PottaFella Posted June 23, 2016 Report Share Posted June 23, 2016 I re-fired the bowl that had a few craters on rim and have now got craters all over! Can someone suggest what I'm doing wrong? Does it look like I over fired this? (standard terracotta with earthenware gloss glaze over) fired to 1100c with 15 min soak PS. I could not add more images to my original post, hence this one. Link to comment Share on other sites More sharing options...
Marcia Selsor Posted June 23, 2016 Report Share Posted June 23, 2016 what temperature was the glaze rated for? You may have over fired the glaze. Marcia Link to comment Share on other sites More sharing options...
PottaFella Posted June 23, 2016 Author Report Share Posted June 23, 2016 The glaze is rated 1060-1160c. There's reviews of this glaze being fired to 1120c with 16 min soak and being v reliable. Has re-firing pushed it too far - too much heatwork? Link to comment Share on other sites More sharing options...
Marcia Selsor Posted June 23, 2016 Report Share Posted June 23, 2016 Maybe it is the clay. How high was your bisque firing? What is the clay temperature rating? Marcia Link to comment Share on other sites More sharing options...
PottaFella Posted June 23, 2016 Author Report Share Posted June 23, 2016 Marcia, the clay is up to 1150c is says. Bisque was to 1000c, quite slow up. Clay: http://www.bathpotters.co.uk/smooth-red-terracotta-vr/p1762 Glaze: http://www.bathpotters.co.uk/leadless-transparent-glaze-powder-b276/p428 Link to comment Share on other sites More sharing options...
LinR Posted June 23, 2016 Report Share Posted June 23, 2016 I have refired many times. Adding even a thin layer of new glaze over the old seems to work best. I had one glaze which could not be refired no matter what. It always ended up with a mass of very small bubbles. I don't use that one any more! Lin Link to comment Share on other sites More sharing options...
High Bridge Pottery Posted June 23, 2016 Report Share Posted June 23, 2016 Not something I have really seen before. It looks closer to underfired than overfired if I am picking but always hard from photographs. I think there could be something in the clay blowing bubbles through the glaze. Coal maybe. The weird pattern on the lower part is interesting. Link to comment Share on other sites More sharing options...
PottaFella Posted June 23, 2016 Author Report Share Posted June 23, 2016 It's very frustrating and has got me down a bit, I have lost a lot of nice work trying to use terracotta and earthenware glaze – I don't doubt it is my technique/I'm doing something wrong Link to comment Share on other sites More sharing options...
High Bridge Pottery Posted June 23, 2016 Report Share Posted June 23, 2016 The glaze besides where it has cratered doesn't look bad. Makes me think it is the clay, not too complicated to try a different bisque firing or even rebisque your pots and glaze to see if that improves anything. Link to comment Share on other sites More sharing options...
glazenerd Posted June 24, 2016 Report Share Posted June 24, 2016 General rule of thumb from my experiences: Pinholes = glaze issue, and craters = clay issue. 1020C glaze firing would be a prime off-gassing temp for potassium fluxed clays. So I will throw my hat in with Marcia and Joel on being a clay issue. Try running the bisque firing up some, with a short hold. You could also do an extended hold when glaze firing, and/or program a slow cool from peak down to 900C on the attempt of healing the glaze. At this temp it is not impossible they are using Nep Sy as a flux: but sodium and potassium off gas a lot in certain temp ranges. Nerd Edit: both the clay and glaze are rated up to 1160C- firing higher might be in your best interest. Link to comment Share on other sites More sharing options...
Magnolia Mud Research Posted June 24, 2016 Report Share Posted June 24, 2016 General rule of thumb from my experiences: 1020C glaze firing would be a prime off-gassing temp for potassium fluxed clays. .... but sodium and potassium off gas a lot in certain temp ranges. Nerd What do you mean by 'off-gassing of potassium and sodium'? What species are being given off? LT Link to comment Share on other sites More sharing options...
Min Posted June 24, 2016 Report Share Posted June 24, 2016 I think glaze blisters are one of the harder things to narrow down the cause of since there are so many possibilities for what the culprit is. If you feel like doing some reading there is a link to a Ceramics Industry 4 page article on blisters below, hopefully something seems a logical avenue to explore to fix the problem. page 1 here http://www.ceramicindustry.com/articles/89057-diagnosing-glaze-blisters-part-1 links on lower left corner for parts 2,3 & 4. A separate topic would be the problems of using a clay and glaze with such wide firing ranges as these have. Link to comment Share on other sites More sharing options...
glazenerd Posted June 24, 2016 Report Share Posted June 24, 2016 LT: I have been studying fluxes from sources outside of the clay biz. Without side tracking the topic: potassium and sodium are in gaseous states when they hit melt temps. At certain points they can off gas as much as 20% of volume. Here is a clay test (no glaze) that had been intentionally over fluxed with potassium. The large craters remind me of the problem described in this thread. Nerd Link to comment Share on other sites More sharing options...
Mark C. Posted June 24, 2016 Report Share Posted June 24, 2016 The deal with refires is the success rate goes down with every fire starting with the 1st fire. That is if it is flawed in the 1st fire the second fire may help or hurt it more maybe 50%-the next fire its less than say 50/50 of the second chance you already gave it. The reasons for the flaws is always a crap shoot. I fire in reduction so thats also adds some more variables . Ceramics is a life long learning curve and many of the answerers are not easy to find.Glaze flaws like these only come with lots of testing. I know no short cuts. Link to comment Share on other sites More sharing options...
High Bridge Pottery Posted June 24, 2016 Report Share Posted June 24, 2016 Try sieving your clay through an 80 mesh sieve and see what is left. Link to comment Share on other sites More sharing options...
Babs Posted June 24, 2016 Report Share Posted June 24, 2016 Slow down your approach to target temp for last phase of glaze fire. 60-80deg C / hr soak . However if you hold your soak at limit of clay probs may arise. At the limit of glaze also (I had success refiring. I think I advised you on last post. Wish I had photo. To support my words). my thoughts are the glaze needs the slow top, pos to a lower temp and hold for 15-20min soak Link to comment Share on other sites More sharing options...
Babs Posted June 24, 2016 Report Share Posted June 24, 2016 What is your firing schedule for glaze firing this? Link to comment Share on other sites More sharing options...
Tim T Posted June 28, 2016 Report Share Posted June 28, 2016 I use that clay quite a lot for my earthenware stuff. My firing schedule is: Bisque firing: 4 hours to 600, then max to 1050, then hold for 15 mins Final firing follows the same but the final temperature is anywhere from 1000 to 1150 depending on the glaze I'm using and the colour of clay I want. I use different glazes, but I haven't seen anything like this. A few thoughts: - try a different glaze but the same firing and if you still get the problem then it lies in the clay - try applying the glaze more thinly, as it is easier for gases to escape a thin glaze layer - try a long soak to see if the bubbles smooth themselves out - say an hour initially, and if that works reduce the time - a bit of a cheat, but it may work. I had a glaze that generated small bubbles when applied to e/ware, but they didn't appear when I had applied slip before the glaze. I put a small amount of nepheline syenite in my slip to help it bond to the clay. So perhaps a thin wash of nepheline syenite before glazing may make the gaze a bit more fluid and sort the problem Link to comment Share on other sites More sharing options...
graybeard Posted June 29, 2016 Report Share Posted June 29, 2016 The first thing that comes to mind is that your bisque was under fired, and when you glaze fired the piece it started out-gassing again. graybeard Link to comment Share on other sites More sharing options...
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