Fine hairline cracks in bisque ware

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

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

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

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

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

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.

I suspect that most folks don't attempt to speed up drying. It's more likely to screw things up. In general, most people try to *slow down* drying to prevent stress build-up, especially where some parts (handles, etc) are much smaller or have more surface-to-volume ratio than the main body. Draping plastic over the piece is a common way to do this.

The trick is to get used to the slower work flow. You may need more storage space for drying wares in the meantime, but you will get better results overall in the long run.

Best regards,

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

I use a hot air gun at times, but in a limited way. When I throw and want to alter a piece I dry it enough so that you can touch the surface without marking it, but it is still very flexible and you can alter the shape. Or if I am turning and it is just a little two wet I use the hot air gun very sparingly to firm it up a little. I tried it on porcelain once :o) it will be the only time too. Slow drying is always safest. A