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#21
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Steve Mills....I am having trouble replying to your e-mail you sent me. My
post keeps coming back! JM |
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#22
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On Mon, 29 Nov 2004 09:21:35 GMT, "JM" wrote:
Yeh, I constantly turn the pot so that the temp is even. I also keep the gun away some distance, so the heat is not fierce. My last tutor used to bring a large gas bottle and a blow torch to class, and we used to use that. Those who were not used to using the blow torch method would find that, if you were not careful, small chips would shoot off the ware! It was quicker, but I found my little gun perfectly adequate, and in my mind, a lot gentler on the ware. JM "Bob Masta" wrote in message ... Just a thought, but try skipping the hot air gun on a few pieces. I'd guess that drying the surface much faster than the interior must be building up some stress. Actually, the stress I was thinking about is between the interior and exterior of the clay itself, so constant turning won't help. The idea is that the skin dries out quickly to a certain depth, but the interior is still wet. Then as the interior does dry later, the shrinkage causes stress build-up because the exterior is already dry and can't shrink to match it. Or another way to think about this is that the rapid exterior drying may be causing rapid exterior shrinkage, which causes invisible cracks because the interior is not shrinking to match. The cranks only show up later. At any rate, if you skip the hot air gun on a few pieces and let them dry naturally (however long that takes) and the problem goes away, then you know where to look. Best regards, Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis www.daqarta.com |
#23
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Do I understand you correctly that you are getting the same issue (hairline
cracking) with 2 different clays, fired at 2 different temperatures, with different shaped items? Is this something that is consistently happening or just sporadic in nature? "JM" wrote in message ... What clay are you using and how high the bisque? Steve Bath UK Hi Steve, I dont know what stoneware clay they use at the college, but it is dark grey in colour and fires to a v light grey. They bisque fire to 1100oC (which I think is too high - personally). The clay I use at home, is Potclays 1143M Draycott White and fire to 900oC - I have not had any problems in the past firing at this temp. Mmmm - I could try firing to 1000oc I suppose, and see what happens! But it doesnt explain why the college are getting the same results - esp when they fire at 1100oC. Food for thought. Thanks |
#24
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Do I understand you correctly that you are getting the same issue (hairline cracking) with 2 different clays, fired at 2 different temperatures, with different shaped items? Is this something that is consistently happening or just sporadic in nature? Hi, I know it's baffling! This is why I posted to the site..This is only something I have had happen recently...I have never come across this problem before. The teapot is the only thing I fired at home. This item had been sitting on my shelves for a good couple of months. I cannot, for the life of me, remember whether that had been an piece I had brought home from college to finish off at home. So it IS possible it came from the same clay. Other than that, the only thing in common, is that I used a hot gun on these items...but this has never created a problem in the past. JM |
#25
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HI Bob,
I do use the gun on the interior also..I dont just use on the outside. But I dont hold it close; so it has more the same effect as a warm cupboard would have. My ex pottery tutor used to heat his stuff up soooo much it used to positively steam and he had no probs! Hope this helps to clarify. Thanks JM |
#26
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On Tue, 30 Nov 2004 12:04:29 GMT, "JM" wrote:
HI Bob, I do use the gun on the interior also..I dont just use on the outside. But I dont hold it close; so it has more the same effect as a warm cupboard would have. My ex pottery tutor used to heat his stuff up soooo much it used to positively steam and he had no probs! Hope this helps to clarify. Thanks JM By interior I mean within the clay itself, below the surface, not the inner surface of a pot. You are shrinking the skin but not the guts. Cracks are the expected outcome. I don't think it's safe to assume too much from the fact that others get away with this, since technique and temperature may make a great difference. You are assuming that since you are using low temperatures, you should be less likely to have problems. But that means that to get any given amount of surface drying you are probably penetrating deeper than a hot, rapid technique. That may be working against you. Just try a few pieces with normal drying, and see if the problem goes away. Best regards, Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis www.daqarta.com |
#27
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Maybe when you is doing it you is doing it more..... When i do dry pots on
the wheel using a hot air gun (small one, not the one that does soldering) i only dry them a bit so's they'll retain shape when i pushes them around a bit, ceratainly not enuff for steam..... Plus i then lets 'em dry naturally the rest of the way.... Hugs Eddie "Ken" wrote in message ... I would suggest you allow your ware to dry normally( Slow) Or change your clay body formulation . Your rapid drying has not cracked pots but rather created stress lines that turn to cracks upon bisque firing . Glaze firing rather than curing problem only applifies it as there is difference of expansion during firing that further stresses cracks in ware . Try same clay body dried slower or another formulation that isnt as stress sensitive to rapid drying . Good Luck Ken "JM" wrote in message ... Hi all, I have a problem which has left myself, and my pottery teacher as puzzled as the pots. Recently my ware has developed fine hairline cracking during bisque firing. All ware is thoroughly dried before firing (when pots are both fired at college and at home) - so I don't damp is not the problem. One of the items has been on the drying shelf for about 2 months! I have been using a hot-air gun to get some pieces to leatherhard stage for turning, but have not held it too close - I know a potter who even uses a blow torch. So I am not even sure that this is the problem creater. Thinking that when glazing the glaze might seep in and seal the fine cracking, but the second firing being hotter opens them up even more. The cracks are not localised and run in all directions, so are not caused by lack of compression. If I can't get to the bottom of this irritating and frustrating condition I could always start a line in jigsaw pots. Please come up with suggestions of possible causes - as I have exausted all known possibilities. JM |
#28
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Maybe when you is doing it you is doing it more..... When i do dry pots on the wheel using a hot air gun (small one, not the one that does soldering) i only dry them a bit so's they'll retain shape when i pushes them around a bit, ceratainly not enuff for steam..... Plus i then lets 'em dry naturally the rest of the way.... Hugs Eddie Reply:- That's exactly how I use it...certainly not to steaming point (as my tutor does). I get my ware to the point of holding its shape, then let dry out normally. Jm |
#29
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Another note:
I returned last night to find that the bowl I had left at the college (in the wet cupboard) was as wet as when I had thrown it a week ago...certainly too wet for turning. Therefore I have brought it home to harden a little here. I am using this bowl as an experiment by not using the gun on it; just to see if it makes any difference. Judging by certain comments, it would appear that using gentle heat on ware may be more harmful than a quick hot blast. But the warm cupboard used at college dries ware out slowly - perhaps this works better as it is not localised drying? We have to wait and see! Thanks for everyones input - I'll let you know of my findings. It would be nice to know how others speed up the drying process - I have even been told of placing ware on top of a firing kiln to speed things up. I know some of you are lucky enough to live in warmer climate, and so have no probs drying out. JM |
#30
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In article ,
JM wrote: It would be nice to know how others speed up the drying process - I have even been told of placing ware on top of a firing kiln to speed things up. I know some of you are lucky enough to live in warmer climate, and so have no probs drying out. JM Where I do my work, we have a dry box--a closet with heat lamps and shelves in it. If you leave your pieces in there for just a short while they'll firm up a bit. I try *not* to use it very often. My preferred method of drying is to put the piece on a board and wrap in plastic (like the kind from a drycleaner). In a day or so, I might loosen up the plastic a bit, or if I'm working at home, I'll take the plastic off for a little while, then wrap it back up again. I've found that drying slowly is much better for the piece than drying quickly, and that certain clay bodies (like porcelain) develop drying cracks easier than other clay bodies. Deb R. |
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