Monday, February 10, 2014

More Crystals and Crawling!

6 1/2 x 9 3/8 inches (165 x237mm) porcelain bowl with crawled white glaze around rim.

I am writing this to help me stay awake as I supervise the last two hours of a crystalline glaze firing. I started the firing at 4.30 this morning and it is almost 3 in the afternoon now. The electric kiln that I am using is entirely manual, and has two simmerstat controls that are much the same as the ones that you would fiddle with every day on your electric stove as you cook your porridge or pumpkin soup!

7 1/8 x 7 inches (180 x 177mm) porcelain vase with crawling glaze over chrome tin red glaze. A bit strange this one, but it looked quite nice when I took this photo of it outside. It is growing on me!
To give me some idea of temperature I have a second hand pyrometer, the one that I also use in my wood fired kiln. This has an old fashioned analogue display with a dial. The dial on this one isn't too bad, it's predecessor had to be tapped frequently in order for it to un-stick and lurch to a new reading. Wood firings with the old pyrometer were accompanied by woodpecker noises, and fingers and knuckles became red after 10 or 12 hours! Tap-tap-tap... "Are we going up yet?" tap-tap-tap!

Detail of porcelain bowl with chrome tin red, crawled glaze and thin crystalline glaze.
For reasons of its age, and the multiple repairs that have been inflicted upon the thermocouple and its cable, my pyrometer reads 69 degrees Celsius (156F) too low as the temperature nears the peak of the firing. It is odd, to say the least, seeing cone 8 start to bend at an indicated 1185, and be flat by 1200 (2165 - 2192F), (in a "normal" set up I would be expecting that from 1245 to 1260 (2273 - 2300F), but I am growing used to it! Mr Orton's cones are completely essential for this type of firing, so I set cone 8 and cone 9 inside the kiln where I can see them through a spy hole, and put on my welders goggles to watch for them bend when heat and time have done their work.

Porcelain bowl 8 3/4 x 2 1/2 inches (222 x63mm).

Also essential is my kiln log book; in it is every crystalline glaze firing I have done recorded on graph paper. This serves as a trusty visual guide of the progress of a firing that is taking place, and it is my memory of past firings... and it needs no batteries to run it!!

8 1/4 x 1 1/2 inches (210 x 38mm) Stoneware bowl with tenmoko glaze and crawled overglaze with red underglaze... over the top!
Are you asleep yet?!  You may well be, but I am not, which is the main thing! I have been in and out to the kiln shed frequently to supervise the kiln whilst writing this. Of course, I cannot see the pots or the crystals on them, and have to fire them blind. I have to imagine what the pots might look like, and what the glaze is doing. Have I held it too long at peak temperature, or not long enough? Are there hundreds of closely packed crystals, or very few of them?

Detail of tenmoko glazed mug with brushed rutile decoration. I mixed rutile powder with a little nephelene syenite and some water before brushing it onto a previously fired glaze. I judged quantities by eye. You could just use rutile and water, but I added a little nephelene syenite to make it less likely for the area to look "dry" after it was fired.
I have fired more porcelain and stoneware recently. I have experimented further with crawling glazes over previously fired glazes, and over unfired. Some results have been quite nice, and others a bit repellent, but it is interesting. I keep feeling the need for texture.

Porcelain mug, 3 3/4 inches (95mm), tenmoko glaze with rutile decoration.

The large bowls and platters that I have been firing recently were made a long time ago - it must be a year - and it is nice to finally have them finished.

14 x 2 3/4 inches (355 x 70mm) Stoneware platter with tenmoko glaze.
Well, half past 4 is approaching, and I will be shutting off the kiln then.12 hours is a short firing compared to what I usually would do with crystalline glazes, but the pots are small and don't need really large crystals. Also I was able to fire the first part of the firing quite quickly so I saved a couple of hours at that end of the firing too.

Unpacking a crystalline glaze test firing. Only 2 out of 7 pots had satisfactory crystals... some had none at all! Crystalline glazes are demanding and can be lots of work.... Not for the faint hearted!
I hope I have a better success rate than the test firing that I fired a couple of days ago where only 2 out of 7 crystalline glazes worked as they should..., but that is why it is a good idea to test!

I'll sign off now.... Thanks for keeping me awake!


Anonymous said...

Some weird and whackie ones here Peter... Interestingly different and no possibility of being confused with mass-oroduced stuff from the place where "everybody gets a bargain"!
What a shame about only 2/7 test results. The All Blacks have done rather better against India, that's for sure.
That's a superb blue... I do hope it turns out right for the order. D.

Peter said...

Hello "D",
I had an amusing vision of a cricket match being invaded by a rugby team when I read your comment about the "All Blacks", I think you intended "Black Caps"... Very confusing (and daft) this corporate branding of our National teams. I gather the PM would like us to have a new flag as well... Let's see, a silver fern on a black background...! Ha!
Anyway, the cricket match was most exciting, and it was a great relief that our team finally struggled their way to a victory. I actually felt sad at the prospect of either team losing that one, as India did a great job in their second innings.
Really hoping the blue does turn out... it has been a long day with the kiln today!

Michèle Hastings said...

OUr electric kiln is also an old manual version. We only use it for bisque firing but have been thinking about doing some ^6 glazes, especially with the price of propane this winter!
That tenmokku mug is a beauty!93289996

Pat - Arkansas said...

Fascinating, and somewhat daunting, information, Peter. Your efforts produced some beautiful pottery! Love, love, love that mug!

Peter said...

Hi Michèle,
Good to hear from you, thank you for your encouragement, it is greatly appreciated. You have a manual electric kiln too! I think there should be a special society for manual electric kiln firers! I get tempted from time to time to buy a controller for mine, but the cost of them is enormous in this part of the world, and I am so used to managing without one that I'll probably keep going this way.

Hi Pat,
Lovely to hear from you. Sorry that the info was a bit "daunting" this time around, but my attention was mostly with the kiln that I was supervising, and my multitasking skills were being called upon! I keep having a "crisis of confidence" with my pottery, and sometimes wish I could settle for one glaze, but I seem to be always striving for something that I can't quite "see".
It is quite a relief when something turns out! I love the mug too. Easy to take mugs for granted, but I was quite surprised with how nice it looked when photographed. Sometimes the camera allows one to have a fresh look.

Amy said...

Peter- Am catching up on blogs as I've been away for awhile. Have you ever posted about how you remove the pots from those holders that catch the extra glaze? And what are the holders called? Your post reminds me that pottery is filled with opportunities to be patient- and that you are!

Peter said...

Hi Amy,
I have probably mentioned about how I remove pots from their glaze catchers from time to time, but I have never really done a post about it. It probably is a good idea for a post of its own.
I usually refer to the glaze catching bowls under the pots, as glaze catchers. The little stand that the pot actually sits on, I call a stand. Not sure what the "experts" call such things!