High fire clear glaze over under glaze - cloudiness

[Hendrixl114]

Hello,

I am doing sgraffito on c10 porcelain using Speedball black and Amaco Velvet  underglaze and high fire clear glaze my instructor makes.  I’m the first student she has had to do this and my first few pieces came out of the (gas) kiln cloudy and bubbly over the black.  Does anyone have thoughts on why this might be happening?  The clear was applied in one dip so I don’t think was too thick.  Oh, and this happened with both bisques and dry greenware.  Are there other glazes I could buy/try?  Should I thin the clear before applying?  Thanks very much for any advice!

[Bill Kielb]

Pictures for sure for ideas. One experience  - Not all clear glazes melt well over underglazes so the underglazed decorated area can be locally refractory. The end result for this defect, bumps, orange peel, glaze that does not cover and melt well to a smooth finish. Often mistaken as over fired and bubbling. Additionally if heavily underglazed, when applying glaze over the top the underglazed area is far less absorbent than naked bisque so it can be hard to apply glaze over the top evenly. Spray applying overglaze and drying is one way to get a pretty even coat over the entire ware.

Pictures for sure here as there can be other reasons for sure. Most underglazes need to be tested with their overglazes in the manner they will be used to be reasonably sure the combination works as intended.

[Hendrixl114]

Thank you!  I am out of town but will provide pictures when back.  I applied 2 coats of the underglaze before carving and bisque firing. You are right,  the clear glaze only bubbled and looked like orange peel and was cloudy where the black underglaze was applied, not on the bare clay areas.  I will run some more test tiles and try fewer coats of underglaze and spraying clear glaze.
 

do you have any suggestions for a clear glaze that might work better over underglaze that I can buy?  I have seen several commercial glazes (laguna I think) that I was thinking of trying.  Also, does it matter whether I am using porcelain or stoneware?  Is this just a glaze issue or a glaze + clay issue?  
 

very much appreciate your feedback!

[neilestrick]

Many commercial underglazes just don't hold up at cone 10. They start to flux out and that causes them to behave poorly when you put a glaze over them. If you're just looking for black or blue, you'd probably be better off making your own underglaze or slip. If you do want to use commercial underglazes, each color will need to be tested to see how it holds up and cone 10 and with your clear glaze.

[Bill Kielb]

On 4/22/2024 at 10:14 AM, Hendrixl114 said:

do you have any suggestions for a clear glaze that might work better over underglaze that I can buy?  I have seen several commercial glazes (laguna I think) that I was thinking of trying.  Also, does it matter whether I am using porcelain or stoneware?  Is this just a glaze issue or a glaze + clay issue?  

I do not have a preferred store purchased overglaze. I spent a whole summer trying to resolve this for our cone 6 glazes so our artists could paint freely with most underglazes. The solution was to modify a clear with a bit more boron to more completely melt. If this is your issue then testing other commercial. over glazes  as well as lighter coats of underglaze  are probably your best bet.

I have never noticed a difference between clays but only anecdotally tested on the variety of studio clays used at the time. Color, thickness applied and even underglaze supplier had the greatest effect - all reasonably resolved with a slightly lower melting temperature of the modified clear. Not all under glazes will go to cone 10 as well with significant color change often being the effect of higher temperature. All our artists, especially sculpture test their underglazed for color. It was not uncommon for a sculpture artist to say, I need to stop at cone 2 - no higher as the perfect color of a full sized human bust that took them a month to make might change from what they wanted. A bit tedious, but an example of all tested in advance.

Edited April 24 by Bill Kielb

[Kelly in AK]

Commercial glaze makers have tuned in to the cone 6 market very well in the past twenty years. You can be assured the claims they make hold true most of the time. I’m impressed to see color charts and samples fired to cone 6 of products originally marketed as low fire. The formulas are proprietary so there’s no way to know if or how they’ve changed them to accommodate the higher temperatures. They “just work.”

Cone 10 is different. In fact, they don’t make many claims about that, other than “it might work.” This kind of firing usually happens in a gas kiln and a reduction atmosphere. Cone 10 firing in an electric kiln is unusual. The atmosphere makes a difference. Finding a different clear glaze may be a worthwhile chore. 

Another avenue to pursue is using slip. The easiest course is to use your clay body and add a significant amount of black Mason stain (6600 is my go to).

Unfortunately, I can’t suggest a percentage because I haven’t personally fired those test tiles yet (Coincidentally, they’re slated for Saturday, 20% 6600, 80% B-mix5. It’s in a soda firing at cone 6, rather different from what you’re experiencing, but I’m still shooting for clean blackness like you. I’ll let you know), fortunately, you shouldn’t have to worry about fit issues because the slip is mostly the clay body, unfortunately, you’ll have to apply it in a wet or leather hard state to be safe, fortunately, you’re doing sgraffito which suggests you’re already doing that, unfortunately, you’d have to mix up the stuff yourself, fortunately if your current clear glaze works with the clay body it should work with a slip made mostly from the clay body, unfortunately, maybe those bubbles were there already and you didn’t see them until they were over a large black surface and it is just the glaze…

Fortunately or unfortunately, this is ceramics. It only gets better. 

[Hendrixl114]

Here is a picture…

IMG_0370.pdf

[Min]

Do you know what the specific gravity of the glaze is that you did the one dip of? How long did you did it for? Have you tried a much thinner coat of the same glaze?

Amaco does publish a colour chart with all their Velvet underglazes fired to 05, 5 and cone 10. If you scroll down to the blacks it looks like both the Jet and Velour Blacks survived cone 10. From the little inset blurb about clay and glazes used it appears that HF9 was used on both the cone 5 and cone 10 samples. If you try this commercial glaze please do check with Amaco on this as this is actually a cone 5 glaze.

https://cdn.shopify.com/s/files/1/1023/7667/files/Velvets_Underglaze_all_temps_brochure.pdf 

Edited Sunday at 11:44 PM by Min

[Hendrixl114]

Thank you Madeleine!  I don’t know the SG of the glaze, and it was a very quick dip to coat it.  Thinning it out and spraying on a thin layer were 2 thing I thought of.

thanks for the Amaco information, I will reach out to them and ask their thoughts on the HF clear.

 

Edited Monday at 09:14 PM by Hendrixl114

[oldlady]

the glaze used was made by the instructor, did i miss where it is actually a commercial glaze?

[PeterH]

42 minutes ago, oldlady said:

the glaze used was made by the instructor, did i miss where it is actually a commercial glaze?

No, Min suggested switching to a commercial glaze used in an Amaco underglaze colour chart.