Monday, 7 March 2011

your help needed...

Here's the firing results from K9 on the wkd.... as you can see we have a rather large crack in the big pot and a tad over fired as I knocked the cones from their perch whilst adding some twigs to the firing only the cone 10 remained but i wasn't sure which cone it was until the unloading today.... belinda's dragon now has extra claws....

We did a pre warm up on friday evening and the pots were still warm sat morning when we lit the kiln again, it was still a long firing the kiln only has one pilot and one main burner its a 'port a kiln' model
and the firing log is below and ideas on the cracking issue...
any comments greatly appreciated.... I have done this to one of my pots before in a gas kiln and just assumed the firing was too fast and the shock too great....or is it a cooling crack???
Will get details about the clay body used looks fairly dense to me..


  1. Hi Ang,
    Firstly, I'm so sorry for you, it will be really frustrating having work breaking like that. I realize that you have also had problems with big platters a little while back too.

    First question..., is the pot raw glazed or did you bisque fire it before glazing?

    It is a very clean looking break, and some would say that a clean break with no glaze in the crack would indicate that it happened on the way down. From the distortion that has taken place though, I would have thought that the pot would need to be cracking on the way up, and then moving and continuing tearing as the stresses in the clay work through the pot when the clay is elastic and near top temperature. Has any of the brown (manganese??) shown any sign of moving into the crack at all.. not sure if it would have been runny enough?

    From your graph of the firing, you look to be climbing at about 250 degrees per hour after you light the big burner at about 350 degrees Centigrade. I know that some potters happily fire that part of the firing at speeds higher than that, but I tend to be more boring... and everyone will laugh at me when they read this, but I almost always keep to below about 150 degrees per hour maximum when I am below 650 degrees Centigrade, and only climb above 200 degrees per hour when the kiln is clearly glowing inside. I have been experimenting with firing faster with small pots, and so far OK, but I know that I have ruined a couple of larger pots in the past by cranking up the kiln too soon.

    There are two critical temperatures to watch out for, they are approximately 225 degrees C and 575 degrees C. At these points the pot will expand when the kiln is being fired up, and will contract when it is cooling down. If your clay is unforgiving (and I suspect that is is), you may find breakage can occur at either of those points in the firing. My guess is that you were in trouble at 575 degrees.

    The other possible cause of trouble for you could be cold air leaking into the kiln as it is cooling. I did help someone once who kept losing pots, and it turned out that she always fired her electric kiln with the middle bung open. She got away with it until she switched to a different sort of clay, then she frequently got daunting cracks. When she took my advice to fire with the bung closed, she had no further trouble. With your kiln being gas, you could easily have air leakage around the burner ports, and if there was any gap in the chimney damper after you clam up the kiln at the end of the firing, cold air would travel through the kiln. If you can really seal off the burner ports after firing, you would rule out that potential issue.

    A couple of other problems could be, too fast a firing when the work was bisque fired (if it was), or some issues with raw glazing (if the pot was raw glazed).

    Anyway, some thoughts for you to play with. Hope that it all sorts out for you soon..

    Kind Thoughts, P

  2. thanks peter, a whole bunch of thoughts to go on there....I really was following a firing cycle of much smaller pots so that need to be considered in the future and yes i usually fire my big works on low overnight before bringing up the temp in bisque or glaze....this is not my pot and i'm not sure of the bisque firing conditions either...will sort that info out this week...this kiln has a tendancy to jump when the big burner is brought in even when we wind back the gas pressure, so thats a problem...and the burner ports dont have any closing up mechanism i'll find some fibre to help with the flue I just used old kiln shelf across that, so it can be sealed better :)) thanks again peter!! I think you are right about the clay body too...

  3. I haven't got a clue, but I sure learned a lot from Peter's comment. My first thought was the clay body, but I'm certainly no expert. Like Peter, I fire really slowly, I ramp up 200 degrees an hour, I have a gas kiln and this seemed the only way to get rid of my pinhole issues.. cool dragon by the way! Such a shame to lose such a beautiful pot, sigh...

  4. hey trace the body I'm thinking is part of it too, I agree 'slowly as she blows' with the firing of these bigguns..cheers dragon boy came out a treat!! raku clay body!!! thats the secret....

  5. Hi Ang - I'm afraid I don't have any help to offer... only lots of empathy and sympathy. I'll leave the expert advice to the firing experts! Hope it gets worked out.

  6. How sad to open and see that breakage on great peices :( interesting that the break is so clean -almost a shrinkage/distortion break and not an explosion type of break that usually comes from clay body issues. Looks thermal(?) related.

  7. How sad to open and see that breakage on great pieces :( interesting that the break is so clean -almost a shrinkage/distortion break and not an explosion type of break that usually comes from clay body issues. Looks thermal(?) related. I enjoyed reading Peter's comment -always fun to learn so much from our Blog buddies!

  8. sorry to say, i have no idea. it is amazing though that it is so huge a crack. maybe that indicates that it's an anomaly of some sort. crazy!

  9. thanks patricia I appreciate it :))

    hey cindy yep a wealth of info flowing in and I will consider all of them my main points of interest so far are rate of climb and clay body issues..

    howdy jim an anomaly indeed, it's a very jagged break...