Tuesday, 25 May 2010

A Nasty Outcome

This was the best shelfload in the refired kiln. I refuse to show the nasty stuff that was below it.
I lined them all up, grouped in sickly colours, bubbled glaze, nasty textures. Ugly.
The new sister-in-law said "I can take them and sell them in my dad's shop". Oh no. Oh no no no. She did not understand why I had a hammer in my hand.
There was NOTHING in that kiln that I'd want to sit behind at a craft fair and take responsibility for. Nothing I'd feel even satisfied by, let alone proud of. Nothing I wouldn't feel ashamed to sell.
And it's the glazing, and the glazes that have killed them. The dark blue plate  had rounded  edges on the break, so it broke in the re-heating, not through over vigorous cooling. More even glaze thickness would help, but again, some of the glazes don't seem to have matured, and definitely don't match the descriptions in the catalogue.
So the next stage will be test tiles, new mixes, line-blends, triaxial blends... and an attempt at better control of thicknesses, I stuck one mug VERY badly to the shelf, it was double dipped.
I'll need to grind that bit. 
I'm a bit despondent right at this minute, to be honest. 
I've always been more interested in the making than the glazing, but this is ridiculous.

Bash bash smash, gimme a beer.


  1. that sucks. any idea why that happens? i lose alot of my plates like that and not sure what i'm doing wrong.

  2. How did you do that? Faster than light commenting, before I've finished correcting my spelling!

    This particular break definitely occurred in the heating cycle before the glaze re-melted. The plate had already been fired once to cone 9. As a result, it would have been less able to stand differential expansion stresses than a more porous bisque plate.
    The shape of the break line is a classic "dunt".
    Plates are more susceptible than smaller, or more spherical pieces, but at two points in the firing, there are points where molecules in the clay change "phase", and suddenly alter in size by about one per-cent. If it all happens at the same time, no harm is done. In a fast rise or cooling of temperature, it can happen that one side of a plate is trying to be 1% bigger than the other. BANG!
    My bet would be that this occurred during the quartz inversion at 573 degrees C (1063F) Slower firing at this point would have saved it.

    Cooling pots, and refired pots are at increased risk when passing 226C (439F), where the silica/cristobalite molecule suddenly changes between Alpha and beta form with a massive 3% volume change.
    Don't take vent plugs out or lift the lid on cooling until well below that point.
    Good luck!

  3. boy, i must have hit it just right, when i commented there was only the picture lol. you're a little over my head on the technical aspects, but i'm going to save your comment and do some research. it does make sense and i think it's time for me to get a pyrometer. that and get my husband to go ahead and make the kiln controller for me he keeps talking about. i'm losing about 40% of my plates and i'm getting frustrated.

  4. I have a few nice recipes I would be happy to share :)

  5. Kim: I've been to your site... You do all that without a pyrometer?
    Wow. I'd almost say you don't need one, you're doing so well without it.
    However, the plate loss-rate calls for desperate measures, pyrometer would help.
    Unless Hubby is a very very patient genius, I'd recommend just buying a ready-made controller. U.S. prices seem better than over here, bigger market, I suppose. Ebay's a good source. The pyrometer and thermocouple type must be matched, or it will give false readings.

  6. Gary:!!!!!!! Yes Please!
    I'm a desperate muppet here.
    I fell for the pictures in the catalogue, though they were postage stamp sized.
    I'm mourning the loss of my tatty old glaze book. And I'm almost ready to take up wood-turning instead.
    I'd be very grateful for a recipe or two.
    Hey! I've got a Gary Rith dragon mug now!!!!

  7. thanks! lots of experimenting w/ that darn kiln sitter and temp knobs. i figure if i'm doing ok w/o a pyrometer, imagine how i'd do if i was using one. i've taught myself everything so far, but some things are a bit technical for me. i live close to the Archie Bray Foundation and i'm taking a class this summer. maybe they can help w/ my plate problem. i've been thinking of resulting to slab plates which are ok i guess, but feels like cheating for some reason.

    as for the kiln controller, they're around $900 for the after market ones from skutt, i can get the controller part for $200 from Bartlett Controllers, and they give you the diagrams to build the whole thing along w/ part numbers. my husband's pretty handy and has an electrician friend who said he'd help him if i make him a mug. worth more than a mug to me, that's for sure! i've seen the ones on ebay, i just don't know what i'm looking at, the Bartlett ones are used in the controllers you get w/ your kiln. if you know what i should look for on ebay i'm all ears! i'm sick of treking out to the studio in the middle of the night to fiddle w/ the controls. (needless to say i get a little freaked out sometimes lol)

  8. Commiserations.
    Definitely too much heatwork, so overfired body.

    Keep on going!!.

  9. Try a little sand under the plates too, that sometimes helps them move. Good luck, that's way too much loss. I may copy all the details of firing for when I get my new kiln.

  10. Overworked and overtired,
    will reply to comments tomorrow, but thanks, everybody, for them!

  11. Make it a ginger beer, and I'll commiserate with you.

  12. Kim H, your kiln controllers are a bit different to ours, as are your thermocouples, so far as I can see.
    I see a lot of U.S. kiln manufacturers, like skutt, do built-in models. Ours tend to live outside the kiln, and connect through a standardised multi-pin connector.
    As little as 25 dollars will buy you an industrial controller/pyrometer on ebay.
    The one I'm using is a Safe-fire 3000, from Cromartie Kilns. It has a built-in bunch of programs, which comprise a delay, 1st temperateure ramp in degrees per hour to first set temp, second ramp to peak temperature, soak period, cooling ramp, off.
    All of these can be used as set by the manufacturer, or altered as desired. Switch it and forget.
    I think I messed up by setting the second ramp too fast.
    However, some potters out there fire as fast as 55 minutes to cone ten, and get away with it.
    Kiln sitters are things I quite admire, because they switch when the cone bends, and that has to be a good thing, a hundred percent reproducible heat-work.

  13. gz. Wise words. If you could see the really horrid pics, you'd see the bloaty body.

  14. Linda Starr:I used to use the sand method, but I kept tipping it onto stuff beneath. In this case it wasn't that kind of crack though, the foot ring was intact, just the rim that parted.
    Good idea, though, I'll get some for when I do big things.
    Thanks for commenting.

  15. RDG. You drink the Ginger Beer, I'll have a pint of Black Sheep Brewery's "Riggwelter".

  16. Kim H:
    This from the archives might amuse you:http://gritinthegears.blogspot.com/2009/08/of-kiln-programmers-pyrometers-and-past.html

  17. kiln sitters are great but once you hit temp the kiln shuts off and cools naturally. fine if you want transparent glossy glazes, not so much if you're looking for matts which i prefer. right now i let the kiln do it's thing keeping an eye on it, when the lever drops i turn it back on and fiddle w/ the controls for a 3-4 more hours. it's working but a pain, i'd love to set it and forget it. checked out ebay and the link you gave me. a little technical but i'm working on figuring things out, if i can get one of those controllers off of ebay for as cheap as it looks i'll be super happy, and have you to thank for pointing me in the right direction. :)


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