Jump to content
Sign in to follow this  
Ginny C

Why New Blisters On Re-Fired Glazed Pot?

Recommended Posts

I had a lovely bowl with one small glaze flaw/small blister crater inside. I dremeled it slightly and put a bit of clear glaze over the spot and re-fired it, again at cone 6.  Now it has blisters all over, inside and out! Ruined.  Why?

 

Laguna B-Mix cone 6 clay. Coyote's Ice Blue glaze.  One difference in the firing—fast speed the first time (although I forgot to take the prop out until late, so the lid was slightly open during the first 1700 degrees F)  and medium speed the second time.  Surely that couldn't have caused the new blisters, could it?

Ginny C.

 

 

Share this post


Link to post
Share on other sites

Refires usually are always different the 2nd time around-sometimes they are great others they get worse. Its always a c--p shoot.

I refire a few items every week in my cone 10 car kiln(reduction fire) andI'm not farmiliar with electric cone 6 but all refires in most enviroments are a gamble to some degree.Some galzes work well some do not and thats true with clay bodies as well.

One general rule is heat them up slower as the body is tighter.

Hope this helps some.Maybe a cone 6 person can add more.

Mark

Share this post


Link to post
Share on other sites

I find have better luck with refires of blisters or pinholes to glaze the entire surface with another glaze you know runs a little bit. Yes the color will change but its better than your one little flaw turning into a dozen little flaws.

Share this post


Link to post
Share on other sites

I have had the same thing happen. I just fired a B-Mix vase for the third time, as there was one blister on the outside. I ground it off, added glaze to that spot and refired. The outside, a runny glaze, came out perfectly. The inside, lined with Clear Liner Glaze from Mastering Cone Six Glazes, is covered with blisters, the first problem I have ever had with this glaze.

 

I have had best luck with a very slow glaze fire, slow to heat, short hold at the top, and controlled cool. I have really had no glaze problems on B-Mix other than this one piece.

Share this post


Link to post
Share on other sites

This is almost for sure a body outgassing issue.  Likely caused by the BISQUE firing from a lack of enough oxygen present for enough time, not from the glaze firing.   The cumulative heatwork from the second glaze firing just allowed the more melted pyroplastic clay body to show what was hiding inside the body to start with. 

 

You likely can't fix it at the glaze firing point.

 

Many "glaze firing defects" come from the bisque firing, not from the glaze firing.  They only SHOW UP in the glaze firing.

 

Tightly stacked bisques, particularly fired in electric kilns, often have the issue of two things that combine to cause issues in the GLAZE firing.  On is poor oxygen flow and dispersion in the load.  The other is the thermal lag of the load.  Put em' together.... and whammy.

 

There are numerous reactions that have to happen to the clay body in the bisque firing.  Some require oxygen to be present (inside the clay body walls).  Others just need to have enough time for evolving gases to migrate out thru the clay body.  Some just need to have herat energy applied.   All have a specific temperature or temperature range at which these reactions happen.

 

Cones are usually out in an "exposed area" (in the open) where you can easily see them.  Thermocouple probes for controllers are also.  These devices are measureing the heatwork and temperature in a location very different from the interior of the walls of a piece of ware, particularly if the loading of stacked up wares is densely packed.  Therfore, sometimes the work is not fired to the heatwork that you THINK it is, no matter what you cones or controller are telling you.  This issue is called "thermal lag".  That's the amount the load is "lagging" behind the indicated temperature on a measuring device.  There is ALWAYS some thermal lag.

 

Then there is the penetration of oxygen into the load.  The edges of a stacking get this O2 pretty well.  The interior of a dense load does not.  If the kiln does not have good air circulation from an active draft of some sort, this issue gets worse.  Electric kilns without local pickup vents are very difficult to get "right" with a dense load.  Local pickup vents improperly installed or too small for the kiln unit are also causing issues with inadequate airflow.

 

If you tend to nest bowls one inside the other in large stackings in an attempt to have an efficient use of space... this can make this issue worse, affecting both oxygen penetration and thermal lag. It can also impede necessary outgassing.

 

This stuff is an IMPORTANT understanding to mastering firing operations.  A main point from my ceramic materials and also kiln design and operation courses....... There is no such thing as a cookbook firing schedule.  Firing is specific to the particular material being fired, the particular kiln being used, and the specific stacking job.  You need to know when you have to alter thngs to get optimum results.

 

SOMETIMES, you can fix the poor bisque by dragging out the lower (bisque range) part of the GLAZE firing.  Effectively you are doing what you should have done in the first place.  Glaze firings are almost always stacked more openly....so heat penetration into the load and air circulation is better.   However if you have a glaze application on the pieces that tends to become gas impermeable (particularly to O2) at a low temeperature (like American soda based shinos in highfire), then this can block the gas exchange.

 

Slow down your bisque firings, and make sure the local pickup downdraft vent is actually working as intended to get airflow.  If this happens to you a lot, don't cram in as much ware in the bisque loads.  If it is the occasional piece........ it likely was a happenstance of the way that one particular piece was located in the bisque firing....and comes under the category of "%$#@ happens." ^_^ .

 

Hope that explanation helps.

 

best,

 

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

timbo_heff, GEP, Patsu and 1 other like this

Share this post


Link to post
Share on other sites

Thanks John, I had an "aha" moment while reading your post. I recently refired a pot and got a whole bunch of blisters. I've also been stacking my bisque loads a lot tighter than before. I'm going to try slowing down my bisque firings.

Share this post


Link to post
Share on other sites

And thanks from me, John!  That's good information and advice. I will set my kiln control thing for slow for all bisque firings now, and maybe a short hold at the top?  Also probably I should leave the top and bottom bung out throughout the bisque firings—does that sound right?  (It's a small kiln, I'm not at home and forget the brand, but the interior is only 15 inches across by maybe 20 deep.) So yes, I often stack the pots to be bisqued. And then do two glaze loads from the one bisque load.

 

You didn't mention bisque firing to a higher temp. I always set it for cone 04. I won't change that...unless you say to OR I continue to have blisters even following your suggestions. :)

 

Thanks to all responders!

 

Ginny C

Share this post


Link to post
Share on other sites

I also thank you John that information is invaluable. I've thought about switching from a slow bisque to a fast one but after reading your explanation think I will stick with the slow I'm not in such a hurry to chance losing all of it just to save a few hours. My pieces tend to be all different sizes too with coil and slab as well so I think taking it slow makes sense to get that temperature as even as possible all the way through. The only pinholes I have been having issue with is with the Palladium glaze I simply cannot get it to come out without at least 1 noticeable pinhole, have tried reglazing the pinhole and refiring but then as mentioned above I get more pinholes not less.

 

Terry

Share this post


Link to post
Share on other sites

I use Laguna B-Mix, cone 5 as my primary white clay and Plainsman M390 as my primary red. I've learned the hard way refiring the Bmix is a crap shoot, with blistering happening more often than not - I've experimented with bisque temps and firing schedules as well. On the other hand I can refire the M390 more than once with no issues.

Share this post


Link to post
Share on other sites

I should add that the issue with refiring hasn't deterred me from continuing to use BMix. It has many positive qualities and continues to be my clay of choice. I've just learned its best to toss the pots that might have benefited from a refire.

Share this post


Link to post
Share on other sites

Ginny C, try to slow down your bisque firing after you reach 600C (1112F) or so, and go to 850C at 100C per hour so carbon and sulfur have time to leave. When you reach 900C (1652F) most of the carbon and sulfur will be burned out.

If you got some really thick pieces, you probably want to do a soak at 800C, to make sure most of that stuff is gone.

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  

×