David, while I agree with you almost wholeheartedly, I will add the exception that proves the rule. Properly formulated and fired bone china really is fully vitrified. Now obviously that almost certainly means commercially produced, but from my reading it's very close to glass.
Now arguably bone china stretches the definition of "clay", but ...

On Sat, 22 Jan 2022 at 09:56, David Woof <woofpots@hotmail.com> wrote:
"I don't recall if he gave any reasoning. " James said.

Weigh on gram scale and subsequently Boil a bisqued slab bar in water for four hours.  Then high fire to intended cone. (not to a set temp) use a cone as true "Heatwork" Baseline. Weigh again and do the math for percentage of absorption.

My clay​ body absorption % must be 1.5 or less depending on the constituents of the body.   I prefer 1.0 to a 0.5.
​Some bodies will leak if there is one half cone variance if kiln is firing unevenly between zones and the firing didn't take the clay into the middle of the proper vitrification zone.

And "Completely Vitrified" is a non-accurate description of properly fired clay..  As in Like "Kind of Pregnant" or "Kinda happy" etc etc.

When I hear "Completely Vitrified" it sets my teeth on edge because that condition in clay would be a melted slumped gob or pool that no longer resembles a functioning vessel.
(Snake oil and paregoric for what ails ya)


Completely Woof......................................or at least 1.5 %........some days...............................
From: Clayart <clayart-bounces@lists.clayartworld.com> on behalf of James Miner <woodfirejim@gmail.com>
Sent: Friday, January 21, 2022 12:54 PM
To: Clayart international pottery discussion forum <clayart@lists.clayartworld.com>
Subject: Re: [Clayart] Wide range clay

Regarding acceptable porosity of fired clay for functional ware, I talked
to one of the head people at a large clay company many years ago to find a
good body for cone 10, and he said it should be 2% or less.  I don't recall
if he gave any reasoning.  I won't give you the names involved in case I
have misremembered, but I still go by that myself.  YMMV.

Anybody else have a different answer?

On Fri, Jan 21, 2022 at 10:34 AM Vanessa Greene <bequiet@att.net> wrote:

> Any thoughts from all you experienced potters about those clays that are
> marketed as a wide ranging temp clay, cone 6-10…..are they really fully
> vitrified at cone 6? I hear very different opinions about this…..what
> absorption rate is necessary for functionally food safe pots?  (
> continental clays B clay, Buff etc…. )
> I have always fired these clays to cone 10 but some people suggest that
> these clays are suitable for both cone 6 and 10 firings. Thoughts?
> Vanessa Greene
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