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Liquid seeping through side of mug


Jane W.
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Hello, I've been making mugs with the two glazes, as shown in the first photo.  A customer contacted me to say that their coffee had seeped through the outside wall, photo attached.  I asked for some inside photo's as well, but I can't seem to add any more.  This is a stoneware clay, bisque fired to 05, glaze fired to 9. 

I've never had this before with this mug.  Can anyone cast any light on what's happening here please?  I should say that they have said they only used the mug twice, with instant coffee (I don't think the instant has any bearing!)  I asked if it had been used in a microwave, but they said no.  Thanks in advance.

photo 2.jpg

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Your clay is too open at cone 9, it's weeping because it isn't vitrified enough. Which clay are you using? Need to look for absorption figures of 2% or less and then run your own absorption tests to get a more accurate figure. It could be that firing to cone 10 is enough to tighten it up but that won't work if your clay is still too porous (absorption too high). Tests for doing your own absorption testing here about 2/3 the way down.

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Most likely your clay body is not fully vitrified, and the liquid is seeping through the crazed glazed and through the semi-porous clay body. You need to run an absorption test on your clay body to see if it's tight enough. You want an absorption rate of under 2%, ideally under 1.5%. To do the test, make a bar of your clay body about 11cm long and 1cm thick and fire it to the cone you normally do (raw, no glaze). Weigh it, then boil it for a couple of hours or let it soak for a couple of days, then weigh it again to see how much water it absorbed. Again, you want under 2%. More than that and it's too porous and will weep. As long as you're doing the absorption test, it's also a good idea to do a shrinkage test. Mark a 10cm line on the bar when you first make it, then measure it once it's fired to see how much shrinkage you're getting. It won't have any affect on the absorption issue, but it's a good thing to know.

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Hi Neil   and thanks for your very quick reply, so testing will commence.  I'm wondering if it's just from the last firing as the cone 9 didn't bend fully anyway.  And it's only been this one mug, I've got another one here from that firing so I may test it anyway.  many thanks for your time

Edited by Jane W.
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Hi Min, I'm nost sure if I'm now replying to you.  Anyway thank you, the clay is a UK clay:

Stoneware V9G

CATEGORY: Stoneware
METHOD: Throwing, Modelling
COLOUR: Buff
TEXTURE: Smooth textured
FIRING RANGE: 1120°c - 1280°c

The addition of grogg gives good resistance to cracking and warping.  V9G behaves well under oxidising and reduction conditions.

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4 hours ago, Jane W. said:

clay is a UK clay

I make it a point to not buy clay from a supplier who doesn't provide easy to find approximate absorbtion and shrink rates.

Maybe they can't lawfully?

Here, seems things with under 2% numbers will test vitreous.

Sounds like you just didn't get hot enough.

Cheers!

 

Sorce

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16 hours ago, Min said:

 

Your clay is too open at cone 9, it's weeping because it isn't vitrified enough. Which clay are you using? Need to look for absorption figures of 2% or less and then run your own absorption tests to get a more accurate figure. It could be that firing to cone 10 is enough to tighten it up but that won't work if your clay is still too porous (absorption too high). Tests for doing your own absorption testing here about 2/3 the way down.

Hi, thanks for your reply.  Where do you get your clay from in the UK?  I've only tended to see clay sold with the firing range?  many thanks in advance

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24 minutes ago, Jane W. said:

Hi, thanks for your reply.  Where do you get your clay from in the UK?  I've only tended to see clay sold with the firing range?  many thanks in advance

I found this technical information sheet from Valentine Clay but it doesn't include the absorption figures for their clays. I'ld really suggest doing the absorption tests regardless of whether they gave the information or not. Your firing specifics will all play into how porous the clay will be when fired.

 

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Looks like some crazing there - fwiw, we use crazed ware in our own kitchen, from elsewhere, and from our own studio - however, ware with crazed food surfaces do not leave the studio as for food ware; we keep them, else drill a weep hole, voila, planter! Furthermore, crazed on one side is likely weaker than well fitted on both sides.

...that clay is awake now, if those weep points ain' decaf...

Edited by Hulk
:|
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1 hour ago, Min said:

I found this technical information sheet from Valentine Clay but it doesn't include the absorption figures for their clays. I'ld really suggest doing the absorption tests regardless of whether they gave the information or not. Your firing specifics will all play into how porous the clay will be when fired.

 

Thanks for this, actually that missing bit of info would have been handy!   Is it possible to do the absorption test on one of my mugs, they have unglazed bottoms? Or does it have to be a bar, with no glaze at all? 
also, could I re-fire the mugs I have,  it’s only four of them, at cone 10? Would that solve it?  Many thanks for your time again 

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50 minutes ago, Hulk said:

Looks like some crazing there - fwiw, we use crazed ware in our own kitchen, from elsewhere, and from our own studio - however, ware with crazed food surfaces do not leave the studio as for food ware; we keep them, else drill a weep hole, voila, planter! Furthermore, crazed on one side is likely weaker than well fitted on both sides.

...that clay is awake now, if those weep points ain' decaf...

Yes crazing, Mike Dodd did mention that crazing is always there, but you just can't see it.  I don't work with crazing on my pottery, unlike someone like Florian Gadsby, he's all about the crazing! high fire, functional ware.  but my not intentional!  Thanks for being in touch

 

Edited by Jane W.
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2 hours ago, Rae Reich said:

You might find that using a liner glaze inside the mugs that matures with the clay and fits properly will prevent seepage and still allow you to use exterior glazes which are beautiful but have some flaws.

Yes, thank you,  I've got a cream liner, which this is and a shiny white, which I think is better, thanks for your help

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5 hours ago, neilestrick said:

Crazing can be beautiful, but you need to have a well vitrified clay under it to prevent weeping. Refiring to cone 10 may help, but you don't know for sure if it will be tight enough even then without testing.

Yes it can be, but like you say it can't be porus underneath.  I think I'll re-firng anyway as I'll have the space.  thanks again

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On 4/2/2020 at 1:14 PM, Jane W. said:

Is it possible to do the absorption test on one of my mugs, they have unglazed bottoms? Or does it have to be a bar, with no glaze at all? 

To get an accurate test of the clay itself, your test will need to be done on a piece of fired, un-glazed clay.

The un-glazed bottom is a relatively small portion of the overall surface area of the mug (probably around 10%, or less, depending on dimensions).  Even though the glaze is 'leaking', the glaze will still slow the absorption enough to give you an artificially low result - and lead you to believe it's tighter than it really is.

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On 4/2/2020 at 2:32 PM, Sorcery said:

I make it a point to not buy clay from a supplier who doesn't provide easy to find approximate absorbtion and shrink rates.

Maybe they can't lawfully?

Here, seems things with under 2% numbers will test vitreous.

Sounds like you just didn't get hot enough.

Cheers!

 

Sorce

ok, thanks

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1 hour ago, Rockhopper said:

To get an accurate test of the clay itself, your test will need to be done on a piece of fired, un-glazed clay.

The un-glazed bottom is a relatively small portion of the overall surface area of the mug (probably around 10%, or less, depending on dimensions).  Even though the glaze is 'leaking', the glaze will still slow the absorption enough to give you an artificially low result - and lead you to believe it's tighter than it really is.

Yes, I think really I knew this, but as one of the muds is here!  Anyway test bars are ready, thanks for your help 

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45 minutes ago, Babs said:

Get a few of you "ok" mugs. Fill with water or hot coffee. Leave on a piece of paper for 48hrs and have a look.

Just saying.

Friend was shocked when we did this.

She also thought just one or two faulty mugs. 

Better do it before your mugs are placed on the grand piano ..

Especially if it's a Steinway! Thanks Babs, I'll do that as well. 

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  • 1 year later...
4 hours ago, Anonymous said:

I have the same problem with a mug I bought. As a person who does not have a kiln or anything ceramic related can I fix this by myself at home?

There is probably no  'food-safe' way to fix a leaky mug without re-firing - and even that is iffy at best.   I've had some success sealing a 'weepy' vase, by coating the inside with water-based polyurethane - but definitely would not do that with anything I'm going to use for food/drink.

I would suggest contacting the potter that made it and see if they will replace it.  I wouldn't want someone to show one of my mugs to a friend and say "I really like this mug that Jim made - but I can't use it, because it leaks".  I'm betting most potters would feel the same, and would happily exchange it.

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