Martin A. Arkowitz on thu 6 nov 97
Ron Roy wrote:
Just in case - anyone who tries this glaze - there is a good possibility
this glaze will break ware - it has a low expansion and may present fit
problems with many clay bodies. It is also oversupplied with boron which
makes it a "soft" glaze - likely to leach and perhaps discolour. I
recommend you test for both fit and durability before committing to it.
>----------------------------Original message----------------------------
>here is a clear glaze that i got years ago from ceramic monthly. i found it
in
>an article by dorothy haffner. it is a ^5 - 7 oxidation. it works very well
>over underglazes. it has never crazed over porcelain, but i have never tried
>it over stoneware.
>Gerstley borate........50 gms
>EPK............................17.5 gms
>Flint............................32.5gms
>i use it very thinly on bisqued ware.
---------------------------
Ron::
i have used this glaze on porcelain mugs which i sell to a local research
company. the only addition to the mug is a raised logo which has luster
applied and then the entire mug is refired at ^017. i have never received any
complaints, but now i am curious. i took a mug and subjected to a 2 minute
immersion in ice water and then plunged it into boiling water for two minutes
and the glazed mug rang true and seemed fine. is there any other test that i
could do to check its durability and fit??
TIA
eleanor arkowitz
<Arkowitz@Dartmouth.EDU
Ron Roy on mon 10 nov 97
Hi Eleanor,
I'm late getting back to you on this so I left your post intact.
Did you see the post by Don Jung? - he mentions having had a shivering
problem with this glaze. I think you are not having a problem because you
are using it thinly and on porcelain. If you used it thicker on stoneware
you might have a problem - or maybe even if you use it thicker on
porcelain. This is certainly not the kind of glaze you want to use as a
liner for tea pots.
My concern is that someone will just start using it - not realizing that
every clay we use has a different thermal expansion/contraction profile.
The thickness of the glaze is a factor as well - thicker glaze = more
stress. Using the same glaze inside and out is also a factor. Pots with
glazes under compression (glaze winds up bigger than the clay leading to
shivering or dunting) are stronger up to a certain point - if glazed with
the same glaze inside and out - but there is a limit. But take that same
low expansion glaze on the inside and (worst case) a crazing glaze outside
and get ready for a demonstration of what can go wrong when fit has not
been considered.
When you test a glaze and clay for this kind of fit fault the test should
be designed to aggravate the tension already in the ware. Freezing the ware
simply continues the cooling down from room temperature - Glazes and clays
develop tension because they contract at different rates - the cooler they
get the greater the incompatibility. When frozen, the stresses within the -
dare I call it the system?, are at their upper limit for kitchen ware.
Pouring warm, hot or boiling water INTO the ware will heat up the inside
glaze layer first - expanding it - it happens instantly - if the ware
survives you can sleep soundly.
I just checked the glazes than Don posted - Patti's Crystal clear and
Clear 4321
(Don - the total for Patti's is only 96.48 - am I working with a full deck?)
My limits say they are durable glazes but the expansion is a bit low on
both - test em and don't go adding any low expansion materials to em - like
pax, silica, magnesium, clay or lithium. Does anyone have any information
about what contraction rates different stains have.
I sometimes get to the end of these and start wondering - what was the
question and have I answered it yet - I'm sure someone will know.
>----------------------------Original message----------------------------
>
>Ron Roy wrote:
>Just in case - anyone who tries this glaze - there is a good possibility
>this glaze will break ware - it has a low expansion and may present fit
>problems with many clay bodies. It is also oversupplied with boron which
>makes it a "soft" glaze - likely to leach and perhaps discolour. I
>recommend you test for both fit and durability before committing to it.
>
>>----------------------------Original message----------------------------
>>here is a clear glaze that i got years ago from ceramic monthly. i found it
>in
>>an article by dorothy haffner. it is a ^5 - 7 oxidation. it works very well
>>over underglazes. it has never crazed over porcelain, but i have never tried
>>it over stoneware.
>>Gerstley borate........50 gms
>>EPK............................17.5 gms
>>Flint............................32.5gms
>>i use it very thinly on bisqued ware.
>---------------------------
>Ron::
>i have used this glaze on porcelain mugs which i sell to a local research
>company. the only addition to the mug is a raised logo which has luster
>applied and then the entire mug is refired at ^017. i have never received any
>complaints, but now i am curious. i took a mug and subjected to a 2 minute
>immersion in ice water and then plunged it into boiling water for two minutes
>and the glazed mug rang true and seemed fine. is there any other test that i
>could do to check its durability and fit??
>TIA
>eleanor arkowitz
><Arkowitz@Dartmouth.EDU
Ron Roy
93 Pegasus Trail
Scarborough,Canada
M1G 3N8
Evenings, call 416 439 2621
Fax, 416 438 7849
Studio: 416-752-7862.
Email ronroy@astral.magic.ca
Home page http://digitalfire.com/education/people/ronroy.htm
| |
|
