It is often possible to fire cone 6 porcelain higher, since the flux used is sodium spar. As I understand it, sodium spar will only begin to flux at ^ 6 and mature at ^9.

Almost all the dishes in my kitchen are porcelain these days. I do not cook every day as I used to do when my children was in the house, so we often eat microwaved leftovers. A bowl may get warm as heat is transferred into the clay walls, but the handles do not get hot. 

On the other hand , I cannot always say that about stoneware and earthenware. 
Every once in a while I will use a dish from either my own stoneware days, or one that I have from other potters and every once in a while the handle heats up. This happens
when liquid seeps into the clay walls and heats up along with the food. 

I also have stoneware soap dispensers that I’ve been using for years. The soap seeps through the basis.   

Although there are many good things about firing to ^6, I also think that potters should not have a false reassurance about the clay they are using. As Ron pointed out, clay can vitrify over a range of temperatures. 

Also, good glazes are only good if it work on the clay body it is used for. There is a misconception about the temperature range of a clay body vs the clay that it is used on. For instance if a glaze works well on stoneware, it may run badly on porcelain. 
I just recently loaded this techno file on my website. It explains how I went ahead to create ^6 porcelain clay bodies, by mostly manipulating the fluxes.



International Academy of Ceramics

Mississippi Arts Commission

On Jul 11, 2021, at 5:18 PM, paul gerhold <> wrote:

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.


Sent from my iPad

On Jul 11, 2021, at 4:06 PM, 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.


Quoting Madeleine Hall-Arber <>:

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 Hall-Arber
Madeleine?s Mud Pie Studio
(617) 510-5955
Instagram: madeleinehallarber

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