Cone 5 vs. cone 6 glaze differences, strength, and bloating

[plumcreative]

Hi all

I fire an electric kiln at cone 6 for glaze and have a few questions.  We are trying Standard 266 dark brown clay and are experiencing bloating in about half the work.  Standard says to fire at cone 5 for the 266 but, in order to not fire almost empty kiln, I would need to combine with projects made with other clays.  Also, we use a lot of different glazes and have tasted them at cone 6.  So my questions are:  1- Would changing to cone 5 or 5.5 change the results of my glazes?    2- How different is the strength of pottery fired at cone 6 vs. cone 5 ? ( Is there a chart somewhere that shows the difference in strengths for different cones?).

I also read somewhere that the bloating can be fixed with change to the bisque firing schedule-- ie: staying longer at highest temp to give the clay time to release the gasses that cause the bloating.  Has anyone had experience with this?  

And, if we decide to try firing to cone 5 or 5.5 can you recommend some firing schedules that would work best for our cone 6 glazes?

Cheers,
Amy

[Denice]

I have been working with C5/6 since the 70's and have never had trouble with bloating.  I fire a regular bisque schedule to C04,  it will take  about 7 to 8 hours depending how full it is and if I remember to turn up the kiln.   It could be a bad batch of clay I have run across that in low fire clay,  I would contact your supplier.   Denice

[plumcreative]

I have contacted Standard and they say bloating is definitely an issue with this clay-- so not just a bad batch.  My studio-mate fires at another location sometimes and does not have problem with the clay bloating there.  They fire at about 5.5.  Standard also indicates to fire at cone 5 but I am trying to ascertain the repercussions of doing so.  We use a lot of cone 6 glazes which I have tested at cone 6 and would prefer not to redo all that testing.  We use different clays in the studio and I fire all together to get full kilns.  I don't want to fire the 266 alone in almost empty kiln-- and also, since I am the one doing the kiln watching, I don't want to do more firings than necessary.

I really want to know about other peoples experiences with this particular clay and their experiences with differences of glazes when firing cone 5 vs. cone 6.  And also if there is any kind of chart or information giving info on strengths of final work fired at the different cones.

[neilestrick]

That clay usually bloats at cone 6, although I have heard a couple people say it works for them at 6. I've always just done a separate firing for that clay, and my students understand that we have to have a kiln load if they want it fired. As to whether or not a higher or longer bisque will help, it's worth a try. You're either going to have to change your bisque or change the glaze firing if you want to use it. Personally, I change the bisque, because dropping your glaze firing will affect your glazes, unless you tweak them.

[yappystudent]

I've noticed very dark clays tend to be fussy in general during firing, bloating and not playing nice with many glazes. Oregon Brown is a clay I like to use because it seems more stable than the others, I'm not sure it's available away from the west coast US. Ironically georgie's in Eugene doesn't carry it, maybe their Portland store does?  they have their own version which I haven't tested yet. I bought my last block of OB in CA, (humboldt county, store called phoenix). OB turns cocoa brown in bisque and dark brown with quite a lot of grog at ^6. If you fire it again at ^5 it's essentially blackish w/ brown overtones. I found it a decent dark sculpture clay. Never really finished my experiments with it but it's good stuff, stable for a dark clay and kinder to glazes than say Cassius. 

[PeppernPatches]

I also use Oregon Brown and haven't had bloating issues in cone 5 or 6. I did have bloating with Electric Brown clay. I bisque to cone 06/07.

[plumcreative]

Thanks for the input everyone.

Oregon Brown sounds like something we would like to try but we're all the way on the other side of the country in DC area.  

 

[Min]

9 hours ago, plumcreative said:

I fire an electric kiln at cone 6 for glaze and have a few questions.  We are trying Standard 266 dark brown clay and are experiencing bloating in about half the work.

@plumcreative, the bisque cycle is really important with dark clays. The kiln needs to be well vented up to around 1650F. Lots of space around the pots if possible, vent on, or peeps open if you don't use a vent, and you need to go really slowly up to that temp. Lots of oxygen is needed to get a clean burn out of the carbons. If not well vented the carbon atoms will try and pull oxygen from whatever they can, including the red iron in the clay. If that happens the red iron oxide (Fe2O3) converts to black iron oxide (FeO) which will flux the clay. This fluxing will tighten up the clay and won't allow the gasses to pass through the clay walls. If the gasses can't escape they will collects under the clay wall and cause bloats during your cone 5-6 glaze firing.

[S. Dean]

My experience  is that 266 bloats at Cone 6 .  I have significantly less problem at Cone 5.5 but even then it can have intermittent issues.  And ....don't even think about re-firing it as it will almost certainly bloat in a second glaze fire.   I bisque to Cone 05 and would be interested in knowing if folks get a better result at Cone 6 if they bisque to Cone 04. 

It is a beautiful clay.  

-SD

[PotterPutter]

I might be oversimplifying this, but isn't the groundwork for bloating created when the bisque temp is too low?

@plumcreative, what Cone are you bisque firing to? I use 266 almost exclusively and haven't had any problems. I bisque to Cone 04 and glaze Cone 5.5. I have fired to ^6 and re-glazed and re-fired pieces up to 3 times without any issues.  

 

[plumcreative]

PotterPutter, we normally fire bisque to 05.  If we go to 04 will we see much change in glaze absorption? And do you think we would see much difference in glaze going from cone 6 to cone 5.5?

Timbo Heff (in another thread) seems to have similar thinking as Min— he said it helps to hold bisque at 1525 (below point of sintering) for a couple hours to give gas more time to escape.  (He indicated that this can be done at any bisque firing from 08 to 02.)

I really appreciate everyone’s great input. I am starting to formulate a test plan.  

Cheers, Amy

1 hour ago, PotterPutter said:

I might be oversimplifying this, but isn't the groundwork for bloating created when the bisque temp is too low?

@plumcreative, what Cone are you bisque firing to? I use 266 almost exclusively and haven't had any problems. I bisque to Cone 04 and glaze Cone 5.5. I have fired to ^6 and re-glazed and re-fired pieces up to 3 times without any issues.  

 

 

[Babs]

Read Curt's reply in the rebisquing post you created

[plumcreative]

Thanks Babs.  I found it.  Super helpful.

For anyone else interested you can read some really detailed info here: