Leslie St.Clair on thu 2 mar 00
Hello everybody.
A glaze I've been using for a few years has started shivering lately. I
know I should change the coefficient of expansion to try to correct this,
but I don't know which materials have a high and which a low coefficient.
Here's the recipe:
Temmoku Gold Cone 6
cornwall stone 61
whiting 8
gerstley borate 3
lithium carb 5.5
dolomite 7
flint 5.5
red iron oxide 10
I'd appreciate if anyone had any suggestions. Thanks.
Leslie St. Clair, Ft. Mitchell KY
Robert Friedman on fri 3 mar 00
The same thing happened in our studio with coleman purple I did some
research and testing and began to add more feldspar to my body. It solved
the problem. You can also need to adjust the silica in your clay if it is
short of silica. The testing i did was out of an old studio potter (it was
far beyond my understanding of physical clay properties but I could follow
directions) and used 10 glazes with specific coefficients of expansion then
observing how they perform on you clay and then adjusting the clay based on
the results. Very time consuming process. In short if you have no feldspar
in your body try adding some 5-10%. I could find that article with just a
little trouble if you are interested.
----------
>From: "Leslie St.Clair"
>To: CLAYART@LSV.UKY.EDU
>Subject: Help-shivering glaze
>Date: Thu, Mar 2, 2000, 6:36 PM
>
> ----------------------------Original message----------------------------
> Hello everybody.
> A glaze I've been using for a few years has started shivering lately. I
> know I should change the coefficient of expansion to try to correct this,
> but I don't know which materials have a high and which a low coefficient.
> Here's the recipe:
> Temmoku Gold Cone 6
>
> cornwall stone 61
> whiting 8
> gerstley borate 3
> lithium carb 5.5
> dolomite 7
> flint 5.5
> red iron oxide 10
>
> I'd appreciate if anyone had any suggestions. Thanks.
>
> Leslie St. Clair, Ft. Mitchell KY
Peter Jones on fri 3 mar 00
Leslie: My guess would be the lithium carb listed in there. Lithia is the
main ingredient in flameware/pyrex, etc. It goes through an irreversible
expansion when it is heated beyond a certain point and as such, it will
prevent the glaze from shrinking back down at the same rate as the clay
body. Because it will not shrink (the lithia that is) it makes for an ideal
solution to rapid heat change as experienced in flameware. Since there is
no expansion or contraction when heated or cooled rapidly, it has less
tendency to crack. The problem is that it can created such stress in
improperly formulated clay bodies that pots have been known to crack like a
rifle and send parts flying across rooms at bullet speeds at room
temperature. Frankly I wouldn't touch flameware with a ten foot potters
wheel!
Back to the basic question: I suggest lowering the lithia content by a
percentage point at a time until you have less problem with it. Shivering
occurs when the clay body shrinks more than the glaze does, placing the
glaze under compression and causing little flakes of glass/glaze to come off
in someone's mouth, not a pretty thing.
Peter Jones
David Hewitt on sat 4 mar 00
Leslie,
If you have been using this glaze for a few years without a problem and
it has now started to shiver, then something has changed. My calculation
of the coefficient of expansion of your recipe comes out at 4.91
x10-6/oC linear English & Turner. This value, from my experience, is
not likely to shiver. Would I be correct in thinking that you have not
changed the clay body you are using? If this is so then the next
question is, is it just the current glaze mix that is giving the
problem? If it is just the latest glaze mix that has given the problem,
try a new mix before changing the recipe.
As to which oxides do what as far as expansion is concerned, you might
like to look at my web site and click on Calculating Crazing.
http://www.dhpot.demon.co.uk
hope that this will be of some help.
David
In message , Leslie St.Clair writes
>----------------------------Original message----------------------------
>Hello everybody.
> A glaze I've been using for a few years has started shivering lately. I
>know I should change the coefficient of expansion to try to correct this,
>but I don't know which materials have a high and which a low coefficient.
>Here's the recipe:
> Temmoku Gold Cone 6
>
> cornwall stone 61
> whiting 8
> gerstley borate 3
> lithium carb 5.5
> dolomite 7
> flint 5.5
> red iron oxide 10
>
> I'd appreciate if anyone had any suggestions. Thanks.
>
> Leslie St. Clair, Ft. Mitchell KY
>
--
David Hewitt
David Hewitt Pottery ,
7 Fairfield Road, Caerleon, Newport,
South Wales, NP18 3DQ, UK. Tel:- +44 (0) 1633 420647
FAX:- +44 (0) 870 1617274
Web site http://www.dhpot.demon.co.uk
Ron Roy on sun 5 mar 00
Hi Leslie,
The expansion looks OK for this glaze - there is some evidence that Lithium
Carb can have an effect on the boundry layer between clay and glaze and
this can lead to shivering - and even crazing and shivering on the same
pot.
I recommend reducing the Lithium 1% at a time to see if that will help.
This glaze is very high in alkalies and is probably suseptable to leaching
with certain foods - I would be concerned about using it when it can come
in contact with food.
I have recalculated it but don't think it will look the same and most
certanly will craze.
G200 spar - 47.5 ( custer is OK)
Whiting - 11.0
Gerstley borate - 4.0
Lithium Carb - 2.5
Dolomite - 7.0
Old Mine 4 - 2.0 (use the ball clay you have)
Re iron - 10.0
Total 100.0 ( have included the irom in the total as per your original)
RR
>----------------------------Original message----------------------------
>Hello everybody.
> A glaze I've been using for a few years has started shivering lately. I
>know I should change the coefficient of expansion to try to correct this,
>but I don't know which materials have a high and which a low coefficient.
>Here's the recipe:
> Temmoku Gold Cone 6
>
> cornwall stone 61
> whiting 8
> gerstley borate 3
> lithium carb 5.5
> dolomite 7
> flint 5.5
> red iron oxide 10
>
> I'd appreciate if anyone had any suggestions. Thanks.
>
> Leslie St. Clair, Ft. Mitchell KY
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
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