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How To Minimize Cracking In Repeat Firings


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#1 Isculpt

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Posted 21 July 2013 - 12:53 AM

I'm making slab sculptures, with walls that aren't of consistent thickness.  (I know it's preferable, but sometimes it's just not doable.)  I bisque to 06, apply copper carbonate and refire to 06, 05, or 04.  I'm ending up with some cracking occurring in the second firing regardless of temperature.  I would like to fire once or twice more to 06 -04 with underglaze, but I'm worried about the pieces cracking more each time they're fired.  I have a two part question.  (1) Would it help to fire higher on the bisque, lower on subsequent firings?  I2) Why would the clay survive 06 temperature with no cracks only to crack when it's subjected to the same temp again?  I haven't found anything in my pottery books or online to explain why it does that... 

Jayne



#2 mregecko

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Posted 21 July 2013 - 02:08 AM

What do your firing schedules look like?



#3 OffCenter

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Posted 21 July 2013 - 09:29 AM

Maybe you're firing too fast the second time. It seems that since it is already bisqued you would be able to fire as fast as possible but thick pieces may not be able to take it, especially if the clay is near maturity at 06.

 

Maybe it is still slightly damp from the copper carb when you fire the second time.

 

Jim


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#4 Bob Coyle

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Posted 21 July 2013 - 09:50 AM

I would agree with Offcenter... you may be firing too fast on the second firing.

 

Couldn't you add your copper carb to the leather hard or bone dry ware before you fire? That way you could only fire once.



#5 Isculpt

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Posted 21 July 2013 - 11:50 AM

I may indeed be firing too quickly.  I have always used a really long bisquing schedule, and a pretty long second firing schedule, but recent posts (in the past month or so) have made me think that I'm firing unnecessarily slow on the second and subsequent firings.  Maybe I need to back up to my earlier, slow schedule. 
 
As for allowing the copper carbonate to dry, I figured that the copper carbonate would dry really quickly, since it's sprayed on and wiped off immediately, with copper carb staying only in the crevices.  So I didn't allow a long drying time there at all. 
 
As for doing it all at once before the first firing, I can't do that because I do need to wipe off the excess, and doing so would potentially rub away some detail -- like a nose, say!
 
Can anyone suggest an appropriate firing schedule for the second firing?  I'm using an old kiln, and my current schedule looks something like this: Lid down, vent holes plugged
 
Start with 2 of the 3 elements on low for 1 hour
Add 3rd element for an hour
Turn all elements to half way between low and medium for 2 hours
Turn all elements to medium for 2 hours
Turn elements up to halfway between medium and high until temperature reaches 06 or 05
 
I have a sculpture that I'd like to refire today with additional copper carbonate, but I'm afraid that the cracks will worsen.  Would slowing things way down help to minimize that possibility?  It isn't necessary that I refire it, but it would help matters.

 

I'd like to include an image of the cracks, but I can't find an option to insert an image in the new format.....



#6 Min

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Posted 21 July 2013 - 04:19 PM

I think you answered your own question, if your old schedules were working then don’t change them. I think you are now firing too fast for sculptural work. A fast firing is usually okay for work that is of even thickness, I think the potters advocating a fast glaze fire are for the most part talking about dishware of even thickness top to bottom.

 

I had a look at Nan Smith’s schedule for sculpture. She does use a computer controlled schedule but you can get an idea of how slow she fires and cools. The slow cooling down part is just as important as the slow rise up in temp. The extra cost in electricity is nothing compared to the cost of lost work.

 

If you were having cracking with your former schedules with 2nd and subsequent firings the other way to fire would be to fire low the first time (like in the ^012 -  ^ 010 range) then apply your copper/underglaze and  fire to your top temp.

 

Nan Smith’s  article on firing sculpture: http://www.mnclay.co...aces_smith.html

 

Min



#7 Mark C.

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Posted 21 July 2013 - 05:30 PM

A few other points-hopefully your clay body is an open one with grog. The tighter talc bodies will not hold up as well as to many fires.

Also the more fires  on the same clay the more issues will pop up. In functional wares this makes for weaker pots. In sculpure it will not matter. I used to fire a fellows sculpture . Ever few months he would fill my small 12 cubic foot gas cone 10 kiln. I did this for about 3 years so I saw a lot of hand build issues.

The more he would fire them the more issues came up.

Mark

This one one of his (he gave me this one) its about 14 inchs tall- cone 10

non glaze finish.

 

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#8 Isculpt

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Posted 22 July 2013 - 01:30 AM

Min, Thanks for the link to Nan Smith's article.  I have not paid attention to the cool-down time schedule at all, trusting the kiln to cool in a sufficiently slow manner.  Doing some quick math, I realize that my kiln is cooling at something like 275 degrees an hour, rather than the 150-200 degrees that Nan recommends.  I think it's time for me to give in and buy a decent programmable kiln!  

 

And Mark, my clay is groggy -- in fact, it is raku clay.  I don't do raku (though I would love to find a workshop and learn about it), but I like the feel and the working strength of the clay.  I had thought that being Raku clay, it would be less likely to crack during standard electric kiln firing, but I'm not finding that to be the case.  Your observations about your friend's sculpture firing that the more the sculptures were fired, the more issues would come up, was interesting.  That's good to learn from someone ELSE's experience! 

 

And while we're on the subject of firing, while my kiln was being repaired I took some work to a community handbuilding clay group for firing in their electric kiln.  Four of my eight sculptures had major cracks and blown off parts.  I learned that the group leader doesn't candle at all - insurance issues, I was told.  She looked at me like I was nuts when I mentioned starting the bisque fire with the lid open for at least several hours.   I've gotten the impression here on the forum that everyone candles if they're building pieces with thick or inconsistent walls.  Am I incorrect about that?  



#9 OffCenter

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Posted 22 July 2013 - 08:39 AM

When people here are discussing bisque firings and glaze firings in general, I think most of us are thinking of pots, not sculpture, but I think that as long as we're not talking massively thick pieces, most of what works for pots works for sculpture. (Some of my large bowls have as much variation in thickness and thickness at the foot as a lot of sculptures.) Sometimes it is shocking to see the ridiculous bisque schedules mentioned here. A week or so ago someone here was wondering if closing the lid at 1700 degrees was too early! Candling is for wet pots. It's not part of the bisque firing; it is drying pots so they can be bisqued. I would have looked at you like you were crazy, too, if you had asked me about firing with the lid open. Why not just wait until the pot or sculpture is dry instead of wasting a lot of energy and heating the studio up by drying in the kiln? When I did all the firings for 3 beginning sculpture classes on work-study in college, nothing was allowed in the kiln room until it was bone dry and then, just to be sure, it had to spend a couple of days on the shelves with a fan blowing on it. I hardly ever blew up anything--and when I did I told them it was because of an air bubble.

 

Jim


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#10 Benzine

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Posted 22 July 2013 - 08:55 AM

I hardly ever blew up anything--and when I did I told them it was because of an air bubble.

 

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#11 Isculpt

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Posted 24 July 2013 - 11:48 AM

Jim, the reason I candle my sculptures is that I live in South Carolina, where the humidity is almost invariably 98% for most of the year.  Mold grows on everything!  My house, which is well insulated and centrally air conditioned, has a humidity rate of 85% or thereabouts.  My studio falls between those two, usually closer to 98%.  The last three months have brought rain 25 days out of 30, and many of those rains were HEAVY rains that lhave eft the ground a semi-permanent sponge.  Are you suggesting that under these conditions, I don't need to candle? I'm open to learning......



#12 OffCenter

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Posted 24 July 2013 - 01:30 PM

Jim, the reason I candle my sculptures is that I live in South Carolina, where the humidity is almost invariably 98% for most of the year.  Mold grows on everything!  My house, which is well insulated and centrally air conditioned, has a humidity rate of 85% or thereabouts.  My studio falls between those two, usually closer to 98%.  The last three months have brought rain 25 days out of 30, and many of those rains were HEAVY rains that lhave eft the ground a semi-permanent sponge.  Are you suggesting that under these conditions, I don't need to candle? I'm open to learning......

 

Isculpt, I live in Georgia, near Macon, so I'm experiencing the same monsoon. June was wettest on record and so far July has been wetter. I'm having a problem drying pots so have sealed up an old shed and chased most of the snake out of it and put a dehumidifier and fan in it. That gets my large bowls bone dry.  But, except for the big bowls everything else drys on shelves that are almost outside and I've had no trouble bisque firing as normal. I have had trouble glazing because my bisqued pots stay too damp to absorb glaze and I have to sometime dry them with a heat gun before glazing. Of course, your sculpture is probably more like my big bowls in that the thickness varies and at the foot they are very thick so maybe you do need to dry them more. I prefer a dehumidifier to candling.

 

Jim


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#13 Wyndham

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Posted 24 July 2013 - 03:02 PM

I noticed you mentioned copper carb, are you doing a post fire reduction in anyway?

 are firing a low fire talc body or a mid or hi fire clay in your work

Rod's Bod from Laguna has been a successful cone 10 clay for raku as it has some sand and grog that allows for a raku firing.

Even if you don't do post fire reduction you might consider an alt clay body. This body has been refired more than a few times by different potters.

Just a thought.

Wyndham



#14 Mark C.

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Posted 24 July 2013 - 03:19 PM

You said

( Four of my eight sculptures had major cracks and blown off parts.)-this can all be from to wet sculptures to -to fast firing.

I know this was done elsewhere but its shows that slow and dry are better. I think a change of of clay body as noted in above post would not hurt as well.

Mark


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#15 oldlady

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Posted 24 July 2013 - 03:38 PM

to the folks in georgia (i know it isn't on topic) we in the rest of the country heard about your terrible long term drought that even dried up lakes in georgia a few years ago.  is this year's rain ending the drought and filling up the lakes?  haven't heard anything on weather channel re this.


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#16 Isculpt

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Posted 24 July 2013 - 04:05 PM

I know that for those of you who have ceramics degrees or many years of hard won knowledge, it can be annoying to deal with people like me who have come late to clay, with no potential of formal education and limited time to learn all that needs to be known.  Recognizing that fact, and apologizing in advance, and without asking you to take your time to provide me with a complete education regarding clay bodies -- I need to understand my clay choices better:  I thought by choosing a raku body (over the earthenwares I was using before) I was choosing a clay that would stand up to potential issues like cracking and so forth.  It appears from what Mark and Wyndham have said, that I was way off base there.  I live in SC and buy clay in Charlotte NC (to avoid shipping costs, since I don't have the space to store huge quantities and I don't use huge quantities, anyway).  It's my assumption that Laguna is sold in the west; is that right?  If so, can anyone suggest a clay that is available here?  I could drive 2.5 hours to Asheville NC to Highwater Clay if there is a clay body from them that would perhaps suit my needs better.  I very occasionally use the trashcan reduction method for my work, so that is why I thought that Raku clay would be the way to go for everything....  

thanks for all your patience and helpful advice - Jayne



#17 Mark C.

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Posted 24 July 2013 - 04:29 PM

Laguna is made on the west and in the East (Ohio) and they also make some in Florida(southeast) I think as they bought up Axner in Florida some years ago..

 What I'm suggesting is trying another clay body not  particularly Lagunas but anyones body. Maybe one made for sculptures and handbuilding.One that can take the refire stress of many fires.

Mark


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#18 Pugaboo

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Posted 24 July 2013 - 05:08 PM

to the folks in georgia (i know it isn't on topic) we in the rest of the country heard about your terrible long term drought that even dried up lakes in georgia a few years ago.  is this year's rain ending the drought and filling up the lakes?  haven't heard anything on weather channel re this.


Yes oldlady we are out of drought status at least here in the northern part of the state. In fact I think we left drought status back in April or May and are now way to the other end. We are hugely high on water and every time it rains we get the flash flood warnings and such.

My little dogs have suggested I build them an ark for their daily potty runs since the ground is so wet here it is like a lake out there most of the time. A few weeks back I walked them out there and it was raining AGAIN and I swear they turned and looked at me then the sky as if to say "seriously?" Lol poor babies.

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#19 Idaho Potter

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Posted 24 July 2013 - 05:27 PM

Jayne,

Have you had a good talk with your supplier?  If they are local, then they are aware of the weather problems, and should have insight on what would work for your sculptures.

 

Because I live in the mountain west, I use Laguna or Clay Art clays (like for you, shipping is expensive).  Most of my sculpture pieces are produced from Coleman's raku (cone 06 to 10), or Laguna's paper clay (cone 6).  One of my students does life-size busts, and her work--after hollowing out--is uneven at best, so her work is ALWAYS candled.  I don't like cleaning up exploded work in my kilns if it can be avoided.  She's been using the raku clay and B-mix smooth, no grog

 

I think your first post had a good solution--fire your bisque to 04 with all subsequent firings at lower temps.  Jim's solution about firing real low then applying your copper carb and re-bisqueing to 04 should work as well.  In multi firings, it's been my experience that I fire bisque to 04 then fire to vitrification--if it's for functional use--otherwise 04 is as high as I go.  Then I work my way backwards.  Layering glazes that fire at 05, then 06 and so on down to adding lusters or china paints.  Every subsequent firing is lower than the one just completed.

 

Thankfully, I don't worry about humidity (it's been running about 6%), but if it worries you, you have to find the firing schedule that works for the clay and for your peace of mind.

 

Shirley



#20 Isculpt

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Posted 24 July 2013 - 05:52 PM

It sounds like I need to have a more thorough discussion with the folks at the local supplier, but since they are the ones who have led me thus far, I'm not terribly hopeful. I will mention the clays that have been suggested along with the general suggestions, though, and see what happens.  When I read books that mention the clay bodies that other sculptors are using, I ignore those formulas, since -- am I right about this? -- they are having large quantities of clay made up for them.  For a small user, (sounds like a dwarf addict) that option is not available, right?  Words like kyanite or talc are just something to look for in the description of the clay body, correct?      






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