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GoodKarma

Only glazing half of mug, susceptible to stains/moisture?

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Hi Good!

Potential for stain and/or mold greater for clays with high(er) absorption (at full maturity); darker clays may not show staining as much...

Probably a bad idea to let any unglazed piece that has high absorption soak in dishwater, whether it's just a bare foot ring or a lot more, as in your pic. Wash, dry.

Another concern may be lower strength, particularly at the glazed/unglazed transition. Where the glaze ends at a buttressed point - at the foot ring or opposite the transition arc bottom to wall - lower strength less of a concern? If I were planning to leave a large patch of bare clay, slightly thicker clay at that transition, and a thorough burnishing of the surface as well.

What constitutes "low absorption" rate then? ...oh, probably less than two percent? Aardvark SBF is rated 1.5% - bare and burnished, it's almost like matte finish glass... 

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@GoodKarma, I would really suggest testing the claybody and the glazes you are thinking of using before doing this. To prevent staining look for a body that has a low absorption figure (1 1/2% or less) and then test it in your kiln with your firing methods and measure what absorption figure you get. If this checks out okay then make sure the clay can take the added stress of having only one side of the pot glazed. To do this take a thin walled cylinder and glaze the inside only with the glaze you are thinking of using as the liner glaze and glaze it as thickly as you would an actual pot. Freeze the test cylinder(s) then put them in the sink and pour boiling water in them. If you will be doing some glaze on the outside then do a test cylinder like this also. If the clay is vitrified then it will be food safe but chances are you won't be eating off the outside of a pot so no worries there.

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4 hours ago, liambesaw said:

Only weakness is slight structural weakness and +5 damage to sponges. I do a lot of my bowls this way for extra grip and tactile texture

What if the sponge uses a Spell of Resiliency?

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@liambesaw @Benzine I've found that if you apply Sherril's Scarlet Kidney of Shining to the projected exposed parts at leather hard, sponges gain a +5 resistance to shredding damage. Your gaming table still will still likely appreciate a coaster.

For those non-Dungeons and Dragons players, burnish the exposed part with a little red rib to smooth it out if you're worried about causing damage to sponges or tabletops. Work clean to keep the feet clear of burrs and crumbs, and give your pots a quick pass with some 220 grit wet/dry sandpaper, used wet, after the glaze firing.

My clay is fired to around 1% porosity or less, so I have no trouble leaving exposed clay on the bottom 1/4 to 1/3 of the pot. I haven't tried to test where the line is when a mug will break more readily if too much is left unglazed. I know if it's only lined, it's a lot more fragile.

image.jpeg

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On 10/19/2019 at 10:02 AM, Callie Beller Diesel said:

@liambesaw @Benzine I've found that if you apply Sherril's Scarlet Kidney of Shining to the projected exposed parts at leather hard, sponges gain a +5 resistance to shredding damage. Your gaming table still will still likely appreciate a coaster.

For those non-Dungeons and Dragons players, burnish the exposed part with a little red rib to smooth it out if you're worried about causing damage to sponges or tabletops. Work clean to keep the feet clear of burrs and crumbs, and give your pots a quick pass with some 220 grit wet/dry sandpaper, used wet, after the glaze firing.

My clay is fired to around 1% porosity or less, so I have no trouble leaving exposed clay on the bottom 1/4 to 1/3 of the pot. I haven't tried to test where the line is when a mug will break more readily if too much is left unglazed. I know if it's only lined, it's a lot more fragile.

image.jpeg

Delicious!!

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I bought a porcelain mug with a lovely frosty blue glaze and an exposed 1 1/2" bottom area. First use with Lemon Zinger tea revealed a hairline crack going up the exposed area. I suspect that the thickness of the wall as well as the tension between glazed and unglazed both contributed to the fail, but also  the relative density of porcelain  

Exposed porcelain will never really look pristine again after it leaves the kiln. Granite stoneware still looks fine after 20+ years. Most red stone wares also still look good with no staining. 

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On 10/19/2019 at 12:25 PM, liambesaw said:

I'm guessing it was first used by a manager, to slowly sip, while critically looking at their employees,  "Yeah, I'm going to need you, to get  those manuscripts turned in, before you leave today..."

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On 10/22/2019 at 3:13 AM, Benzine said:

I'm guessing it was first used by a manager, to slowly sip, while critically looking at their employees,  "Yeah, I'm going to need you, to get  those manuscripts turned in, before you leave today..."

And chiselling on rock or making firinf a clay tablet and then mining the ink stone makes for a big day at work.

Certainly need that cuppa

Benzine  of bizarre brain..ah you teach teenager's art classes..

Edited by Babs
errors

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I glaze most of my cups and mugs like this with no issues at all so far. Even put them  in the dishwasher and so far so good.

My question is though, I made some bottles in this manner but did not glaze the inside. Only 3/4 of the outside. I used laguna 50 stoneware fired to cone 6. Is it still foodsafe to be used as say, an olive oil bottle? And will it be vitrified? I’ve looked around and can’t seem to find a straight answer 

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1 hour ago, Melissa F. said:

I used laguna 50 stoneware fired to cone 6. Is it still foodsafe to be used as say, an olive oil bottle? And will it be vitrified? I’ve looked around and can’t seem to find a straight answer 

Is this the clay you used? http://www.lagunaclay.com/clays/northeastern/wc606.php  If so then no I wouldn't use it for an oil bottle. Either way you need to do your own testing and not go solely on what the clay companies post as the absorption figures. Your kiln, your pots, your firing schedule etc are all going to make a difference. Clay absorption test in this link, about 2/3 the way down the page. I would add an 8 hour simmer to the test then turn off the stove and leave the sample piece(s) in the water for a total of 24 hours then continue with the drying and weighing. I'm not fond of the idea of leaving an oil bottle unglazed on the inside, it will be very hard to clean should you have oil go rancid inside.  

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15 hours ago, liambesaw said:

Whoops, I was thinking of #65, sorry about that.  Yeah, 3% absorption would get nasty quick.  Any reason you want an unglazed interior?

I just didn’t have enough glaze to do the interior. I might re-fire now with clear on the inside if I can get my hands on some! 

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5 hours ago, Melissa F. said:

I just didn’t have enough glaze to do the interior. I might re-fire now with clear on the inside if I can get my hands on some! 

If the bottle is used to contain oil, I don't think a refire with glaze would work, the glaze can't penetrate the clay enough to be a good coating.

What's the point of testing for absorption with water when you intend to use oil?

I learned from my mentor that oil lamps, even when glazed with a reliable liner glaze inside, will seep oil through the unglazed base. We brushed two coats of slightly thinned white glue on the bottoms and that sealed against the oil seepage. It dries waterproof, if it isn't soaked in water for extended periods. However, we can assume that the lamp oil still penetrated the liner glaze and remained harmlessly in the body of the piece.

For food grade oil, I would not store it, even in any fully glazed ceramic piece, because of the likelihood of oils remaining in the body of the piece and becoming rancid. Possibly for short-term usage, like serving. .

Put a flower in that bottle, it's a vase! 

Edited by Rae Reich

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44 minutes ago, Rae Reich said:

What's the point of testing for absorption with water when you intend to use oil?

I think it's important to do the absorption test with water not to rule the pot as tight for oil but to rule out pots that will probably leak. It isn't possible to do a boil and soak test with lamp oil for example but if you know the absorption is much more than zero with water it will probably leak. I don't have issues with my veg/olive oil bottles weeping and the porosity/absorption of my clay is around 1%, it isn't tight enough for lamp oil.

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16 minutes ago, Min said:

I think it's important to do the absorption test with water not to rule the pot as tight for oil but to rule out pots that will probably leak. It isn't possible to do a boil and soak test with lamp oil for example but if you know the absorption is much more than zero with water it will probably leak. I don't have issues with my veg/olive oil bottles weeping and the porosity/absorption of my clay is around 1%, it isn't tight enough for lamp oil.

@Min    Agreed that is a good reason to test with water, too. Not sure I'd want to boil lamp oil, but that should work for food oils. 

Maybe the volatiles in lamp oil make it more prone to seepage.

What do you use for a liner glaze? Cone 10?

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53 minutes ago, Rae Reich said:

@Min    Agreed that is a good reason to test with water, too. Not sure I'd want to boil lamp oil, but that should work for food oils. 

Maybe the volatiles in lamp oil make it more prone to seepage.

What do you use for a liner glaze? Cone 10?

Yeah, I think oil lamps are one of the most problematic things to have not leak. I think it has to do with the polarity of the lamp oil molecules versus that of water molecules or veg oils. I remember about 20 years ago there seemed to be a lot of potters making oil lamps and running into problems with weeping. All sorts of sealers were tried then it seems we just moved on from making them. Too much liability.

I've made oil bottles at  ^10 and 6, for ^10 I used Robins Clear and for ^6 I use my low COE clear. I don't think the glaze is hugely important as the oil molecules will find their way through any glaze flaws or pinholes over time, can't see any flaws that will be inside the bottles. I think the clay itself has to be able to resist the oil from weeping through.

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