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.
How high are you bisque firing to? What glaze is this happening with (is it all glazes, just one, a mix)? We need to know more before suggestions would be of much use. 40 minutes of hold time is way more than you should need so something is not as it should be.
It usually happens with the really glossy glazes, but I actually had one bubble on a matte glaze this time as well.
Since the bubbles go all the way in to the clay, do you think I might be bisque firing too low, so that there are "substances" left in the clay before the glaze firing?
I do think my oven's thermometer might show a bit higher than it actually is, so maybe just increasing to 1060 or 1070 would help?
Once again - any input greatly appreciated! Especially since I have a full load for bisque firing ready.
I'm home sick with flu and not functioning 100% so bare (or is that bear) with me...
First, you should use cones on all firings or at least do regular spot checks. You need to know what heat work you are getting not just what temperature you are going to and you need to know that for different spots of your kiln.
You can try bisque firing to cone 04 (1060 C). I only fire to 06 (995 C) and have no problems but I mix my own glazes and know the materials I am working with. Firing to 04 is often recommended. Do you make up your own glazes or are you buying glazes? If you don't fire with cones, see if you can beg, borrow or steal some until you can buy your own, at least to check your next bisque firing.
You can get bubbles before if your pot is very, very dry when glazing. I am one of the few potters out on the eastcoast that I know of who rinses off their bisqueware before glazing. It is a really, really quick rinse so the pots are still absorbant. I was taught in the desert environment where it was so dry you could soak your bisqueware in water and five minutes later they would be bone dry. In fact you had to dip them in water or the glaze bubbled up. You can wipe down your pots with a damp sponge to both clean them off and decrease bubbling on the dry glaze surface. This is probably not the problem with your glaze however. It is something you would notice by now.
Let us know about the glazes you use. (if you mix your own, what ingredients - better yet what recipe are you using).
I think it is "bear" - as in to bear a burden, not the animal - hehe!
Okay - I haven't even seen anyone using cones over here in Europe - my teacher doesn't even with her new kiln, and neither did an old teacher of mine. I can't remember seeing any in the extensive catalogue from my dealer, either.
I was told that sponging off bisque-ware was only to remove the dust. My workshop typically has a humidity of around 50 %. I guess maybe that should be enough.
But, when you mention during glazing, I do sometimes se that just little spots aren't covered - like a little speck of grog. I just dab it with my brush with glaze on it, but could that be part of the problem? I like your suggestion of a little quick dip in water, and will try that later on. I need to get myself one of those potter's pencils that doesn't burn away - and start numbering my pots and writing down their whole way through the processes.
Hervorragender Drehton, sehr plastisch, gute Standfestigkeit, auch für sehr grosse Stücke. Einfach und gut im Trocknen. Guter Glasurträger. Raku möglich.
I'm stuck on translating any of this at the moment, but will try if you don't get the general gist of it.
These are the glazes I use.
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one I have most problems with at the moment is 101 with a glaze improver over. The improver - number 100 - makes it go different blues and greens and blacks when I add it on top. Fantastic colors, but the bubbles keep showing up. Mind you, I had a bubble on a matte glaze - 404, I think - on this last high firing. I don't have this problem at all with low-fire stuff - at 1050 C, same as my bisques (that way I can mix them a bit and get a full kiln faster).
My kiln is teeny - only about 2 feet wide and maybe 3 feet tall inside - so I wouldn't expect huge temperature ranges inside except when firing with the holes open - on in the lid, one right down at the bottom. possibly even below the bottom shelf. I open those about up to 650 then close them. I can't remember any bubbly piece specifically being by the open holes, and glazes only really start to melt well above 650C, don't they?
I'm going to go fire my kiln now. It is a bisque, and I will check my glazes to see if they can be fired higher than 1050 - there are a couple I have that shouldn't be, which is why I have stayed at that temp, and I have some glazed stuff going in now.
Thanks again for your time and input!
***virtual chicken soup for you to get better on***
I'm assuming you are also posting on clayart where you might have more luck of overlapping with someone using these glazes.
With that high of humidity don't dunk the pots (I only rinse mine under the tap very, very quickly). Dunking them is going to empty the air out of them and you don't want that. I would though, wipe them off with a damp sponge.
How are you venting your kiln? Clay and glaze materials put off some not good stuff...
Since you are buying a commercial glaze material you should be able to call them up and get some support. Since you don't know the ingredients and they do it is their responsiblity to give you help on this. Generally people who sell glazes are all to happy to do all they can to keep their customers happy and buying their product.
IMO - There really isn't anything anyone else can do to help unless they are familiar with the glaze or know what it is made up with. As I said a 40 minute soak is to me overkill.
No, I don't post on clayart. Is that a newsgroup, or a forum? I don't like forums, I keep forgetting to keep up and it ends up that I just drop it.
Hmmm - venting my kiln? Well - I told you about the holes. The kiln is in the laundry room, so I make sure there are no clean clothes in the room and then open the window. The room does get warmer, but not too bad.
All the glazes I use are so-called "poison class free" - ie. they are supposed to contain no poisonous materials. Still, the room really stinks, especially when I am glaze firing, of course, so I always close the door to the hallway and have the window open in there.
There is a hitch with me trying to discuss this with the makers of the glaze, because my German really isn't up to par to discuss this! I tried talking about it with the boss where I buy the stuff, but even face-to-face, I didn't understand everything he said, and it just got so complicated that I gave up. I did catch a couple of ideas, but I thought the gang on here could get into this in general, as it is happening on other glazes than the
101+100 combination, though that one is the worst.
We were fighting bubbles too (not you - them), when we first got into lowfire clay. It seems that there is some kind of material in the clay that volatilizes at the maturation point, causing holes in the glaze, and bubbles, which are really nasty. The solution, using cone 04 clay and 04 glaze, was to bisque to 04 and hold it there a while in the bisque fire, to let it do its thing. It did make the ware a little less absorbant when glazing, but it beat the bubbles.
I am also wondering if the bubbles are coming from the ware itself or from the glaze. Could it be that I have more trouble on the glossy glazes because they just hold the bubbles better?
The data on my clay says bisque 950-980 degrees C, and I am already firing it to 1050 (again because then I can combine low-fire glaze and bisque in the same firing). So then I would have thought I have gotten what needs to be gotten out of the clay?
The bisque pieces are very nicely absorbant fired to 1050, so I guess I should try 1060 or 1070 on my next batch?
*reminder to self - scratching head too much may cause splinters in fingers*
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
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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.
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)
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 :-)
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.
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.
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.
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!
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.
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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...
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.
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).
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! :-)
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.
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
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