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Air Bubbles In Clay

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#41 Mart


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Posted 17 October 2013 - 02:43 AM


So the firing speeds that we normally use are too fast, and the remaining water in the pot turns to steam and BOOM! Thick pots need a slower ramp up in temperature during the first 200 degrees, to make sure they dry out. This is why people preheat overnight.

What length of preheating would you recommend for hand built items including coil and slab?

Also I just finished a sculpture of a dog sitting it is about 8x8 inches. I tried to get much of it to the same thickness throughout but there were places where it just had to remain a bit thicker. I plan to let it dry for about a month in my studio with a dehumidifier running to try and lower the overall humidity. When I go to bisque it how long of a preheat would you suggest I try?



Huh.. one month and dehumidifier running? Talk about overkill and massive waste of energy :)
Set your kiln to 82C (180F) and let it sit there for an hour or 2 and it will be fine.
I use ambient to 82C in 30 min and then few hours for drying (only if needed)

If you have a top loader, you can stick your hand in there and feel is it "steamy" or not.

I had this 45x38 cm (before firing) coil built vase in the kiln for 4 h ... just in case, because I finished it only few days earlier and it was dry outside but not in the inside.

Attached File  IMG_2789_v2.jpg   155.84KB   3 downloads

So I used:
* ambient to 82C - 0.5 h
* 82C-82C - 4 h
* 82C-600C - 5 h
* 600C-1200C - 4 h
* 1200C-1200C - 0.25h
* cool off

I did not to bisque fire, because there was no need and this clay is fine at 1200C

Someone told me that preheating for longer than 20 minutes was a waste of time and energy, ...

If it still steams, it needs more time. Period. :)
And the energy wasted is way less than "wasted" on building the piece, blowing it up and then building another one and so on...

#42 Pugaboo


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Posted 18 October 2013 - 05:42 PM

I should note I ALWAYS have a dehumidifier running in the studio. :) Georgia seems to be quite humid or at least my studio and garage are. I dry pieces in a cabinet to slowly let them dry and I like to err on the side of caution. I use little loafers and several of the potters at the group studio have horrible issues with cracking while drying. I dry slowly and have had no issues with cracks during drying. I am hoping this means that drying LL is better done slowly rather than quickly. OR I've just been lucky so far! ;) Non sculpture pieces I can dry in a week sometimes less if small ornaments sometimes a tad more if a large coil vase. With sculptures I try to be very careful with as they are a lot of work to get to that point and it seems silly to risk all that work by rushing the drying process.

I tend to preheat for 2 hours and then slow bisque. I've done a 4 hour preheat when I had some extruded pieces and I could not check the insides of to tell if dry. I've thought about skipping the 2 hour preheat but so far have not felt like I knew what I was doing well enough to KNOW everything was 100% dry. Maybe one day when I am braver and know my clay a bit better I will. Oh I should note I've fired less than 10 bisque loads so am still new to firing.

Well that's it I'm off to my 2nd craft fair with pottery in the morning! The weather looks like it's going to be nice so keep your fingers that the sales will be good.

The world is but a canvas to the imagination - Henry David Thoreau

#43 Isculpt


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Posted 23 October 2013 - 12:21 AM

Terry, y'know how potters are always telling you to test, test, test?  Well, as a sculptor, I have to say that the idea of making a sculpture for the purpose of testing whether a preheat is necessary sounds just a little bit painful.  But I inadvertently ran just such a test.  My work never blows up or blows out a section when I do my usual 8+ hours of preheat, but when my kiln broke, I took my work to a community pottery place for firing.  I assumed that since all work fired there is handbuilt, and most of it is made by newbies, they would preheat the kiln.  When I went back for my work, I found that 4 out of 6 sculptures (about 2 weeks' worth of work) had blown. THEN I found out that they don't preheat.  I know some people insist that preheating is unecessary, but I have learned my lesson and I don't think I'll run THAT test again!

#44 Pugaboo


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Posted 23 October 2013 - 06:48 AM

Isculpt - Thank You! I'm sorry to hear your pieces didn't make it through their kiln but I'm glad you said something here. With sculptures I think I will continue to err on the side of caution in my own kiln AND I will remember your woes and not fire sculpture pieces anywhere that I cannot verify a paranoid kiln director is in charge. How awful for you to lose all that work!

The world is but a canvas to the imagination - Henry David Thoreau

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