[Clayart] Low fire, functional, and vitrification

Tue Nov 8 12:13:18 EST 2016

Hi Robert,

Lets be careful and say - every clay will need a different amount of  
vitrification to not leak. The number I first started using was under  
5% and I got that from the Rhodes book. That is clearly not good enough.

I don't use a pressure cooker by the way - I just boil for two hours -  
got that from the Rhodes book as well - it was the only book at that  
time that had anything about that stuff.

Since then I have found that test to give different results under  
different circumstances. If you don't cool the bars off after boiling  
water will evaporate and give uneven results. If you put the bars in  
hot out of the kiln you get different results. Load the bars in and  
start cold or dropping into boiling water gets different results. In  
the end you have to do everything the same to get comparable results.

Anyone who wants my testing procedures - email me.

RR

Quoting Robert Harris <robertgharris at gmail.com>:

> Hi Paul,
>
> As far as I am aware it's all pretty arbitrary. I know that I used a
> commercial Cone 10 clay body that had 2.5% absorption (Moon White from
> Highwater - which has kyanite in it) and I could tell the difference
> between that and my clay body that has less than 1.5% absorption, and I
> didn't like it.
>
> Purely anecdotal evidence but after a few go rounds in the dishwasher it
> had enough residual water soaked into it that the footring felt a bit damp
> and it seemed to have problems heating up too much in the microwave. Like I
> say this only happened after a few weeks of use and a few go rounds in the
> dishwasher.
>
> The only reason I know of Ron Roy's acceptance of 2+% was that (to the best
> of my recollection) he stated exactly that on Clayart when I made an issue
> of complaining about 2.5% absorption not really being properly vitrified.
>
> I do think that accurately measuring absorptions of less than 1% is
> starting to get tricky to do accurately at home. Things like thickness of
> clay sample etc start to matter at that point. I know that Ron Roy uses a
> pressure cooker to measure absorption for his samples.
>
> I suspect that ceramic engineers (and maybe geologists?) probably have a
> very specific definition of "full vitrification" that relates to
> sintering/melting of the matrix and could be tested by looking at porosity
> under a microscope, but for all intents and purposes absorption is a crude
> test for that, but one where the numbers for "functional vitrification" are
> arbitrary.
>
> My two cents worth on that topic.
>
> On Tue, Nov 8, 2016 at 7:17 AM, Paul Gerhold <gerholdclay at gmail.com> wrote:
>
>> Since vitrification is to make like glass and 1.5% or 2% water absorption
>> in clay is nowhere near the water absorption of glass I am wondering where
>> the standards for absorption in clay actually come from.  Are these just
>> arbitrary numbers or is there a functional reason.  Will pottery seep water
>> at 2.5% and not at 2%.
>>
>> Paul
>>
>> Sent from my iPad
>>
>> > On Nov 7, 2016, at 10:13 PM, Robert Harris <robertgharris at gmail.com>
>> wrote:
>> >
>> > Hi Deb - in short, yes you are confused.
>> >
>> > Only stoneware and porcelains vitrify. Earthenware (whatever temp it is
>> > fired to) is never vitrified. Vitrification literally means to make like
>> > glass. In other words the particles melt together enough to eliminate any
>> > porosity. In modern parlance we measure the amount of porosity by water
>> > absorption. A fully vitrified clay has less than 1.5%  absorption of
>> water.
>> > (Although I know Ron Roy is more generous than me and allows 2%+ or so).
>> >
>> > Earthenware (which includes your mica clay) is always porous ... its
>> > vitrification temperature is about the same as its slumping temperature.
>> >
>> > When we are making things from stoneware then the cone rating on the side
>> > of the box ought to be the temperature at which vitrification is such
>> that
>> > water absorption is less than 1.5%. That is the crux of the current
>> > argument about "Cone 6-10" clays. There is no way that a clay can have
>> 1.5%
>> > water absorption at the lower boundary and not melt or bloat at the upper
>> > boundary. Therefore these clays are unlikely to be properly vitrified at
>> > Cone 6.
>> >
>> > I hope that makes sense.
>> >
>> > Robert
>> >
>> >> On Mon, Nov 7, 2016 at 7:16 PM, Deborah Thuman <debthuman at zianet.com>
>> wrote:
>> >>
>> >> I thought that clay was vitrified when fired to the cone for which the
>> >> clay was designed. ^6 for ^6 clay, ^04 for^04 clay. Or am I confused?
>> >>
>> >> I have a mica clay bread pan that I dearly love. Because mica melts at
>> >> about ^2 or so, this is ^04 clay. I glazed the inside of the pan
>> because I
>> >> tried, several times, making bread in an unglazed ceramic bread pan. It
>> can
>> >> be done, but a generous helping of C-4 is needed to get the baked loaf
>> out
>> >> of the pan. I didn’t glaze the outside of the mica clay pan because I
>> like
>> >> how mica clay looks. I’ve been baking bread in that pan for…. maybe 4
>> years
>> >> now??? I bake in it, I clean it in the dishwasher, the glaze is as good
>> as
>> >> the day it came out of the kiln, the pan is holding up well. I’ve put
>> that
>> >> pan in an oven already preheated to 425 degrees. I’ve put that pan in
>> the
>> >> microwave/convection oven and preheated to 425 then set the timer for 30
>> >> minutes. Either way, I get great bread. I’ve made rice pudding in that
>> pan
>> >> (I had made a cover for the pan) and taken pan and pudding to pot luck
>> >> meals.
>> >>
>> >> Deb Thuman
>> >> debthuman at zianet.com <mailto:debthuman at zianet.com>
>> >> https://debthumanblog.wordpress.com <https://debthumanblog.
>> wordpress.com/>
>> >>
>> >>
>> >> You can’t have a light without a dark to stick it in.
>> >> Arlo Guthrie
>> >>
>> >>
>> >>
>> >>
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Ron Roy
ronroy at ca.inter.net
Web page ronroy.net