natanata Posted March 31, 2020 Report Share Posted March 31, 2020 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 Link to comment Share on other sites More sharing options...
oldlady Posted March 31, 2020 Report Share Posted March 31, 2020 so sorry this is happening. i have had something like this happen in the past and the folks here have answered the question several times. they will wake up in an hour or so and i am sure they will have an answer for you. Link to comment Share on other sites More sharing options...
carolross Posted March 31, 2020 Report Share Posted March 31, 2020 I've had the same problem and still do sometimes. The bubbles on my work looked just like yours. Although there were suggestions about needing a longer bisque for outgassing, I did 12 hr bisques (elec) and that didn't help. Certain underglaze colors produced the bubbles more consistently than others. I spoke with a chemist at Amaco who couldn't resolve it for me. What finally made a difference for me was changing the commercial underglazes and glaze I use, from Amaco to Mayco. Perhaps an incompatibility with the clay I use? I don't know if that info will help... Link to comment Share on other sites More sharing options...
Min Posted March 31, 2020 Report Share Posted March 31, 2020 Looks like a classic case of blebbing (bloating) from over firing the clay. Body is breaking down from being over fired and is off gassing into the walls of the pot. Were the pots all made from the same batch of slip? How big is your kiln, what's your firing schedule and were the ones that don't have the problem closer to the center of the kiln shelves? Do you have some cones to verify what the kiln is actually firing to? What does the clay manufacturer say the top temperature for the clay is? Link to comment Share on other sites More sharing options...
neilestrick Posted March 31, 2020 Report Share Posted March 31, 2020 I agree, it looks like bloating. Are you using cones to determine heat work, or just going off your pyrometer? Pyrometers aren't always precise, and they lose accuracy as they wear. Try firing cooler and see if that helps- it should. Link to comment Share on other sites More sharing options...
natanata Posted March 31, 2020 Author Report Share Posted March 31, 2020 23 minutes ago, neilestrick said: I agree, it looks like bloating. Are you using cones to determine heat work, or just going off your pyrometer? Pyrometers aren't always precise, and they lose accuracy as they wear. Try firing cooler and see if that helps- it should. 2 hours ago, Min said: Looks like a classic case of blebbing (bloating) from over firing the clay. Body is breaking down from being over fired and is off gassing into the walls of the pot. Were the pots all made from the same batch of slip? How big is your kiln, what's your firing schedule and were the ones that don't have the problem closer to the center of the kiln shelves? Do you have some cones to verify what the kiln is actually firing to? What does the clay manufacturer say the top temperature for the clay is? 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. Link to comment Share on other sites More sharing options...
Mark C. Posted March 31, 2020 Report Share Posted March 31, 2020 No the slip would not go bad from age and start to bloat. As long as the slip is defocualted corectly and mixed right with a flow cup it should be fine. If on the other hand you are not measuring the slip well then its just harder to work with but should not bloat. Looks overfired to me like others have said Link to comment Share on other sites More sharing options...
Min Posted March 31, 2020 Report Share Posted March 31, 2020 1 hour ago, natanata said: 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 The nearly 1/2 hour soak at the end of the firing plus what you said about your kiln reading cool is likely taking you over the 1250C mark. Get some cones in the kiln to measure what the heatwork actually is. When work is thin I've noticed it tends to bleb/bloat more if it's overfired than a thicker walled piece. It sounds like your glaze needs to be fired hotter than your clay. Link to comment Share on other sites More sharing options...
natanata Posted March 31, 2020 Author Report Share Posted March 31, 2020 35 minutes ago, Min said: The nearly 1/2 hour soak at the end of the firing plus what you said about your kiln reading cool is likely taking you over the 1250C mark. Get some cones in the kiln to measure what the heatwork actually is. When work is thin I've noticed it tends to bleb/bloat more if it's overfired than a thicker walled piece. It sounds like your glaze needs to be fired hotter than your clay. 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 Link to comment Share on other sites More sharing options...
neilestrick Posted March 31, 2020 Report Share Posted March 31, 2020 @natanata It's not about temperature, it's about heat work, which is temperature over time. Adding a hold to the end of a firing increases the heat work, which affects the clay and the glaze both. A 26 minute hold will get you about one additional cone of heat work. So even though you're going to a lower temperature, you're still getting too much heat work for the clay. As for why it's suddenly happening, it could be due to poor mixing of the slip, where something has settled out and now that you're getting to the bottom of the batch there's too much of it. It could also just be a case of your thermocouples not reading accurately any more. Have you put cones in recently? Link to comment Share on other sites More sharing options...
Mark C. Posted April 1, 2020 Report Share Posted April 1, 2020 In ceramics when things go wrong after doing the same thing for years means chages have occurred . Things are not static in ceramics . Neil expressed some reasons for this and Min did as well. In my short 40 plus years with clay full time things go to crap in one firing which has happend many many times in my career .The first thing we tend to do is think the clay or glaze is bad or whatever often thats not the case. Often its not one thing but two or more or a perfect combination of subtle changes .First is cone -they measure what everyone above is calling heat work. your thermocuple does not measure heat work-its a static temperature reading and its often wrong as the thermocouple that it gets its reading from ages over firing and degrades giving off poor (not exact temps) cones do not do this. Many who fire electrics believe the number on the screen which is a crap shoot at best and mis really is just a guide. Cones are exact and measure the time temperature (heat work) of that fire. Your glaze and clay react to that heat work.You mentioned some variables like slip mixing and firing cooler or longer or shorter soaks (using a temp number from a screen which relys on thermocouples which age and send out different signals as time goes by.Hence inacurate readings. The differance of cones is very close temp wise really (look at a cone chart) to see that minor difference -Meaning a small soak time means a lot more temp even say 10-15 degrees. We often say test- test-- test here and in this case thats the way to get back to making things work once again.Cones will help now that you need exact info on what s happening in the kiln as nothing is as good as cones. Let us know the outcomes as testing goes by. Link to comment Share on other sites More sharing options...
natanata Posted April 1, 2020 Author Report Share Posted April 1, 2020 Dear Mark, Neile! Thank you so much for your time and your thoughts on this. I found this an interesting thought: 9 hours ago, neilestrick said: @natanata As for why it's suddenly happening, it could be due to poor mixing of the slip, where something has settled out and now that you're getting to the bottom of the batch there's too much of it. It could also just be a case of your thermocouples not reading accurately any more. Have you put cones in recently? 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! Link to comment Share on other sites More sharing options...
Bill Kielb Posted April 1, 2020 Report Share Posted April 1, 2020 4 hours ago, natanata said: 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! I’m not sure what the soak provides you but it often exacerbates things like bubbles. Folks tend to think it might smooth things out but these are thin walled items. Often adding holds at the top temperature only makes these problems worse. Here is an interesting thing to contemplate, if your kiln was +20c higher at some point previously, your elements continue to wear with each and every firing. When that occurs, it takes longer to fire as the element output decreases so If you check with cones today are you worn out to +30c? No mention of the time it takes to fire. It’s always good to keep track. With new elements your kiln may take hours less than with elements that have 150 firings on them. All those extra hours struggling to achieve a temperature ends up to be one of over firing. Add in the variability of mixed product and all of a sudden you have multiple issues in effect. I would suggest you start to take notes: time, temperature, actual witness cone from the current firing........... it’s hard to speculate whether your slip was not mixed as well and therefore slightly more sensitive to over firing. It also seems your glaze is not so compatible with your clay temperature wise. You may want to find components that are more temperature compatible so you have a bit of room for safety. My first guess, if you were +20c when you program to match you cone chart end of firing rate your elements were fairly worn then. Check how far out of spec your elements are now, It may be time to change them. Too many things possible here to not just check off each one. These firings seem to be sensitive, maybe more than one thing going on here. Regardless, it all seems to center on temperature / heatwork so it sounds like figuring that out first would be productive. Link to comment Share on other sites More sharing options...
curt Posted April 8, 2020 Report Share Posted April 8, 2020 If you are recycling clay it could be contaminants getting picked up in your clay body from somewhere in your process, which may explain why some pots have it and some pots don’t. Have you tried firing with brand new clay right out of the bag. Not thrown, not even wedged, just straight out of the bag and in to the kiln? Link to comment Share on other sites More sharing options...
Mark C. Posted April 9, 2020 Report Share Posted April 9, 2020 I suggest less soak time-My experience is in gas kiln firing to cone 11 not electronic screen settings but its the same thing really. Less soak time lowers the heat work.I fire with cones and the time it takes to get to that cone (thats the heat work) say you get to cone 6 in 5 hours then you fire again and take 10 hours -that time spent at the end of the fire close to the end point (the place where the cone gets to be where you want it) thats the heat work. more soak at the end really is more temp and some glazes like this and some do not-same with clay bodies so its really more complex with both clay and glaze together . This takes time learning whats best for each. Link to comment Share on other sites More sharing options...
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