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SeaTree

Clay changing colour with hot water

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Please advise. I use earthenware clay. I made some mugs with coloured brush on glaze inside and transparent glaze outside and underneath. When filled with boiling water, I see the clay (I presume) gradually changing colour, darker, rising slowly up the cup. There is no leak or condensation. I reglazed and refired some but the same things still happens.

What might cause this?

Does this happen in every mug but you don't normally see it under a coloured glaze?

Are they safe to use?

Thank you

Michaela

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It's because the inner glaze is crazed and the liquid is seeping into the partially vitrified earthenware.

Try filling a cup with ink or drawing inside with a sharpie to see where the cracks are.

As far as safety goes, they're not safe to microwave, and bacteria can grow in the clay body, so if you want to use them, be sure to clean in a dishwasher to sterilize.

And they will eventually weep through to the outside, fill one with water and put on a piece of paper over night.  I'm willing to bet the paper will be wet in the morning.

Edited by liambesaw

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Temp. you are firing to is not hot enough to vitrify your clay to an acceptable level.

Lots of posts re. this on the forums here.

I would not sell them.

What temp does it say to fire your clay to and what temp are you firing to?

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2 minutes ago, liambesaw said:

Earthenware doesn't normally vitrify, you will need to address the glaze fit if you want to seal these up.  I wouldn't sell them, they are leaking 

Ok thank you. They don't leak out - it stays inside the glaze. It's an odd effect. If it was a coloured glaze I don't think I would know it was happening. 

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3 minutes ago, SeaTree said:

Ok thank you. They don't leak out - it stays inside the glaze. It's an odd effect. If it was a coloured glaze I don't think I would know it was happening. 

Yeah, I meant leaking into the clay body.  But in my experience if it's leaking into the clay body it will eventually weep out of the foot of the pot too.

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On 8/2/2019 at 8:37 AM, SeaTree said:

It is KGM earthstone so can be fired at earthenware or stoneware temperatures. I am firing to 1160.

 

Unfortunately this problem comes up time and again with claybodies that are sold as ones with wide firing ranges. For a clay to be mature it needs to fired to the higher temperature they recommend which is 1260C or approximately cone 9. At 1160 (roughly cone 4) you are underfiring it for the purposes you are using the clay for. It might be fine for sculptural work at 1160C but not for functional pots.

On 8/2/2019 at 9:08 AM, SeaTree said:

If it was a coloured glaze I don't think I would know it was happening. 

I think you would as the cups are rapidly soaking up moisture. The glaze will craze, the cups will get very hot in the microwave and they will leak. It's not worth selling them and having unhappy customers. Either use a clay that matures at earthenware temperatures or fire the KGM Earthstone to maturity, ie 1260C (cone 9) with appropriate glaze.

Edited by Min
temp typo corrected

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10 hours ago, SeaTree said:

Hi. I am firing them within the temperature range on the clay. It is KGM earthstone so can be fired at earthenware or stoneware temperatures. I am firing to 1160.

 

If clay has a temp range to cone 10 it is not earthenware. Therefore if it withstands the temp of c 10 firing it will be no where near vitrified enough at low temp.

Do tests for absorbency to start with.

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