[Clayart] Cracks and slow seepage- porcelain

Madeleine Hall-Arber arber at mit.edu
Sun Jul 11 19:49:11 UTC 2021

Thank you, Ron.

This is what my supplier says about the clay: #16 Grolleg Porcelain Cone 5-6
Grolleg Cone 6 porcelain. similar to #15 with more ball clay, making it less white, more plastic and forgiving. Additional silica reduces crazing in some glazes. good for slab forming.
sh 13.9% ab. 0.12 @ C/6

I do use cones on every shelf, bisque to cone 04, fire to cone 6 (cone 6 bent to touch the shelf, cone 7 half over).

I just did the microwave test and alas the 2 pots I tested did get hot.  Not burning, but noticeably hot.  I guess the next step is to test the absorption.  I would appreciate the instructions, or are they in the book?  (I don't have it in front of me.)

I will try calling Laguna and let you know if they shed any light on the subject.

Thanks for taking the time to respond!

On 7/11/21, 2:01 PM, "ronroy at ca.inter.net" <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.
    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

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