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About tomhumf

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    Sheffield, UK

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  1. tomhumf

    Help with raw glaze bubbles

    Thanks all. I can see firing higher could help, but already burning through more propane than I'd like so hopefully I can fix this with the glaze. The bubbles in question are all sealed over. There are a few tiny pinholes in other areas but I'm not too concerned about those, I'm not expecting a perfect finish with this body. I would have thought increasing the fluxes and reducing quartz a little would create a more fluid glaze that would get rid of the bubbles. Problem is the CaO and B2O2 levels are already over the suggested limits according to insight live. https://digitalfire.com/4sight/education/limit_formulas_and_target_formulas_206.html Perhaps this is contributing to the problem. I'm thinking maybe another flux like talc or frit3110, may be needed...mainly because I have those in stock.
  2. tomhumf

    Help with raw glaze bubbles

    Bubbles are worst on the inside. I've just read it could be to do with firing schedule. Maybe I need to hold at lower temps before glaze is melting while gases are escaping from body.
  3. I've been working on a raw glaze recipe for application to leather hard pots. I'm firing to ^8 in a new gas kiln. The tests pictured had some reduction during firing. The clay is speckled stoneware. My recipe currently is : Hyplas71 ball clay 30 quartz 15 calcium borate frit 15 wollastonite 15 Soda feldspar 30 total 105 It's looking ok, I just really don't like the bubbles where it's thick - see the top of handle attachment. Any ideas on getting rid of the bubbles? Maybe more frit or wollastonite? thanks!
  4. I think I got very lucky considering I have no clue what I'm doing. The chimney cross section in your kiln looks a lot smaller than mine - if it's still the same as the photo of the round opening. Also, couldn't you just prop your lid for your earlier stage firing? You should keep your thread going, it was an inspiration for me. I read this just as I was starting reduction at ^05 for the second time. This evening I've watched a load of YouTube videos of reduction firing. They all start around 800 Celcius, with hard reduction, then lighter reduction to end of firing. Like LT said though I need to burn off carbon before reduction though as I'm doing single firing. I used to bisque to ^05 with this clay in electric kiln as I had problems with pinholing - though I'm not sure the bisque temp was definitely the issue. So I'm a bit confused but I suppose I'll get some ^08 cones and try it out next time. And maybe do the rest of firing with the chimney a quarter closed or something. I've used a lot of propane so far. The tank was freezing up when I was finishing today. I ran hot water over it, and seems about 1/4 of a 19kg tank left after 2 and a half firings! I was very cautious first time though, and quite cautious today.
  5. Thanks oldlady, I did take some photos when I unloaded so maybe will be able to tell from those. I did a video for my own reference but maybe someone reading this thread may find it interesting.
  6. Clay goes more grey rather than tan in reduction? These two are same clay, must have been in different locations..
  7. It's this one https://www.valentineclays.co.uk/product/stoneware-special-fleck The speckles showed on the bases of previous electric fired pots, but they seem to be showing through the glaze more after the gas firing.
  8. Well I unpacked the kiln this morning. The firing went fairly well I suppose. It did take 10 hours, but I'm sure I can do it faster next time. It seemed to fire fairly evenly. The bottom got colour first, but ^05 dropped on top shelf about 20 mins before the bottom shelf. I think ^5 was about the same, and I missed the ^8 dropping because I didn't expect it to get there so fast. The cone packs look pretty much the same, ^8 is really snakey so maybe I got ^9. The only problem I think was I didn't get enough - or any reduction? All I did was close the chimney so it was about 1 inch left open. And left it for 45 mins around 1060C, then I opened the chimney fully up to finish firing. Perhaps I should have pumped the gas up more, or left the chimney partly closed for the final part of firing... I didn't get any reds from glazes I thought might go red - although they are all trial recipes so it might just be the recipe. But the clay doesn't seem to be reduced - not that I really know, maybe someone else can tell from the picture? Thanks everyone for the amazing help on this project. I certainly couldn't have done it without you all!
  9. Thanks magnolia, I'm going to try a hold at around ^05 I think, and then reduce. I've just lit the kiln. Everyone cross fingers please!
  10. Thanks! I'll move the burner further away for sure then. I'm planning to fire raw glazed pots so I suppose I'll have to have the lid propped wide, and maybe look into some kind of pilot attachment... I plan to try follow what I read Neil suggest in another post about the firing schedule. I'll start firing with the lid off, then shut it and make sure no water coming from pots before increasing temp as fast as I dare without stuff blowing up. At cone 08 I plan to start reduction for 45 mins. From what I can tell the best way for me to do this is to cover the chimney with a brick gradually until flames are coming evenly out of the top and bottom spy holes? And to 'stall it out' I suppose adjust burner power to maintain temp... After 45 mins I plan to go to neutral atmosphere, so uncover the chimney most of the way I suppose. Hopefully the temp will get high enough to bend cone 8. Not sure if I'm going to be able to tell what's going on with the atmosphere really. Apart from reading reduction firing has long smokey flame, I don't know if you can tell difference in oxidation vs neutral without an oxygen probe?
  11. I've just done a little test firing. I was mainly wanting to try my burner out but ended up getting the kiln to about 600 degrees, with no pots in. Having never done a gas firing before I'm not sure, but it seems the kiln climbs in temp fairly fast - I know it will get a lot slower at higher temperatures. I struggled a bit to keep the kiln below 100 celcius, even with the lid propped a few inches. Is this standard practice when getting rid of water? I know my burner is quite oversized for the kiln. I couldn't get my flame any lower without it blowing out. In the end I just about managed to keep it just below 100 degrees. I couldn't feel any heat coming out of the chimney join, even at the end of the firing, so I'm pleased and will leave the ceramic fibre off there for now. One thing I'm not sure of is how far my burner should be inside the kiln. The attached image shows where I started it, and I moved it in about an inch when I turned the gas up. I did a spreadsheet log of my temp rise but not sure how to post it here. Basically it took about 40 minutes to go from 200c to 600c , with the gas still very low. The flame was still mainly yellow throughout, which I think is normal at lower temperatures?
  12. tomhumf

    Recent pinhole struggles

    Just to throw a spanner in the works... Aside from your bisque ideas, does your pinholing occur on all glazes? Maybe try some other glazes, or try longer glaze firing to see if the pinholes heal over?
  13. Just to attach to a handheld pyrometer, so I can check temp rise. I'll be using cones too.
  14. Stupid question alert! I've never used a thermocouple. I'm guessing they are intended to be pushed in all the way so the collar bit is flush... There is some plastic around the wires which I guess will melt if I do this. It's not a totally snug fit as the ceramic bits are slightly oval.

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