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#11
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Those darned bubbles! - A question
On Tue, 9 Oct 2007 13:01:03 +0200, "Bubbles_"
wrote: Original post below. I am just wondering about something. My wheel is aluminium, and it oxidizes, so that I get a black stain on the back of my left hand from centering - but I also get it in some of the clay that I later reclaim. Is aluminium a possible cause of these bubbles? Should I stick to new-bought clay and/or treat my wheel with something so it doesn't let off particles? No, this aluminum should not cause any problem at all. Note that aluminum oxide melts at a very high temperature (stiffens glazes), whereas whatever is causing the bubbles is something that is actually boiling, or more likely burning, at your firing temperatures. The combustion gasses are released as bubbles that appear in the slow-to-heal glaze. (There are other possibilities, like release of chemically-bound gasses from some minerals, which don't actually constitute "burning" as such, but the result is the same.) Best regards, Bob Masta DAQARTA v3.50 Data AcQuisition And Real-Time Analysis www.daqarta.com Scope, Spectrum, Spectrogram, Signal Generator Science with your sound card! |
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#12
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Those darned bubbles! - A question
On Tue, 9 Oct 2007 13:01:03 +0200, "Bubbles_"
wrote: Original post below. I am just wondering about something. My wheel is aluminium, and it oxidizes, so that I get a black stain on the back of my left hand from centering - but I also get it in some of the clay that I later reclaim. Is aluminium a possible cause of these bubbles? Should I stick to new-bought clay and/or treat my wheel with something so it doesn't let off particles? Marianne Not to worry, aluminum oxide (alumina) is one of the primary components of clay and glazes. And the amount you might get in your clay from your aluminum wheel head is very, very small. My vote as to what pottery "chemistry" book you should get is "Clay and Glazes for the Potter" by Rhodes. The latest edition is by Rhodes / Hopper, but for what you'll want an older edition is fine. But don't expect to be able to jump in and easily fix glaze problems. That's an art as much as a science and pinholing can be caused by any number of problems - underfired bisque, underfired glaze, overfired glaze, the phase of the moon, orientation of your kiln to magnetic north, etc., etc., etc. deg |
#13
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Those darned bubbles!
I just re-read this and am real confused - is that 1270 C? In which case
you are firing to ~ cone 10? Wow that must be hard to get to in a a small hobby kiln. How many hours does it take? Why are you firing that high? They actually have some pseudo porcelain at cone 6 (what I fire to). This is my typical firing scedule (the hold brings it up to cone 6) Segment Rate F hr(C hr) Temp F (C) Hold (min) Time (Hr) 72(22.2) 1 100(37.8) 220(104.4) 0 1.48 2 350(176.7) 2000(1093.3 0 5.09 3 108(42.2) 2185(1196.1) 0.33 1.71 4 -500(-295.6) 1900(1037.8) 0.17 0.57 5 -125(-87.2) 1400(760) 0 4.00 13.34 "Bubbles_" wrote in message ... EWWWW!! I keep getting bubbles on my finished pieces. I have tried up to 40 minutes hold time at 1270 degreess, but they still show up! I am getting rather frustrated by it all. I can "repair" them by adding glaze to the holes (that are more like inverted craters going all the way down to the clay) and refiring once or even twice, but I have a very small kiln at home, and find it both a waste of space and power, while at the same time not wanting to "lose" the pieces. Any ideas on other things I could try? Last kiln was 6 hours to 650, then up to 1270 and hold for 30 minutes. The cooling was over almost a day. Policy being that over about 400, I don't even let the cool air of the laundry room in, but after that, I take out a couple of plugs (top center and bottom) and under 200 I start opening the lid of the kiln a little. That shouldn't affect the bubbles, though, as I am sure they have set at much higher temperatures. BUT - AGH!!! Any input and suggestions greatly appreciated!!! Marianne |
#14
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Those darned bubbles! - A question
"Bob Masta" wrote in message ... No, this aluminum should not cause any problem at all. Note that aluminum oxide melts at a very high temperature (stiffens glazes), whereas whatever is causing the bubbles is something that is actually boiling, or more likely burning, at your firing temperatures. The combustion gasses are released as bubbles that appear in the slow-to-heal glaze. (There are other possibilities, like release of chemically-bound gasses from some minerals, which don't actually constitute "burning" as such, but the result is the same.) Hi Bob If aluminium stiffens glazes, couldn't it be that the release of it from the clay, trying to get out through the glaze - causes the glaze to stiffen and thus the bubbles not to melt back "in position"? I know that may be a stupid question, but I have found that I only learn if I ask questions about things I don't understand :-) Marianne |
#15
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Those darned bubbles! - A question
"Dewitt" wrote in message ... Not to worry, aluminum oxide (alumina) is one of the primary components of clay and glazes. And the amount you might get in your clay from your aluminum wheel head is very, very small. My vote as to what pottery "chemistry" book you should get is "Clay and Glazes for the Potter" by Rhodes. The latest edition is by Rhodes / Hopper, but for what you'll want an older edition is fine. But don't expect to be able to jump in and easily fix glaze problems. That's an art as much as a science and pinholing can be caused by any number of problems - underfired bisque, underfired glaze, overfired glaze, the phase of the moon, orientation of your kiln to magnetic north, etc., etc., etc. Hi Deg I am beginning to wonder if I shouldn't make a blood sacrifice before each glaze kiln now! LOL! Thanks for the book suggestion. Added to my list :-) Marianne |
#16
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Those darned bubbles!
"DKat" wrote in message ... I just re-read this and am real confused - is that 1270 C? In which case you are firing to ~ cone 10? I do think that my kiln-computer actually shows a bit higher temperature than the actual, so 1270 might be 1250. I would love to figure out a way to test the hypothosis. Must ask my teacher on Friday if she has any suggestions. As I said earlier, I have never heard of folk here using cones. Wow that must be hard to get to in a a small hobby kiln. How many hours does it take? Why are you firing that high? They actually have some pseudo porcelain at cone 6 (what I fire to). Well - I stopped watching over the kiln when it is firing, but I would say something close to 10-12 hours to get to top heat. But that is with my firing schedule as well. Actually, I use 2 different ones for high glaze and have been adjusting them a tad each time because of the problems with the bubbles. This is my typical firing scedule (the hold brings it up to cone 6) I don't understand what you mean by segment, sorry! You fire to almost 1200 - my glazes are meant for about 1220 to 1270 according to the lables. I only have 1 hold on my computer, and that is at top temp. I vary between 20 and 30 minutes - having tried 40 once, to no avail. Segment Rate F hr(C hr) Temp F (C) Hold (min) Time (Hr) 72(22.2) 1 100(37.8) 220(104.4) 0 1.48 2 350(176.7) 2000(1093.3 0 5.09 3 108(42.2) 2185(1196.1) 0.33 1.71 4 -500(-295.6) 1900(1037.8) 0.17 0.57 5 -125(-87.2) 1400(760) 0 4.00 13.34 D - thank you so much for taking the time to help me try to figure out this problem! You are an angel, as are all the folks on here who have such patience with a pseudo-beginner like myself! Hugs! Marianne |
#17
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Those darned bubbles!
Let me see if I can rephrase into common ground. There are low fired,
mid-fired and high-fired pottery. Low fire is typically your earthenware, raku, majolica, etc. Mid-fire can be functional stoneware, high fire is stoneware and porcelain of the translucent variety. Mid fire is what many people with small electric kilns are now working it. Your clay is vitrified (if it is a true midfire clay) at midfire range. It is not at lowfire . Typically the midfire range is cone5 to cone7 or 1183C to 1237C if you are firing @ ~60C/hr. If I were to soak at peak temperature I would fire to a lower temperature because the heat work would take me to the cone I want. The cone is a clay made to 'melt' with a certain heat work. What I want to know is when the clay and glaze is mature and this happens with heat work rather than just a certain temperature. That is why cones are important. Segments in my firing are simply different periods in the firing - so if you are running your kiln manually (without a computer program) you might run the first 6 hours with one switch on low and two switches off (assuming you have three separate controls). That would be one segment. The next 1 hour with 2 switches on low. That would be the second segment. The next hour with 3 switches on low. The fourth segment might be 1 switch on medium and 2 on low. etc. So you are slowly coming up to a temperature you want. How fast you go from one temperature to the next is the ramp. Typically you go very slowly until all of the water in the clay has safely escaped from the clay. From there you can go fast until you get to the point with the clay changes physical structure. Here you want to again go at a rate where you do not get sudden fast changes. If you have glazes that form crystals you want to decrease the temperature slowly at the temperature where the crystals are forming. So you would not just turn off the kiln when it reached temperature and leave it. You would monitor it and when it reached the temperature you wanted to slow the cooling you would have it on low for a certain period (how you would do this manually would depend on how quickly your kiln cooled on its own). I know they use cones in Europe. Go to this site. The person is very nice and I am sure would answer questions (I would think she could tell you where and how to get cones). But there you can see what cone 6 glazes look like. She does a lot of testing. If you can fire at a lower temperature until you have more experience, I think you would find life easier. http://www.alisapots.dk/ Donna P.S. How do you know what temperature you are going to if you don't use cones? Are you doing this simply on timing given you are not sure if you are at 1270 or 1250? Communicating hard... "Bubbles_" wrote in message ... "DKat" wrote in message ... I just re-read this and am real confused - is that 1270 C? In which case you are firing to ~ cone 10? I do think that my kiln-computer actually shows a bit higher temperature than the actual, so 1270 might be 1250. I would love to figure out a way to test the hypothosis. Must ask my teacher on Friday if she has any suggestions. As I said earlier, I have never heard of folk here using cones. Wow that must be hard to get to in a a small hobby kiln. How many hours does it take? Why are you firing that high? They actually have some pseudo porcelain at cone 6 (what I fire to). Well - I stopped watching over the kiln when it is firing, but I would say something close to 10-12 hours to get to top heat. But that is with my firing schedule as well. Actually, I use 2 different ones for high glaze and have been adjusting them a tad each time because of the problems with the bubbles. This is my typical firing scedule (the hold brings it up to cone 6) I don't understand what you mean by segment, sorry! You fire to almost 1200 - my glazes are meant for about 1220 to 1270 according to the lables. I only have 1 hold on my computer, and that is at top temp. I vary between 20 and 30 minutes - having tried 40 once, to no avail. Segment Rate F hr(C hr) Temp F (C) Hold (min) Time (Hr) 72(22.2) 1 100(37.8) 220(104.4) 0 1.48 2 350(176.7) 2000(1093.3 0 5.09 3 108(42.2) 2185(1196.1) 0.33 1.71 4 -500(-295.6) 1900(1037.8) 0.17 0.57 5 -125(-87.2) 1400(760) 0 4.00 13.34 D - thank you so much for taking the time to help me try to figure out this problem! You are an angel, as are all the folks on here who have such patience with a pseudo-beginner like myself! Hugs! Marianne |
#18
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Those darned bubbles!
"DKat" wrote in message ... Let me see if I can rephrase into common ground. There are low fired, mid-fired and high-fired pottery. Low fire is typically your earthenware, raku, majolica, etc. Mid-fire can be functional stoneware, high fire is stoneware and porcelain of the translucent variety. Mid fire is what many people with small electric kilns are now working it. Your clay is vitrified (if it is a true midfire clay) at midfire range. It is not at lowfire . Typically the midfire range is cone5 to cone7 or 1183C to 1237C if you are firing @ ~60C/hr. If I were to soak at peak temperature I would fire to a lower temperature because the heat work would take me to the cone I want. The cone is a clay made to 'melt' with a certain heat work. What I want to know is when the clay and glaze is mature and this happens with heat work rather than just a certain temperature. That is why cones are important. Hi D! I fire to 1250 and prefer this because of the life in those glazes and that the pottery is properly vitrified, so it can be used in the garden and I don't have to take it inside in the winter. The mid-fire glazes I have seen up until now have been a bit dull for my tastes. I typically only use max 2 glazes (1 inside the bowl, one outside - possibly 1 on the rim) on my pieces, so I really like for the glaze itself to add life to the pot. Segments in my firing are simply different periods in the firing - so if you are running your kiln manually (without a computer program) you might run the first 6 hours with one switch on low and two switches off (assuming you have three separate controls). That would be one segment. The next 1 hour with 2 switches on low. That would be the second segment. The next hour with 3 switches on low. The fourth segment might be 1 switch on medium and 2 on low. etc. So you are slowly coming up to a temperature you want. How fast you go from one temperature to the next is the ramp. Typically you go very slowly until all of the water in the clay has safely escaped from the clay. From there you can go fast until you get to the point with the clay changes physical structure. Here you want to again go at a rate where you do not get sudden fast changes. If you have glazes that form crystals you want to decrease the temperature slowly at the temperature where the crystals are forming. So you would not just turn off the kiln when it reached temperature and leave it. You would monitor it and when it reached the temperature you wanted to slow the cooling you would have it on low for a certain period (how you would do this manually would depend on how quickly your kiln cooled on its own). I have a thermostat in my kiln. The controller is quite simple, and only lets me put in 4 requirements. time until temperature (650 C always) temperature to be reached soak time This has worked fine for glazes I use without this special "foredler" (beautifier) on top. But the beautifier changes the glaze immensly and makes it so lively with flares and beautiful highlighting colors (such as bright blue in the black of the glaze). I know they use cones in Europe. Go to this site. The person is very nice and I am sure would answer questions (I would think she could tell you where and how to get cones). But there you can see what cone 6 glazes look like. She does a lot of testing. If you can fire at a lower temperature until you have more experience, I think you would find life easier. http://www.alisapots.dk/ WOW! I have driven through Aabenraa and even stayed there once! Every summer when we drive up to Norway, we pass through Aabenraa! I will definitely get in touch with her. Her homepage is really great as well - and how generous of her to share all about her glazes! P.S. How do you know what temperature you are going to if you don't use cones? Are you doing this simply on timing given you are not sure if you are at 1270 or 1250? Communicating hard... The thermostat in the kiln. Although this isn't 100% accurate. But if I have been firing to 1250 and I increase to 1270, I know that even if 1250 is really 1230 in the kiln, then I am increasing the temperature by about 20 degrees. Thus the new temp might in fact be 1250, but the important thing (I thought) was to know what change I am making, and not necessarily the exact temperature. Also - the clay sinters at 1220 C - but can be fired at everything from 1000 to 1300 degrees. That means that I am well within the parameters as far as that goes. The glazes are for 1250 C. Thanks for your time, D. I think that D stands for DEAR! :-) Marianne "Bubbles_" wrote in message ... "DKat" wrote in message ... I just re-read this and am real confused - is that 1270 C? In which case you are firing to ~ cone 10? I do think that my kiln-computer actually shows a bit higher temperature than the actual, so 1270 might be 1250. I would love to figure out a way to test the hypothosis. Must ask my teacher on Friday if she has any suggestions. As I said earlier, I have never heard of folk here using cones. Wow that must be hard to get to in a a small hobby kiln. How many hours does it take? Why are you firing that high? They actually have some pseudo porcelain at cone 6 (what I fire to). Well - I stopped watching over the kiln when it is firing, but I would say something close to 10-12 hours to get to top heat. But that is with my firing schedule as well. Actually, I use 2 different ones for high glaze and have been adjusting them a tad each time because of the problems with the bubbles. This is my typical firing scedule (the hold brings it up to cone 6) I don't understand what you mean by segment, sorry! You fire to almost 1200 - my glazes are meant for about 1220 to 1270 according to the lables. I only have 1 hold on my computer, and that is at top temp. I vary between 20 and 30 minutes - having tried 40 once, to no avail. Segment Rate F hr(C hr) Temp F (C) Hold (min) Time (Hr) 72(22.2) 1 100(37.8) 220(104.4) 0 1.48 2 350(176.7) 2000(1093.3 0 5.09 3 108(42.2) 2185(1196.1) 0.33 1.71 4 -500(-295.6) 1900(1037.8) 0.17 0.57 5 -125(-87.2) 1400(760) 0 4.00 13.34 D - thank you so much for taking the time to help me try to figure out this problem! You are an angel, as are all the folks on here who have such patience with a pseudo-beginner like myself! Hugs! Marianne |
#19
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Those darned bubbles! - A question
On Thu, 11 Oct 2007 02:45:36 +0200, "Bubbles_"
wrote: "Bob Masta" wrote in message ... No, this aluminum should not cause any problem at all. Note that aluminum oxide melts at a very high temperature (stiffens glazes), whereas whatever is causing the bubbles is something that is actually boiling, or more likely burning, at your firing temperatures. The combustion gasses are released as bubbles that appear in the slow-to-heal glaze. (There are other possibilities, like release of chemically-bound gasses from some minerals, which don't actually constitute "burning" as such, but the result is the same.) Hi Bob If aluminium stiffens glazes, couldn't it be that the release of it from the clay, trying to get out through the glaze - causes the glaze to stiffen and thus the bubbles not to melt back "in position"? As Dewitt mentioned, aluminum oxide is very common in clays and glazes, and the amount from your oxidized wheel is trivial by comparison. However, even if you had a lot more (say, if you deliberately worked powdered alumina into the clay or something), you'd find that alumina by itself wouldn't melt at kiln temperatures without a flux. It wouldn't "release" from the clay. Alumina that was on the surface and mixed with glaze fluxes would retard their melting in the mix zone. My guess is that this surface alumina would not migrate far up into the glaze itself... I don't think it would dissolve in it very easily and raise its overall melting (freezing) point, unless you held the kiln at a high temperature that would keep the glaze molten long enough to allow the alumina to mix. my guess is that if you held it that long, assuming the glaze didn't run off the pot, that all the bubbles would have time to work out through the molten glaze, anyway. But to reiterate, I don't think alumina is your problem at all. I know that may be a stupid question, but I have found that I only learn if I ask questions about things I don't understand :-) Marianne More than one instructor has said that "the only foolish question is the one you don't ask"! Best regards, Bob Masta DAQARTA v3.50 Data AcQuisition And Real-Time Analysis www.daqarta.com Scope, Spectrum, Spectrogram, Signal Generator Science with your sound card! |
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