Just glaze inside?

[laughlin]

Hi all! Question:  I want to glaze just the inside of a large vase I've coiled and leave the outside mostly raw, with just some delicate marks outside. I seem to have heard that that can cause cracks or other problems but have been unable to find anything definitive on it. What say you all?  Is some sot of uneven shrinkage a real issue if I do that?

Also - This one is Sheffield 4DB3 clay (red brown) and I've also gotten some Brown Bear, both ^6,  with an eye to showing a lot of bare clay on organic, coiled forms.  I've worked mostly with porcelain (thin) and understand it, so dark clay is a new ballgame.  Like, i'm probably drying it 3x as slowly as I need to, lol. And making a lot of new test tiles to check out the  on the new glaze variables. There's tons of info on beautiful BB's quirks online but  nada about the 4DB3 beyond the shop blurb - has anyone in here played with it? Thank you! 

[Min]

3 hours ago, laughlin said:

Is some sot of uneven shrinkage a real issue if I do that?

It's a glaze fit issue that can cause dunting when only the inside of a pot is glazed. 

Stress testing can exacerbate the problem so it's a method to use to determine if you should rule a glaze out for fit issues. Stress testing for dunting... make several 4-5" tall cylinders thinly out of your chosen clay. Make them as thin as possible, they can be simple slab cylinders, then bisque fire them. Glaze the inside only with a heavy and thick coat of the same glaze you would use on your vase and fire them to the same cone you will fire your vase. Put the cylinders in the coldest part of your freezer overnight then put them in the sink and immediately fill them with boiling water. If they split open the glaze is not a good fit. I know this sounds extreme and no sane person would do this in day to day life but it stresses the clay and fit issues should show up with it. If the cylinder(s) split / crack then the glaze has too low an expansion for that clay. 

[shawnhar]

33 minutes ago, Min said:

Put the cylinders in the coldest part of your freezer overnight then put them in the sink and immediately fill them with boiling water

Pretty sure every store bought mug bowl or plate we own would not survive this, lol.

[Min]

26 minutes ago, shawnhar said:

Pretty sure every store bought mug bowl or plate we own would not survive this, lol.

But they should!

[Bill Kielb]

4 hours ago, shawnhar said:

Pretty sure every store bought mug bowl or plate we own would not survive this, lol.

In reality it’s a few hundred degree difference so not all that severe. Not very comfortable for humans though.

[laughlin]

Min - I've got some test pieces in bisque fire now and there are a few little cups in the lot, just quick-pinched test things, but it'll be easy to run that test on those with.a couple of liner candidates. I hope it works as I want to do brushwork on bare or partially glazed clay on the outside and thinking it needs glazing on the inside to hold water., though absorbency looks good for that I think.  Gift for my daughter - won't see dishware-level  stressful use in rl,  just would be nice if it made it through firing. Thanks, any other input welcome. I don't even know if it's unusual to just glaze inside a biggish moon pot like this, new genre for me. Took forever, too attached.