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natanata

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  1. Dear Mark, Neile! Thank you so much for your time and your thoughts on this. I found this an interesting thought: Yes, I have put cones (I have used PTC Rings) and, as I mentioned before, they showed that, yes, my kiln is firing ca. 20 C more than it shows on a screen, with a hold of 26 minutes. What I already did is lowering the temperature, BUT in this case, the exterior glaze didn't mature well, it was underfired. Bubbles where still there though. What puzzles me - is why did 2 cups out of 5 didn't have bubbles, although they were standing at the same spot, on the same shelves, close to each other. So minor temperature difference. And this already happened before, when one batch of cups was ok, and then the next firing - all cups or almost all had bubbles... It would be clear to me, that yes, it is overfiring issue, if all of them would have this problem, but not when just some do! Anyway, my next step will be trying a new batch of porcelain and see - maybe what Neile's comment is exactly what is happening. Mark, from your experience, could you give some advice on how to lower the heatwork the best? Shall it be done through lowering the soak time? Or leaving the soak time and lowering the temperature on the screen? Like Mark said, if 26 minute soak could rise heatwork for one Cone - lowering soak time could in a way help, if we talk about overfiring. Thank you again to all of you! Its a relief to be sharing this and receiving your support!
  2. Thank you, Min! I just find it somehow unlogical - before no such problem occurred and glaze on exterior was firing fine. now, to solve the problem, probably, I need to fire lower, which will lead to exterior glaze be underfired. If the only reason for the bubbles is that the kiln due to aging is firing higher than it should, I should be firing lower. But what I find puzzling is - how did it all worked before, for several years, meaning, no bubbles and exterior glaze firing well. Whereas now, if I lower temperature, maybe bubbles will go, but the exterior will be underfired... I wonder if changing, shortening soaking time could probably have some kind of impact on the result. Could it possible be that soaking for 1/2 H is too long and this is the reason why kiln is overfiring.. hmmm... Thank you anyway for your fast replies! Feels good to at least discuss this a bit, as this situation makes me crazy at the moment
  3. Thank you all for your replies and hints.. I appreciate it so much and hope to find the answer to this. Yes, over-firing, that is also what I thought! That is why I did fire one kiln with 1220 (This porcelain can be fired up to 1250) - and the bubbles where still there, not as big, as on the photos, but still all over the pot. And the exterior glaze was then underfired... I also fired one cup with no glaze, to see if the bubbles will show up, but cup with NO glaze, had no bubbles. That is why I though that bisque firing was too short. I switched to 1 hour soaking at 1030 C bisque. But it didnt change anything. Yes, I have used cones to test the temperature in the kiln. It showed, that the kiln fires ca. 20 Degrees more than it shows. But as mentioned before, I have already tried firing lower. I have also fired other objects, with just transparent glaze only on interior - at the same temperature 1230/40 - no bubbles! If thinking, that this is overfiring, then those objects should have bubbles too.. Two questions here: Porcelain slip I used for the cups is almost over and I have been using this batch of slip for a longer period of time, could it be that it has gone bad? For those lasts cups with bubbles I have been using really last scraps. Can there be a problem with that? Firing schedule I need to check it again, but as I remember it was 100 C/H until 300 C 130 C/H until 1230 26 minutes soak I have mixed a new batch of slip, and will try it out.
  4. Dear Colleagues, please help! I am puzzled – I didn't have this problem before and in the last months I have been trying to find out, why this is happening but am left clueless, as I have already tried several recommendations. Maybe you have experiences similar problems before. Problem in short: Cups come out with bubbles on the surface and in the clay. See the picture. If you break the cup, bubble is empty inside. Cups are made using Limoges porcelain, slipcasted, fired at around 1240 C. Bisque-firing at around 1020 , soaking for 1 Hour (I am using word "around" because I know my kiln fires ca. 10-15 Degrees more). I have already tried firing lower, but same problem occurs. I have used this batch of porcelainslip before and never had this problem. I have also fired one cup without any glaze - no bubbles. What puzzles me is that in the last test firing I made, I have placed two cups on every shelf to see if maybe slight difference in the kiln temperature could make a difference - and, as seen on one of the pictures, two cups from the same shelve - one cup is having bubbles and another doesn't! What the heck? Feeling really frustrated... As orders are waiting and I am tottaly clueless what am I doing wrong.. Maybe someone knows whats wrong? I would be so grateful for any hints... Hope you are healthy and save...! Many Greetings from locked down Germany, Nata
  5. Hello everyone! Another question from me: is it possible to cast a flat plate (without a foot) with a one-piece plaster mold? I am usually throwing a prototype with a simple white clay on a wheel, then wait until its dry enough and cast a mold, but when i tried making a plaster mold for a flat plate, plaster seemed to heavy for the clay to hold, clay got softer and caved in. As the result: the mold is with a little knob in the middle. Any advices? Thank you! Nata
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