Underglaze blistered and bubbled

[Christy Ann]

I made an underglaze test tile on Minnesota Clay 03 biscware using underglaze with a clear glaze on top and this came out fine. Then, on a sculpture, I applied an underglaze to Minnsota Clay 03 greenware.  This came out fine.  Following bisc, I applied a clear coat of glaze and fired the piece at Cone 6.  The final piece had very large bubbles and blisters on it. My instructor could not explain why this happened. Another instructor thought the problem might be that I overwork my pieces, creating a very smooth outer layer. The piece appeared to be dry for bisc after drying out in the air for several weeks. I wonder if the glaze simply cannot tolerate both the heat from the bisc and from the final, cone 6 firing. Does anyone have a good explanation? This is the second time this has happened to a major piece in the last two years. The first time, I used Amoco Velvet underglazes. This time I used a Bell underglaze.

[Pres]

Christy Ann, could you post a picture so that we can actually see what the results were?

 

best,

Pres

[Bill Kielb]

10 hours ago, Christy Ann said:

firing. Does anyone have a good explanation? This is the second time this has happened to a major piece in the last two years. The first time, I used Amoco Velvet underglazes. This time I used a Bell underglaze.

I have experienced various clears failing to fire smoothly over various underglazes which at first appear bubbled but are often the result of the clear not melting evenly. Usually dependent on color and thickness of the applied underglaze and also a bit brand dependent. I am with @Pres this is very difficult without a picture of the defect. Please post some examples if possible. Folks here will likely have some good explanations.

[Jeff Longtin]

Good Morning Christy Ann. I have years of experience with MN Clay. 

When you mention Minnesota Clay 03 are you referring to Minnesota Clay #3 White Stoneware? It's been a very popular white stoneware for many years. 

Are you using the clay right out of the bag or is it reclaimed clay? Sometimes reclaimed clay can have bits and pieces that bloat in the glaze firing.

I use Amaco underglazes and regularely fire them to cone 6.  I use a cone 6 clear glaze over them.  Is your glaze a cone 6 glaze? Do the bubbles appear to be clear, i.e. glaze like, or does appear that the actual clay piece has bloated/bubbled?

 

[Christy Ann]

I am referring to #3 Minnesota Clay white stoneware, and I was using the clay right out of the bag. My glaze was cone 6. The bubbles appeared to have both the clear and the underglaze color but did not appear to have clay pieces in them.

[Christy Ann]

Unfortunately,  I left the piece at the studio and am in the process of removing all the glaze from the piece using a dremmel tool, so I cannot send a picture right now and any picture I sent would no longer show the problem well.

[Bill Kielb]

On 8/13/2022 at 11:31 AM, Christy Ann said:

Unfortunately,  I left the piece at the studio and am in the process of removing all the glaze from the piece using a dremmel tool, so I cannot send a picture right now and any picture I sent would no longer show the problem well.

Some questions to ask yourself: is it mainly in an area of a particular thickness of underglaze. Maybe thickness and moreso a specific color? When you did test tiles did you replicate the thickness and color? Often Wash applied underglaze  react just fine, solid color applied underglazes can require a clear that melts a bit better. The mug below relies on the rough texture of this very defect  for its outside decoration. It was fired to bubble then in the end down fired with a lowfire clear to make it a bit sturdier. The simple sugar bowl below required about a summer of testing before I realized the overglaze needed to be able to melt better and develop a clear matte and gloss that covered the very solid red and solid black on that pot.

Try and make your test tiles match your desired  application exactly

 

Just to add a note to add since someone is asking, the mug above is my creation, the bowl below was thrown and glazed by me and hand painter in  underglaze by my Marcia  - Marcia Grant under Madison Pottery where we collaborate on many pieces together.

 

Edited August 16, 2022 by Bill Kielb

[kswan]

If you had really burnished the greenware clay, it may have been too solid a surface to absorb the underglaze and combine with it. It may have just been sitting on the surface and didn't melt into the clay particles. I've had the bubbles of underglaze and glaze happen too, so as Bill said, use a watered down first layer of underglaze. That will sink down in between the clay particles and then add another layer or two of underglaze to adhere to that one.  

When those bubbles have happened to me, I break them and sand or Dremel like you're doing, add more underglaze and top with glaze and refire. It usually comes out fine. 

Just remember that you want to get different surfaces to interlock so they melt together, whether it's clay to underglaze to glaze. Bisque is really porous, soaking up the liquid, but a burnished clay may be too tight to absorb underglaze and it sits on top. 

[Babs]

I find some underglazes need a bit of frit added to help with the melt with a glaze. 

Without a pic. hard to determine.

Rest of glazed area ok? Then I would try a bit of frit mixed with that specific u.glaze

[Babs]

8 hours ago, Christy Ann said:

Unfortunately,  I left the piece at the studio and am in the process of removing all the glaze from the piece using a dremmel tool, so I cannot send a picture right now and any picture I sent would no longer show the problem well.

Removing all the glaze wirh a dremmel?

You realise the piece, having been fired to C6 won't accept glaze readily if you plan to reglaze.....