Jump to content
Sign in to follow this  
Joseph

Massive Crack In Single Firing

Recommended Posts

I had a number of pieces crack like this in my last firing.   rapid heating?

Any insights would be appreciated.

Thanks

Wow,

      I've seen this kind of results from cone 10 clay bought from Hobby Lobby, and fired to cone 10. (Amaco?)

I would change clay manufactures if possible... Try Trinity, Standard, Highwater, etc..

Is there a circle of potting friends in your area that will lend you 3 to 4 pounds of clay for

testing purposes?  And make something similiar to what failed.  I don't think it was rapid firing.

     You might be able to take a broken example to a clay distributor and show them.  There are 

clay warehouses in Nashville,TN, Marietta, GA, Florence, AL, and Asheville, NC.

     I suppose it could have been from a bad batch, but I suspect its the clay body.

Who was the mfg? or was this some kind of mystery clay you picked up somewhere?

What cone was the clay?

What kiln temp did you go to?

Did they all break like this?  Were they all on one shelf?  Top shelf or bottom shelf?

Did different sizes break?  One size?  One shape?

What part of the country/world are you in?

     I think I'd change clays by going with another manufacture or make it from scratch

using recipes from another post.

      Answer the questions to try and figure this out.

    I wouldn't feel comfortable trying to make a bad clay good, because in the back of your

mind you'll have doubts.

Good luck,

Alabama

Share this post


Link to post
Share on other sites

 

I had a number of pieces crack like this in my last firing.   rapid heating?

Any insights would be appreciated.

Thanks

Wow,

      I've seen this kind of results from cone 10 clay bought from Hobby Lobby, and fired to cone 10. (Amaco?)

I would change clay manufactures if possible... Try Trinity, Standard, Highwater, etc..

Is there a circle of potting friends in your area that will lend you 3 to 4 pounds of clay for

testing purposes?  And make something similiar to what failed.  I don't think it was rapid firing.

     You might be able to take a broken example to a clay distributor and show them.  There are 

clay warehouses in Nashville,TN, Marietta, GA, Florence, AL, and Asheville, NC.

     I suppose it could have been from a bad batch, but I suspect its the clay body.

Who was the mfg? or was this some kind of mystery clay you picked up somewhere?

What cone was the clay?

What kiln temp did you go to?

Did they all break like this?  Were they all on one shelf?  Top shelf or bottom shelf?

Did different sizes break?  One size?  One shape?

What part of the country/world are you in?

     I think I'd change clays by going with another manufacture or make it from scratch

using recipes from another post.

      Answer the questions to try and figure this out.

    I wouldn't feel comfortable trying to make a bad clay good, because in the back of your

mind you'll have doubts.

Good luck,

Alabama

 

Hey,

      I just noticed this was a SINGLE firing.... In that case, I would either fire bisque and then glaze or

find that small network of potters who single fire.

Sorry,

Alabama

Share this post


Link to post
Share on other sites

Were you firing this ware stacked?

Raw glaze firing   was the ware absolutley dry before firing?

How thick are the walls of your are, temp/hour rise? Ie firing schedule may help diagnose

Share this post


Link to post
Share on other sites

The body was some highfire purge. The pots that cracked like this were on the top closest to the flame path. . I hit 2450 in about 7 hours which is tje same as my last firing where I did not have this problem although my last firing I used phoenix from highwater which seemed far grogier. I candled for the same smount of time 36 hours or so. The fraser book ceramif faults recommends wading the piec es to reduce temp differences. also more grog to reduce crack travel. I have more of this clay so I guess I'll slow the firing and wad. Any recommendations?

Share this post


Link to post
Share on other sites

Joseph did you stack pots together, could be exacerbated by pressure on the rims.  how fast is the starting end of your schedule as this is the critical tiem, or a fast drop after switch off?

Share this post


Link to post
Share on other sites

They were. on shelves single stacked.

 

Schedule 1 2 3 4 5 6 7

First 550 650 850 1125 1275 1750 2450 held for three more hours

Second. 400 700 1150 1300 1650 2100 2450 held for three

 

This is an olson fast fire style kiln. I beleive it is a heating dunt since the glaze meltes after the cracking.

Thanks!

Share this post


Link to post
Share on other sites

For comparison here is Steven Hill's schedule for single fire for the heating up portion.. Only to ^6 but it shows how slow he goes through the lower temps. Considering how substantial your pot looks I would slow down the 1st ramp even more, plus I would change the first ramp to hold at 190. (not sure how hot you candled at for 36 hours)

 

ramp 1 - 200F hr  /  220  /  hold 1 - 3 hours

ramp 2 - 100F hr  / 500  / 0 hold

ramp 3 - 400F - 500F hr  /  2100  /  60 minutes plus or minus as needed to get to ^6

 

he then slow cools on the way down

Share this post


Link to post
Share on other sites

Thanks for the schedule. I'm not sure if I can read it though. 

 

ramp 1 - 200F hr  /  220  /  hold 1 - 3 hours <- this means to hold below 220 for 1 - 3 hours?

ramp 2 - 100F hr  / 500  / 0 hold <- proceed at 100 per hour until 500?

ramp 3 - 400F - 500F hr  /  2100  /  60 minutes plus or minus as needed to get to ^6 <- after 500 go as fast as I can?

 

I candled both batches for 33+ hours the first time with propane which mantained a high 100's temp the second time with wood where the temp swund from 150 to 200 mostly at 200. 

 

thanks

Share this post


Link to post
Share on other sites

Thanks for the schedule. I'm not sure if I can read it though. 

 

ramp 1 - 200F hr  /  220  /  hold 1 - 3 hours <- this means to hold below 220 for 1 - 3 hours?  Yes

ramp 2 - 100F hr  / 500  / 0 hold <- proceed at 100 per hour until 500?  Yes

ramp 3 - 400F - 500F hr  /  2100  /  60 minutes plus or minus as needed to get to ^6 <- after 500 go as fast as I can? No, go at or between 400 to 500 an hour up to 2100 (which will be approx ^5, just go as hot as you need to for your firing. He soaks for a long time to get the look he wants, if you don't need this then just fire to your top temp or near as you need to get and then soak if you do that to get your cones how you want them. I think he goes at that rate because most electric kilns won't be able to go any faster.  

 

I candled both batches for 33+ hours the first time with propane which mantained a high 100's temp the second time with wood where the temp swund from 150 to 200 mostly at 200. I would imagine this would be ample to dry out the pots, so this would be your ramp 1. It's the second ramp that I think you need to follow.  I would err on the side of caution and go slow until you have a schedule that works then if you want to gradually try speeding it up.

 

thanks

Share this post


Link to post
Share on other sites

Looking at the photo image of the first post, you can see that the glaze surface has pulled away from the cracks that you can see.  This tells me that the cracks formed on the HEATING cycle, not the cooling side of things.  The cracks were there when the glaze started to fuse.  So this cracking happened on the front end of the firing.

 

They look like cracks that happen with raw glazing when the body is damaged by the appluication of the glaze...... cracks start forming in the body from the uneven expansion of the part of the pot where the glaze water is penetrating into the surface... and not in other areas.  Uneven thickness of the walls can exacerbate this.

 

Often you can't see them under the new layer of glaze material.... but they are there.  Then they show up in the firing.

 

Are you glazing at leather hard or bone dry?  Leather hard is the best time to do this for fewer issues.

 

best,

 

................john

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

×