^6 matte with zircopax pinholing problem

[Haley]

Hello potters! I have had a rise in pinholes on a clay (Standard 553 Buff) and glaze that I’ve had relative success with for years. I’m hoping someone has dealt with something like this before. I use high Calcium semimatte Base 1 from  Mastering Cone 6 Glazes with 8-10% zircopax. Here is the recipe:

20      3195

29      Wollastonite 

4         Neph Sye.

30      EPK

17      Silica

8-10%  Zircopax

I have always had minor pinholing but recently have had too much to sell the work and feel good about it.  I’ve tried semimatte base 2 from the book also and still get the same amount of pinholing. I bisque to ^05-04ish and I do a drop and soak from:

2185  hold for 10min rate 400 to

2085 hold for 30min 

Slow cool from 1900-1400 rate 80

I hope that is enough info to get started. Let me know if anything is unclear. I’d be willing to use a different glaze if it comes highly recommended but would love to tweak what I’m already using if possible. Thanks in advance!

Haley

[Kelly in AK]

I don’t want to bark up the wrong tree, so I’ll ask this way: What’s changed?

You’re using the same body, but is it possible the formula has been changed? The clay maker might not give you the formula, but they’ll probably volunteer whether and how it’s been changed. Have you used cones lately to verify the kiln is doing what you think it is? Which material you’re using is the newest purchase?

That’s a of questions and no answer. My feeling is you’ve been riding on the cusp of this a long time. The drop and hold schedule suggests you’re already correcting for it (edit: I imagine the drop/hold and slow cool may also be to enhance mattness). Some single variable pushed it over the edge is my guess. 

Edited January 11 by Kelly in AK

[Haley]

Some of the glaze raw materials are new. I also received a new batch of clay so it could be that but I haven’t reached out to Standard yet. I remixed my glaze to rule out a glaze mixing error. 
 

I understand zircopax stiffens glaze melt so I tried mixing samples with less zircopax but the white isn’t bright enough. I’ll try to add a picture so you can see the pinholes. 
 

another challenge is that my test tiles come out just fine when I use base 1 or base 2 with various amounts of zircopax but once I commit to trying one of the glazes I get pinholes on taller forms like a travel cup or kitchen crock. 

[Haley]

[Marilyn T]

Hi Haley,

I don't have an answer for you but maybe my ongoing experiences with pinholes will help somehow.  

I am also working through a pinhole issue on my new-to-me brown clay.  I have eliminated almost every variable suggested and have focused on my firing schedule as the most probable cause of the pinholes.  I also had a discrepancy on results of test tiles and ware, so now I do all my testing on 1 lb. bowls.

If you decide to explore changes to your firing schedule, then my advice to you is to do your absorbency testing at cone 5 6 7 before you begin.  For example my % absorption is .2 at cone 5, 0 at cone 6, and 12 at cone 7.  Therefore I know that heatwork above cone 6 is a no go for testing.  

 Good luck.  I will be following this thread to see what worked for you on your clay.  

[GEP]

Based on simply looking at your photo (which is not very scientific), the glaze appears to have been applied too thickly to me. Over thick application is a common cause of pinholes. Perhaps the thing that has changed recently is the amount of water you are using? Do you measure for specific gravity? 

Another common cause of pinholes is underfiring. Sometimes a 5 degree higher firing can solve a pinholing problem. Perhaps the real issue is that your elements or thermocouples need to be replaced, and therefore your kiln is slightly underfiring? A firing with witness cones can answer this question. 

Edited January 11 by GEP

[Hulk]

Do the holes go all the way to the clay?
...looks like they do (also not scientific!). If so, perhaps there are some larger "off gassing" particles in the new clay?
They appear (also not scientific) to be random - all over the surface. If you're dipping, the glaze layer near the foot should be thinner.

Are the problem forms thicker than your test tiles? If so, that could be another indication of problem particles, for the thinner wares "burn out" more and faster in the bisque.

And so, if the new clay has more/bigger bits of stuff to burn out, a longer more thorough bisque fire - with plenty of oxygen - could help.
See also drop & hold glaze fire: Drop-and-Soak Firing (digitalfire.com)

 

[Kelly in AK]

To be sure, a picture is worth a thousand words. Very helpful. The clay body looks gritty. The pinholes are not like craters at all, no raised rim, not looking like bubbles that popped and didn’t heal. Every one is more like a sinkhole. Widely but evenly spaced, not concentrated in a particular area. Important  diagnostic clues. I can’t say more at the moment, I just know the picture tells a lot. 

[Haley]

Thanks for all the responses! 

Matalyn T - I’m not familiar with absorbency testing. I’ll look it up. 

GEP - I can toy with thickness. It just hasn’t been a problem till recently so it feels to me like there is something else going on, even thin applications are pinholing on me right now. I’m using witness cones and hitting my temps so I know I’m hitting cone 6 for glaze. I could up my bisque since my kiln is really only going to 05 when I set it to 04. 

Hulk - I’ve been wondering if the foreign material in the clay is giving me off gassing trouble. The pinholes go down to the clay and some of them go down into the clay. I can put a needle tool down into the clay. Like something burned out. I took a pic of some of the plastic, rubber and wood that I’ve been picking out of the clay. These crock’s are thicker because they are larger forms so there might be more junk in the wall. 

Kelly - many of them are deep. Even the tiny ones you can’t see the clay in the bottom because they go so deep. They are just black holes. 
 
Here is a pic of some of the debris I’ve been picking out of the clay. I’ve lost a lot of it but started collecting it recently. I found a piece of crumbly black rubber about the size of my fingernail that I lost. That 1/2 inch long piece is wood. 
 

 

[Min]

What is your firing schedule for bisque?

Also, are there more pinholes on trimmed areas? Once the pot is thrown do you rib the outside? Any chance you have some other clay handy that you can try the glaze on? 

[Callie Beller Diesel]

If you’re getting that much crud out of your clay body and the pinholes are going straight down into the clay, that says to me the clay is the problem, and possibly batch related. I’d reach out to Standard with that image as a start. If you don’t want to get rid of your existing clay, you may do well to slow down your bisque if it’s not already moving at the slowest speed. Speed is more valuable in my experience than top temperature. If it’s already going slow, another option is to add a soak hold in the carbon burnout stages, to allow all that junk a chance to escape. 

[Haley]

Callie - I have reached out to standard and I’ll try to figure out what the dact

[Haley]

My mistake. I hit send too early.
- Factory cone fire mode “slow” setting actually is. They both seem like next steps for me. I’ll reach out for feedback after my samples (from another batch of clay) come out of a glaze fire and I hear back from standard.