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pritchpat

Glaze Leeching

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Help please.

 

I've been using Mayco glazes and have been very pleased with most of the results, but I have been having a few pieces crazing when using some Elements so I raised the temperature from 1050 to 1060 as I'm new to firing my own pieces and I had no real idea what would help and thought I would give it a go.

 

At the same time with some of the pieces I was using a new stoneware clay which this temperate firing range fell into.

 

I glazed nearly all of the pieces with Stroke & Coat as part of the design some had the addition of Elements and Clear Cascades which in the past I have had success. I then fired these pieces to cone 06.

 

All but a couple of the pieces have what I can only describe as a damp bottom and the piece with the most design i.e. Elements and Clear Cascade crazed horridly and is now weeping everywhere.  I can't see any visible cracks to the inside of any of the pieces all but a couple are glazed with Stroke & Coat.

 

So my question is would raising the temperature have caused this or could there be another explanation?

 

I live in Turkey so am using clay which I'm sure nobody has ever heard of so cannot compare to anything. 

 

Thank you

 

P

 

 

 

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Hi Pritchpat and welcome.

 

I wouldn't call what is happening to your pots leaching but rather you have an underfired claybody. Does your clay supplier say what the firing range is for this clay?

 

Even if the glaze looks uncrazed there are going to be microscopic voids in it that allow water into the clay. If you are making functional pots I would fire a few unglazed flat bottomed pieces, scatter them throughout the kiln to allow for firing differences, then fill them with water and place on newspaper for a few days. If the paper gets wrinkled or wet then the clay isn't mature. 

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Leaching is when the glaze itself leaches out metals or other minerals. What you're referring to is weeping. They are two very different issues.

 

Low fire clays like what you're using are do not generally vitrify, so you must rely on the glaze to seal them up. If the glaze crazes, then liquids can seep through the cracks in the glaze and into the wall of the pot, and weep out the unglazed bottom. If you're using commercial glazes and commercial clays, then there's not much you can do besides testing other products until you find some that work.

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I took this,

"At the same time with some of the pieces I was using a new stoneware clay which this temperate firing range fell into."

to mean Pritchpat was using a stoneware clay at lowfire temps?

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To see at what temperature your clay is fully matured, make up some test bars, all the same size, say 5mm thick, 12cm long and 2cm wide.

Scratch into each the name of the clay and the firing temperature, e.g. 1050, 1100 etc. I suggest 50 deg.  C temerature increases.

Weigh them each straight away, then let them dry fully (in Turkey they'll probably air dry OK - leave them until they no longer feel cool against your cheek, and aren't reducing in weight any more)

Weigh each one again when dry - this gives a reference for fired weights, and also the amount of water in the clay out of the bag.

Now fire each one to its temperature. If you normally have a soak when firing, include that - main thing is each is fired the same, just to a different temperature.

Once cooled down, weigh it to get the loss on ignition.

Then put into a bucket of water for a minute, take it out, pat off the surface water with a towel and weigh again. (the official tests soak for longer, but I find 1 minute is good enough for comparative purposes).

The weight gain is showing how porous the clay is - generally under 3% is seen as fully vitrified.

Also look at the clay and see if there is any bloating, showing it to be overfired. You can also test this by putting 2 marks 10cm apart when you make the sample, measuring again when firing to get the shrinkage wet to dry, and then measure again after firing. If the length continues to reduce from dry to fired, it is OK, but if it starts going up then the clay is being fired at too high a temperature, even if there are no signs of bloating.

Tim

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To see at what temperature your clay is fully matured, make up some test bars, all the same size, say 5mm thick, 12cm long and 2cm wide.

Scratch into each the name of the clay and the firing temperature, e.g. 1050, 1100 etc. I suggest 50 deg.  C temerature increases.

Weigh them each straight away, then let them dry fully (in Turkey they'll probably air dry OK - leave them until they no longer feel cool against your cheek, and aren't reducing in weight any more)

Weigh each one again when dry - this gives a reference for fired weights, and also the amount of water in the clay out of the bag.

Now fire each one to its temperature. If you normally have a soak when firing, include that - main thing is each is fired the same, just to a different temperature.

Once cooled down, weigh it to get the loss on ignition.

Then put into a bucket of water for a minute, take it out, pat off the surface water with a towel and weigh again. (the official tests soak for longer, but I find 1 minute is good enough for comparative purposes).

The weight gain is showing how porous the clay is - generally under 3% is seen as fully vitrified.

Also look at the clay and see if there is any bloating, showing it to be overfired. You can also test this by putting 2 marks 10cm apart when you make the sample, measuring again when firing to get the shrinkage wet to dry, and then measure again after firing. If the length continues to reduce from dry to fired, it is OK, but if it starts going up then the clay is being fired at too high a temperature, even if there are no signs of bloating.

Tim

 

 

I took this,

"At the same time with some of the pieces I was using a new stoneware clay which this temperate firing range fell into."

to mean Pritchpat was using a stoneware clay at lowfire temps?

 

 

Leaching is when the glaze itself leaches out metals or other minerals. What you're referring to is weeping. They are two very different issues.

 

Low fire clays like what you're using are do not generally vitrify, so you must rely on the glaze to seal them up. If the glaze crazes, then liquids can seep through the cracks in the glaze and into the wall of the pot, and weep out the unglazed bottom. If you're using commercial glazes and commercial clays, then there's not much you can do besides testing other products until you find some that work.

 

 

Hi Pritchpat and welcome.

 

I wouldn't call what is happening to your pots leaching but rather you have an underfired claybody. Does your clay supplier say what the firing range is for this clay?

 

Even if the glaze looks uncrazed there are going to be microscopic voids in it that allow water into the clay. If you are making functional pots I would fire a few unglazed flat bottomed pieces, scatter them throughout the kiln to allow for firing differences, then fill them with water and place on newspaper for a few days. If the paper gets wrinkled or wet then the clay isn't mature. 

Thank you all for your replies I have to admit to my translating the firing instructions from Turkish to English was a little off, they recommend 900 and glaze firing 1180 - 1200 which is not what I did. I assumed that the Mayco glazes would work okay but lesson learned. I have purchased some stoneware glazes so I will carry out some test with that. 

 

I sorry if my description of leaching was incorrect but I was trying to find the best way to explain what I thought it was doing.  I'm new to all this and it's all a bit like alchemy to me. I will take on board all your suggestions for trials and hopefully improve. It gets a bit frustrating over here as stockists of materials are limited and probably not of the quality that can be purchased in the US or UK.

 

Once again thanks for all your help I need as much as I can get.

 

Regards P

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Pritchpat, 

Many times here there are folks with language differences trying to write in a non native tongue. We are used to seeing this, and realize that often the change of one word can mean so much in the difference of the advice. I hope that you are able to find help here in the forums, this is a great place to solve problems and get solutions.

 

best to you on your Ceramics journey(experience)

 

 

best,

Pres

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If I had to translate everything from Turkish, DISASTER WOULD FOLLOW ME EVERYWHERE. Disaster tends to happen anyway. ;) It must be challenging to learn and make things work in another language. Pritchpat, you are smart to ask for help. :)

 

 

Pritchpat, 

Many times here there are folks with language differences trying to write in a non native tongue. We are used to seeing this, and realize that often the change of one word can mean so much in the difference of the advice. I hope that you are able to find help here in the forums, this is a great place to solve problems and get solutions.

 

best to you on your Ceramics journey(experience)

 

 

best,

Pres

Thanks everyone you are very kind. Just to clarify I'm a Brit living here in Turkey and finding my way around the differences in clays and glazes. I just want to play with them all so probably will be on a long steep learning curve, but loving every moment of it. I'm not trying to make a living fortunately or I would most certainly starve haha.

 

Sorry with slow reply I've been sailing for a week, no clay on a boat but lots of time to work out what to do next to get better results.

 

Regards

P

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Honestly with how confusing everything is with pottery when you start out, it might as well be in Turkish! I remember in the beginning I was SO CONFUSED as to why I had to use "low-fire" clay and "low-fire" glaze together. I thought, "WHY? Clay is just clay!!" And the cones gave me headaches. They make no sense at first but eventually your poor brain accepts it. 

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Honestly with how confusing everything is with pottery when you start out, it might as well be in Turkish! I remember in the beginning I was SO CONFUSED as to why I had to use "low-fire" clay and "low-fire" glaze together. I thought, "WHY? Clay is just clay!!" And the cones gave me headaches. They make no sense at first but eventually your poor brain accepts it. 

Gosh I didn't know I needed to engage my brain as well ! I normally just sit in my potting shed and play.  Will need to go and find where I parked it. :rolleyes:

 

Thanks for all your kind words it really makes me feel less like a floundering idiot.

 

The lady that I learned coil and slab building with is a wonderful potter but her glazing techniques left me always asking the question surely there's got to a better way to do it, or is that as good as it gets? Of course it didn't help that the glazes she first gave me to use were probably older than me! Since using more commercially available glazes I've been a lot happier but I still found myself getting annoyed that I couldn't ask the what if questions that are always buzzing around my head, but now I've found this wonderful site the what if questions might find an answer.

 

Regards

P

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Honestly with how confusing everything is with pottery when you start out, it might as well be in Turkish! I remember in the beginning I was SO CONFUSED as to why I had to use "low-fire" clay and "low-fire" glaze together. I thought, "WHY? Clay is just clay!!" And the cones gave me headaches. They make no sense at first but eventually your poor brain accepts it.

 

I couldn't ask the what if questions that are always buzzing around my head, but now I've found this wonderful site the what if questions might find an answer.

 

Regards

P

I love this forum. There are some who agree with me which makes me happy, and enough who disagree to keep me learning and considering different points of view.

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Honestly with how confusing everything is with pottery when you start out, it might as well be in Turkish! I remember in the beginning I was SO CONFUSED as to why I had to use "low-fire" clay and "low-fire" glaze together. I thought, "WHY? Clay is just clay!!" And the cones gave me headaches. They make no sense at first but eventually your poor brain accepts it. 

 

I asked the same question and was told "you don't question that you have to put petrol in a petrol engine, and diesel in a diesel engine, so why don't you accept that you have to use low fire clay with low fire glaze?"

 

Hummmmmm thought I, so I still experiment with clay and glaze to see if the "experts" are right.  And no, I don't experiment with the car, I've seen what the wrong stuff can do!

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Update to my glaze issues.

 

I've re-fired to cone 5, now the jug that leaked like a sieve holds water there is still a little crazing on the section with the Elements glaze, but none of the pinging and cracking noises that went on after first firing.

 

This has taught me something regarding the application of Elements so really happy that I risked a re-fire.

 

P

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Nice!!! :) I'm so glad! 

Thanks it's so good not to

have the sound effects that went on before, just annoyed at myself for not paying attention to the instructions for the clay. I must have been a bloke in a previous life don't read instructions  :P haha!

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