Underglazes "jumping" Off Pots

[kimmyzthings]

Help! I am having issues with underglazes disappearing in spots during glaze firings. I paint the underglazes (usually Duncan or Speedball) onto greenware and fire in a bisque kiln to cone 06. Then I dip my pieces in a clear glaze and fire to cone 6. When the pieces come out of the bisque firing, they are totally fine. After the glaze firing however, some pieces will have small round spots that look like the clear glaze and colored underglaze just JUMPED right off the pot. All my pieces are wiped down before applying the clear glaze, and as I already stated, they look perfect after the first firing.. I have had this happen on the inside of bowls, as well as the outside. Why would this be happening? Thanks in advance!

[Min]

Hi and welcome to these forums.

 

Sounds like the underglaze didn't bond with the clay. Might have been because it went on with too thick a coat. Have you used this glaze alone without being on the underglaze? Any chance you could have missed some slight lifting of the underglaze prior to glazing? Were the glazes dry when you glaze fired or did they go from freshly dipped to a fairly fast fire? Could you post a picture so we have a clearer idea of what is going on? (sorry, more questions than answers)

[Chris Campbell]

My first suspicion is that your underglaze itself is too thick and is lying on top of the clay, not soaked into it.

Or the pots are still too wet when you are applying the underglaze so they are staying on the surface rather then soaking in.

I usually wait until the pieces are bone dry.

 

I would not expect a glaze to pull off a properly applied underglaze, but someone else might have that experience.

 

And, as Min says ... can you post a picture??

[Tim82]

I've had this issue in the past usually one of a few things.

 

Most common reason underglaze is too thick or too many coats.

Is you want a deep solid colour try applying several thin coats and wait for each côté to be 100 percent dry.

In the past this happened with a blue underglaze, what I ended up doing was applying 2 Coates to greenware bisque then apply 1 more thin coat and glaze.

 

9/10 the underglaze is too thick or layers were applied without each layer being dry enough for the next layer.

 

Best way to understand is to test on test tiles. Add various thicknesses of underglazes applied different and do the same with the glaze over the top. Also i find it much better to test on larger test tiles so you can see how it look on a larger surface area.

 

Hope this helps

:-)

[douglas]

Try adding the underglaze at leather hard instead of bone dry. 

[Babs]

If the underglaze has a lot of clay in it, I think that is what I'm remembering and you apply it to a dry pot it will not bond, not be seen at bisque level but can bloat or blow off at glaze time, because it can.

Dampen your pot all over, I do this even at leatherhard, stick it on the wheel and run a brush over hte inside and out, then apply the underglaze.