Underglaze question please

[Bam2015]

Hi All,

As I was painting underglaze on greenware pots today, I started thinking about how underglaze affects pottery seams like mug handles.

So here is my question, does underglaze have a negative, positive, or no affect on greenware pottery seams/joints? In other words, does added moisture from the underglaze affect drying and potentially cause the seam to open up? Or does it help to seal a seam? Or maybe the affect is neutral.

I know that one or more of  you know the answer.   

Thank you,

Betty

[Bill Kielb]

My observation, it has no effect or it’s s minor I have never noticed. We have done thousands of underglazed pieces btw. I can say it changes how glaze is applied in that the underglazed areas are less porous, even when applied on greenware but not as much as when applied on bisque and re bisqued. I almost always spray my glazes though so relatively easy to air dry and spray the next coat.

[LilBlueFrog]

I have definitely applied underglaze too liberally around the base of a greenware handle attachment point, and after bisque that attachment had released. Interestingly, after glaze firing the clay shrank in a way that brought the handle back to the body, and the glaze fused them back together.

[Hulk]

Good question!
...idk, as almost always applying underglaze to bisque ware. I do use sieved slip just after trimming, but not right on joined areas.

[Jeff Longtin]

I've been casting cone 6 porcelain, for years, and never had a problem with underglaze and seams/handles. 

As I cast thin I prefer to apply underglaze, Amaco, when the pot is bone dry.

[Callie Beller Diesel]

If the small amount of water that’s in underglaze affects your handle joins at the green stage, the join isn’t sound to start with. Or you’re working very, very thinly. 

If you’ve done it properly and wiggled the pieces together until they stop moving, mug handles shouldn’t separate from the body at any stage. Because you’re creating essentially one piece of clay instead of 2, the join itself is very reinforced. If there’s going to be damage to a handle, either the handle should snap off leaving a stub, or it’ll break in a U shape around the join on the body. You may still get some hairline cracks around the edges if the attaching slip shrinks, but at that point those are aesthetic problems, not structural ones. You can smooth that kind of crack out with the point of a wooden tool when the piece is dry.