[Clayart] Cracks and slow seepage- porcelain

ronroy at ca.inter.net ronroy at ca.inter.net
Mon Jul 12 16:57:12 UTC 2021


Hi Paul,

I used to think absorption % was a reliable indicator that a clay was  
properly vitrified but not any more. I have seen porcelains with  
absorption below 0.5 that still leak.

If a pot gets hot in a microwave it is a sure indication there is  
water in the clay. Microwave some glass ware and it will not get hot.

There are some exceptions with some high iron glazes and clays that  
also get hot.

Clay bodies are constantly changing because the clays in the ground  
change. Unless you test the raw clays and bodies constantly you will  
not be aware of changes. It's the tradition (with few exceptions) that  
the potter does the testing instead of the clay company.

Laguna's claims depend on a variety of factors - how the clay is fired  
and tested and when. Any reliable data has to have a date and batch #  
attached.

It's the same old story - customer beware.

It's not difficult to test new clay to see if it is vitreous. We  
should all do that if we care about the functional ware we sell.

RR


Quoting paul gerhold <gerholdclay at gmail.com>:

> Ron,
> I agree that it appears she is not actually firing to cone six.  
> Miller #16 has an absorption claimed by the manufactures as .12%  
> plus or minus 1%. There may also be a glaze fit problem if the  
> cracks are not through the pot.
>
> Not sure abot what that microwave test is supposed to show. Just  
> tested three different empty functional  pots I have bought over the  
> years and they all get hot after six ten second cycles in the  
> microwave. Also one of the earthen ware clays I use to make personal  
> functional pieces does the same thing at cone 6 and it has an  
> absorption way below 1%.   I also just tested one that has never  
> been used and even it got very hot.
>
> Paul
>
> Sent from my iPad
>
>> On Jul 11, 2021, at 4:06 PM, ronroy at ca.inter.net wrote:
>>
>> ?Hi Madeleine,
>>
>> The clay is not vitrified enough and water etc. is getting in the  
>> clay over time. Does the description from Laguna say it's a cone 6  
>> clay or does it say it is good over a range of temperatures?
>>
>> It's not uncommon for clay companies to sell clay that does not  
>> vitrify enough at the temperatures they specify. That means whats  
>> in the dish water and food also gets in.
>>
>> If you microwave one of the suspected pots and it gets hot it means  
>> there is water in it. Just microwave the empty pot 10 seconds at a  
>> time for 2 minutes.
>>
>> I can send instructions on how to actually test the clay to see the  
>> percent of absorption.
>>
>> I hope you fire with cones - that is the only way you know you are  
>> getting to the temperature you want.
>>
>> Perhaps a call to Laguna would help. I would like to hear their  
>> explanation for one.
>>
>> RR
>>
>>
>>
>> Quoting Madeleine Hall-Arber <arber at mit.edu>:
>>
>>> For years, I have thrown pots out of Miller/Laguna #16 clay fired  
>>> to Cone 6 with slow cool, primarily using recipes from Mastering  
>>> Cone 6 Glazes, though occasionally experimenting with other  
>>> glazes?some from Britt?s compendium.  My daughter uses a variety  
>>> of my small cups and bowls to prepare her meals in the microwave.   
>>> She never puts them in the dishwasher and handles them carefully.   
>>> We?ve noticed that several have developed long cracks that we can  
>>> see (presumably from food stains).  In several cases, the foot of  
>>> the pot has yellowed.  I?ve also noticed that our soap dispenser  
>>> accumulates a scummy surface at the foot.  Any ideas?   I tell  
>>> customers that my work can be used in the microwave (and  
>>> dishwasher), should I not?
>>>
>>> Thanks for your help and knowledge!
>>> Madeleine
>>>
>>>
>>> Madeleine Hall-Arber
>>> Madeleine?s Mud Pie Studio
>>> (617) 510-5955
>>> Etsy: madmud.etsy.com
>>> Instagram: madeleinehallarber
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
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>>
>>
>>
>> Ron Roy
>> ronroy at ca.inter.net
>> Web page ronroy.net
>>
>>
>



Ron Roy
ronroy at ca.inter.net
Web page ronroy.net




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