Underglaze - exterior of a mug?

[Salt.Forest]

Hi there,

I've recently begun experimenting with Amaco Velvet underglazes and am really enjoying them. If I have a foodsafe liner glaze on the interior of a mug, is there any concern with using only an underglaze on the exterior (with no clear on top)? Would it be dishwasher safe?

 

 

[neilestrick]

Underglazes left bare can be rough. Try it and see what you think. You may not find it to be a very pleasing surface. I would also not expect them to be a durable surface since they are matte and don't go into melt like a glaze. On a vase or something more decorative like that I would say go for it, but mugs take a lot of abuse. Test, test, test....

[Bill Kielb]

The general answer is these are likely safe, especially on the outside but obviously without glaze over the top depending on the underglaze and its colorants could be an issue. If the underglaze derives its color from cadmium, then probably not a good choice. The other thing I would mention is mugs glazed only on the inside can be more fragile than those glazed inside and out. A well fit glaze will typically place the clay in a tiny bit of compression which when applied inside and out strengthens the mug. Coated inside only, there have been mugs that suddenly fracture when something hot or cold was poured into them. So rare, but glazing one side only CAN increase the stress on the mug.

Having said all that, some, folks will compromise and glaze halfway down the outside of the mug.

with respect to dishwasher safe, this would be an absorption issue which actually is sort of a microwave safe thing. Anyway, if the clay is not fully vitrified then it could absorb too much water and as a result failing in the microwave is a real thing as the trapped water expands to steam. 

So without exterior glaze the mug will likely absorb more water and .... underglaze without the protection of an overglaze likely will erode away much more quickly. Add all that to the general wash ability of the surface that is only underglazed and you have another reason against doing this.

There are folks who do this successfully though  and just plain old like the look and feel.

[Salt.Forest]

Thank you all-- I assumed it was fine looking at other's work (for example: https://poepotteryshop.com/ols/all?sortOption=descend_by_popularity ) which look like underglaze transfers on raw clay. Maybe they just have a really low absorption rate...

[Bill Kielb]

1 hour ago, Salt.Forest said:

I assumed it was fine looking at other's work (for example: https://poepotteryshop.com/ols/all?sortOption=descend_by_popularity ) which look like underglaze transfers on raw clay. Maybe they just have a really low absorption rate...

Interesting those examples are glazed half to two thirds down the outside  of the ware as well as the entire handle as mentioned above.

[neilestrick]

9 hours ago, Salt.Forest said:

I assumed it was fine looking at other's work

Yeah, never make that assumption. There's a lot of work out there being done using less-than-ideal methods. Not saying this is necessarily one of them, but you'll need to test it to see.

[out to sea]

I do this method (underglaze on outside and clay left bare) on a brown stoneware that I use that has very low absorption. Sometimes the tactile feeling is not great straight out of the kiln, so I use a little sand paper and sand them gently in a bucket of water which makes the surface nice and smooth. I do glaze the handles and I test my absorption regularly to make sure all is well.

On my white stoneware I use a semi-matte clear glaze (HCSM 2 from Mastering cone 6 glazes) because I don't like a high shine on my paintings.  I feel like it gives me the best of both worlds.

 

[Min]

I only glaze the inside of my marbled tumblers. I would strongly suggest testing the heck out of not just the absorption but also test them for a really good glaze fit. There is a much greater chance of dunting if the glaze is too low an expansion. Thin cup, heavily glazed on the inside only then freeze them in the coldest part of your freezer for 24 hours then pour boiling water into them. I would suggest testing around 20 of the cups/mugs. Thinner the cup/mug wall and the heavier the interior glaze the greater the chance of them dunting, set up that situation so you are testing the worst possible scenario.

[Callie Beller Diesel]

I have a couple of mugs that have used a technique like this: glazed on the handle, lip and inside, but underglaze decoration and bare clay on the exterior of the mug. I bought them from a potter I know who makes truly lovely work. She has stopped working with this exact technique however. The one mug I have developed U shaped cracks in the span of 18 months, and the other I make sure I hand wash and use infrequently because of that. It’s well vitrified and thinly thrown. The glaze fits and hasn’t crazed in 5 years, so I suspect the one that cracked was under compression.

It’s one of those things that you *can* do, but there are always caveats and things to be aware of.

[Min]

I've found that many soda, salt and woodfired pots can inadvertently have close to this same scenario. Liner glaze inside the pots and a very thin glaze from the sodium/ash on the outside.