Glaze for black clay

[Matka]

Hi ceramic community! I'm a beginner potter and I just started to work with black stoneware clay (GSM CH 0-0.2mm), and glazed with some mayco stoneware glaze. It turns out these are not compatible since I got bubbles on the surface. I know the best way is to test glazes, but is there some rule with the compatibility of glazes on black clay, or somebody has experience with this type of clay.

Thank you

[Hulk]

Hi Matka, Welcome to the Forum!

Experimenting with a bag* of black stoneware (cone 5), a few points from my experience:

Thorough bisque, with oxygen throughout, long hold at around 1500F, and peak a bit hotter than my usual cone 04.

No thick parts!

Drop and hold glaze fire.

Just hit cone 5, no more, not even a tiny bit!

I did have luck with a liner glaze, also a red and blue. I'll circle back with details later today.

Dense and durable when fired, very nice - the un-bloated wares with good glaze

...but not worth the problems, for me...

*just one! ...by request from a friend. Expensive, finicky, maybe again, some day.

 

...and later.
Thick parts, more prone to dreaded bloat.

I had good luck with "Wollastonite Clear" - it cleared the bubbles better than the other clears I tried, not perfect, but very serviceable.
"Chrome Red" and Lakeside Pottery "Clear Blue" worked fairly well for me.
The green I like frothed up; I don't remember trying my other glazes.

"Wollastonite Clear" credit Bethany Krull
Found the recipe in a book. It's not in my lineup now...
There are images and the recipe in this thread:
Seattle Pottery Supply - Midnight Black Clay - Page 2 - Clay and Glaze Chemistry - Ceramic Arts Daily Community

"Chrome Red" is in John Britt's book.
The color can wash out some, but I like the look. I got the recipe from the Ceramics lab I took Wheel I and Wheel II in...
Chrome Tin Red | Glazy

 

"Clear Blue" found the recipe on Lakeside Pottery's website
Clear Blue | Glazy

Edited November 28 by Hulk
added detail

[Joseph Fireborn]

Hello Matka,

I wouldn't recommend a black stoneware as a beginner, I say that with a lot of conviction as I went down that path. It was a path of frustration and resistance. Pick an easy to use stoneware that is reliable and glazes come out nice on, and progress from there. It won't take long until you are more comfortable and feel ready to try black clay again. Just be prepared for a letdown. I honestly don't even think the product should be sold without some warnings on it. The only black clay I have ever found to work well is Black Ice by SIO2. It is very expensive but absolutely stunning and the black part has no effect on the glazes on top of it. I will say it is an absolute mess to throw well if you are a wet thrower. It requires learning to throw porcelain that is not really sturdy first and foremost, then move into black ice. In the end I stopped throwing black clay entirely and got much smarter and decided to throw a really reliable brown stoneware and coat that greenware with black slip that I formulated from dried out black ice porcelain  with additives to give it the surface results that I wanted. All of this was years of testing and hundreds of firings to achieve a simple pretty basic result.

If I could do it over, I wouldn't start with black clay. I would start with whatever I find gives me the best results when I open my kiln so that I can be happy. I opened 25-50+ kiln loads with black clay that were bloated or blistered no matter the bisque schedule, no matter the amount of oxygen I sucked into the kiln during the bisque, no matter how long I held the bisque etc. Frustrating.

Edited November 27 by Joseph Fireborn

[neilestrick]

The black clay we use in my studio, Standard 266, tends to bloat at cone 6, so that could be the problem here. When you say it bubbled, do you mean the glaze or the clay? If the clay is bubbling, then it's being over-fired. Use some cones in the next firing to make sure it's firing accurately. With the 266, most commercial glazes work well with it at cone 5, especially some of the Amaco glazes like the Potter's choice series. It will definitely take testing, though, as the black clays tend to make the glazes look a bit differently than you'd expect.

[Matka]

Ok, that was quite useful, thank you. I still don't own a kiln, so I think my friend fired it at cone 7, or maybe 8, since the max temp is 1260°C stated by the manufacturer. The bloating was in the glaze, not the clay. Also, with some transparent glaze it worked fine, but with this specific (Mayco turquoise stoneware) glaze it didn't work. I'm still in the experimentation phase and just trying out things, getting used to failures and lowering expectations  

[Denice]

I tested 40 test tiles of black clay with glazes I already had mixed in my studio,  the clear and black worked well and I had a glaze Flaky Lime Green that turned into a black bronze.   I still use black clay for sculptural projects,   I wouldn't use it for functional pottery too dangerous.     Denice

[Joseph Fireborn]

23 minutes ago, Matka said:

Ok, that was quite useful, thank you. I still don't own a kiln, so I think my friend fired it at cone 7, or maybe 8, since the max temp is 1260°C stated by the manufacturer. The bloating was in the glaze, not the clay. Also, with some transparent glaze it worked fine, but with this specific (Mayco turquoise stoneware) glaze it didn't work. I'm still in the experimentation phase and just trying out things, getting used to failures and lowering expectations  

Something to understand here as a beginner. This might not have happened or it might have happened, but I am going to outline it just in case.

The black body of clay could have been offgassing while the liquid glazes were melting.

Think about it like this. The mayco glaze could have hardened faster due to the formulation, which trapped the defects of the offgassing in the glaze. The transparent glaze could have taken longer to harden which as the clay stopped offgassing the glaze is still melting, which heals over the imperfections and makes it seem like a glaze defect.

This is what I was talking about in my earlier post. It is very difficult to understand all of this at the beginning and why I wouldn't recommend working with a darker body in the beginning. Knowing that you are using someone elses kiln and have no control over the firing, even more so recommend changing bodies.

[Callie Beller Diesel]

+1 for what Joseph said about how the black clay fires. If you have your heart set on using that clay, you’ll have to go into a deep dive on how to clear microbubbles via firing cycles and glaze chemistry. 

If you want the look of black clay, I suggest a black slip, underglaze or engobe applied over a more user friendly clay body. You can get some really stunning results by applying a single glaze over contrasting background texture/decoration. A black or darker clay background will bring out a lot of variegated effects on a number of midfire glazes, whether commercial or homebrew.

If you just want a glaze that *looks* clear over your black clay, add a pinch of iron to a clear that already kinda works and fire to cone 5. The iron acts as a fining agent to release all the bubbles, and the amber colour disappears against the dark background.