide, so the colours came out pretty dark.

Ideally I would like the glaze layer to crawl and take on the appearance of lichen on a rock, in the same way that these previous test pieces have done:

These tests were fired in both electric and wood kilns to cone 9/10. The ash glaze has crawled in a rather beautiful way, but the glaze was applied over refractory slip which had already been fired to cone 10.  So I need to find a way to reproduce this effect without having to high fire twice, which would be even more uneconomical than it already is!

Thanks for reading.