Mayco Fundamentals Underglaze on Standard 266

[Jengin]

Hello.

I’ve been working with Mayco underglazes on Standard 266  green ware with inconsistent results. My process involves a total of 1-4 layers of underglaze in different areas of a piece. The layers can be made up of 1, 2 or 3 different colors. 

The problem that I am having is rough, bumpy areas where the underglaze was applied. It feels like sandpaper and has tiny holes. It doesn’t matter if it was 1, 2, 3, or 4 layers. 

It doesn’t always come out bumpy though, which is why I haven’t yet given up on this technique! I am currently having more failures than successes.

Underglazes  that I apply over slip (oestreich on 266) turn out fine.

I work in a community studio that has 14 kilns and at least 7 different kiln-loaders.  

My pottery instructor has offered to fire some test pieces of mine at different temps to help determine if it’s a firing issue, but I thought I’d check with you all here in the meantime to get your ideas.

Thanks  for your help,

Jen
 

 

[Bill Kielb]

A common cause we have found over the years in this thread. Also, Sue McLeod is currently doing a crowd sourced testing of various underglaze defects I believe. Link here:https://suemcleodceramics.com/troubleshooting-commercial-underglazes/

 

[Min]

Hi Jen and welcome to the forum.

Just to confirm, the clay you are using is this one and the underglazes are fine on the same clay when Oestreich White slip is applied over the same clay then the underglazes applied over that? Slip recipe? Is there a bisque fire after the slip is applied and before the underglazes or slip + underglazes on raw clay?

What is the bisque firing schedule and what cone is the glaze firing done to?

[neilestrick]

266 is not a 'normal' clay. It's loaded up with iron and manganese and it sometimes does odd things to glazes, too. The slip is creating a barrier between the clay body and the underglaze, preventing  the issue, so that makes sense.

What cone is it being fired to? 266 should not be fired above cone 5 or it is prone to bloating. I bet the problem is more pronounced the closer the clay gets to its bloating stage.

Are specific colors having the issue or is it random?

[Jengin]

21 hours ago, Bill Kielb said:

A common cause we have found over the years in this thread. Also, Sue McLeod is currently doing a crowd sourced testing of various underglaze defects I believe. Link here:https://suemcleodceramics.com/troubleshooting-commercial-underglazes/

 

Thanks, Bill. 

I hadn’t thought of the clear glaze possibly being the culprit. There are so many variables to consider! I do use Amaco Zinc Free Clear though, so after testing firing schedules, I’ll look into that possibility if needed. There are a few of the Mayco underglazes that I have decided to stop using altogether because they don’t play well with others. It’s the ones that “sometimes work” that are throwing me. 
I’ve checked out Sue’s website, too.

Thank you for your help.
 

[Jengin]

21 hours ago, Min said:

Hi Jen and welcome to the forum.

Just to confirm, the clay you are using is this one and the underglazes are fine on the same clay when Oestreich White slip is applied over the same clay then the underglazes applied over that? Slip recipe? Is there a bisque fire after the slip is applied and before the underglazes or slip + underglazes on raw clay?

What is the bisque firing schedule and what cone is the glaze firing done to?

Hi, Min.

Yes, that is the correct clay.  Oestreich white is applied over the 266, and underglaze on top of the oestreich.  Underglazes, resists, then oestreich, then more underglazes on top of the oestreich. All on raw clay prior to the bisque fire.  
I do not know the firing schedule at the community studio, but I will find out. I do know that they recently changed from cone 6 to cone 5 due to some problems, but I don’t know any specifics on that. 
Thank you for your reply,  Min.

Here is an example of one of my pieces:

 

 

[Jengin]

10 hours ago, neilestrick said:

266 is not a 'normal' clay. It's loaded up with iron and manganese and it sometimes does odd things to glazes, too. The slip is creating a barrier between the clay body and the underglaze, preventing  the issue, so that makes sense.

What cone is it being fired to? 266 should not be fired above cone 5 or it is prone to bloating. I bet the problem is more pronounced the closer the clay gets to its bloating stage.

Are specific colors having the issue or is it random?

Hi, Neil. 

I’m beginning to reconsider my clay choice! 

Good point about the slip barrier preventing the rough spots. That does make sense. 

There are a few colors that consistently failed for me, so I no longer use those. Unfortunately, the other 6-8 that I use have the issue randomly.  
 

Thank you so much for your response. I am going to start with your cone 5 advice and hope to get positive, consistent results. Stay tuned.
 

[Min]

For "dirty" claybodies like the one you are using it's really helpful to allow enough time and ventilation for the carbons etc to burn off during the bisque. If the bisque schedule is rushed and/or not vented well then the gasses that should have been cleared off during the bisque will try and find their way out during the glaze firing. Organic carbons bur out from arund 300-600F, inorganic carbon in the 1300-1650F range. Going slowly and/or doing a soak/hold to allow the carbons to burn off, especially for the inorganic carbons will help with pinholes etc. Venting the kiln will allow better burn off also, try and request your pots aren't stacked inside other pots.

+1 for not firing manganese bodies to cone 5.

Nice pot!

 

 

[Jengin]

Min, thank you so much for your detailed and informative response. I think I have a specific direction to go now.
I really appreciate it!