Barry Salaberry on sun 2 nov 03
I tried the post firing soak at 1900 F., and have experienced shivering =
in my usually very stable clear liner glaze:
K-22 Clear Cone 10 reduction
Cornwall Stone 51.6
Dolomite 14.8
OM#4 10.3=20
Wollastonite 3.2
EPK 9.7
Silica 10.3
This firing was unique in a few ways:
I used a post firing soak at 1038 C. (1900 F.) for 2 hrs. 40 mins.
My climb from 1000 C. to 1300 C. was 8 hrs. instead of 12 hrs.
I used my regular clay body, Plainsman H-550, but added med. grog =
(IONE), 6% of dry wt.
As suggested by Hank, Mel and others, the colour development was =
outstanding.
I followed Lou's advice to crank up the speed of climb to melt.
I maintained my typical reduction regime, and in the post firing soak, =
the oxy-probe reading hovered at around .5
Cardew recommends that when shivering occurs, discard the clay body and =
proceed.
But, in this case,
I am=20
using the rest of a batch of clay which was not shivering this glaze.
And I this glaze batch was not shivering in previous firings which =
didn't=20
utilize the post firing technique.
Since excessive crystobalite formation is the cause of shivering, I =
figure the extended firing=20
resulted in more than usual crystobalite development.
I have not come across any reference to grog addition being a typical =
cause of shivering.
Since I really:
want to explore this richer colour development and
want to continue using the grog,=20
then,
=20
HOW DO I MODIFY THIS GLAZE TO FIT THE "NEW SIZE" GROGGED BODY?
IF I AM TO REDUCE THE AMOUNT OF SILICA, HOW MUCH WOULD BE A GOOD =
STARTING POINT?
There are a couple of glazes in this firing which (so far) haven't shown =
shivering.
SHOULD I COMPARE EXPANSION RATES OF THESE GLAZES WITH THAT OF THE =
SHIVERING GLAZE,
AND
SIMPLY MIMIC THAT NON-SHIVERING RATE?
any ideas will help
Barry
First snow and "killing frost" this week,
But snatched a last bouquet of red and white dahlias from its murderous =
grasp.
Barry Salaberry
3642 Glencoe Rd.
Westbank, B.C. V4T 1L8
250-768-3268
Stephani Stephenson on sun 2 nov 03
Barry wrote:
"I have not come across any reference to grog addition being
a typical cause of shivering"
Stephani wrote:
Barry
though this is just one part of the puzzle , I HAVE noticed
instances where grog seemed to be related to problems with
shivering.
The problem showed up when the grog was exposed or made more
prominent, on /near the clay surface,
especially on convex surfaces, (for example , a convex rim
of a sink or fountain bowl.)
The problem occurs when the rim has been sponged or
handled in the green stage, to perhaps alter or touch up an
irregularity in the shape. One person in the studio tended
to sponge or sand the rims after the forms were dry
,exposing the grog .On one batch of basins, shivering
occurred in those areas on those basins, but not on others.
This is just anecdotal, but the increased presence of grog ,
in this case as a result of post pressing handling did seem
to have an affect . Though I cannot explain this with
regard to shrinkage and fit
I think it affects the Physical bond between the glaze layer
and the clay layer...As if the two cannot develop a good
interface.
Of course wheneve you do see shivering, the glaze can pull
up quite a top layer of the clay, and one will simply
'see' more grog. but this seemed , as if the glaze just
never was able to melt or bond properly with the clay....
this is just visual information though.
I did not find shivering on rims of basins which had not
been handled , especially sponged, after pressing.
Note: this glaze is a rather low expansion glaze and so is
somewhat 'on the edge'.
Stephani Stephenson
steph@alchemiestudio.com
Carlsbad CA
Dave Finkelnburg on mon 3 nov 03
Barry,
I like your posted thought to mimic the expansion of your non-shivering
glazes on this body. That has worked for me.
You ask about revising the glaze. Your suggestion of reducing silica
will raise expansion of the glaze. You may preserve the character of the
glaze better, though, by replacing part to all of the Cornwall Stone with
another, higher expansion, feldspar. If you sub in Custer feldspar plus 5%
silica the expansion goes up, but the silica:alumina ratio remains the same.
You are just replacing some calcium with higher-expansion potassium as the
glaze flux.
I respect your observation that your glaze was not shivering previously
from your clay body. Still, Plainsman H-550 clay is not represented by Tony
Hansen as a low-expansion clay, but rather as a fairly typical stoneware.
The expansion of the glaze recipe you posted is quite low, more in the range
of a porcelain glaze. I would expect this glaze could shiver off a typical
stoneware body.
In theory, soaking well below 2,000 F should not increase crystobalite
formation. You need to be above 2,000 F for that. Of course, it is
possible you soaked at a higher temperature, that you were getting
information from a faulty thermocouple. I am not saying that is the case,
just that is possible.
All the best with what must be a frustration for you.
Dave Finkelnburg, envious of your dahlias!
----- Original Message -----
From: "Barry Salaberry"
Sent: Sunday, November 02, 2003 5:08 PM
I tried the post firing soak at 1900 F., and have experienced shivering in
my usually very stable clear liner glaze:
Ron Roy on mon 3 nov 03
Hi Barry,
First of all your glaze does have a below "normal" expansion rate - I would
have it raised if doing any extended time firing above 1100C - it is
probably the residual stresses between clay and glaze that the extra firing
triggered - or maybe just the double cooling through the quartz and
cristobalite inversions.
Yes - if I knew the expansion rates of the other glazes that did not shiver
I can adjust yours for you. It's simply a matter of raising the KNaO - as
some one else suggested - G200 or Custer for Cornwall - but it should be
done with calculation.
Some grogs do have cristobalite in them - grogs made from used crushed fire
brick can be loaded with it. I don't have any experience with IONE - maybe
it was not calcined high enough and there is some cristobalite in it. IONE
is out of production by the way - better find another grog.
If you have a firing going in you can try this revision of your glaze. I've
raised the expansion by quite a bit.
G 200 - 54.0
Dolomite - 15.0
OM#4 - 8.0
Wollast - none
EPK - 7.0
Silica - 16.0
Total - 100.0
If it's Custer then -
Custer - 56.0
Dolomite - 15.0
OM#4 - 10.0
Wollast - none
EPK - 6.0
Silica - 13.0
Total - 100.0
You could do a 5 part line blend with yours and either of these and find
out when the shivering stops for instance - but err on the crazing side -
shivering is the last fault you want in a liner glaze.
Let me know what the results are please.
RR
>I tried the post firing soak at 1900 F., and have experienced shivering in
>my usually very stable clear liner glaze:
>
>K-22 Clear Cone 10 reduction
>
>Cornwall Stone 51.6
>Dolomite 14.8
>OM#4 10.3
>Wollastonite 3.2
>EPK 9.7
>Silica 10.3
>
>This firing was unique in a few ways:
>
>I used a post firing soak at 1038 C. (1900 F.) for 2 hrs. 40 mins.
>My climb from 1000 C. to 1300 C. was 8 hrs. instead of 12 hrs.
>I used my regular clay body, Plainsman H-550, but added med. grog (IONE),
>6% of dry wt.
>
>As suggested by Hank, Mel and others, the colour development was outstanding.
>I followed Lou's advice to crank up the speed of climb to melt.
>I maintained my typical reduction regime, and in the post firing soak,
>the oxy-probe reading hovered at around .5
>
>Cardew recommends that when shivering occurs, discard the clay body and
>proceed.
>
>But, in this case,
> I am
>using the rest of a batch of clay which was not shivering this glaze.
>And I this glaze batch was not shivering in previous firings which didn't
>utilize the post firing technique.
>
>Since excessive crystobalite formation is the cause of shivering, I figure
>the extended firing
>resulted in more than usual crystobalite development.
>
>I have not come across any reference to grog addition being a typical
>cause of shivering.
>
>Since I really:
>want to explore this richer colour development and
>want to continue using the grog,
>then,
>
>HOW DO I MODIFY THIS GLAZE TO FIT THE "NEW SIZE" GROGGED BODY?
>
>IF I AM TO REDUCE THE AMOUNT OF SILICA, HOW MUCH WOULD BE A GOOD STARTING
>POINT?
>
>There are a couple of glazes in this firing which (so far) haven't shown
>shivering.
>
>SHOULD I COMPARE EXPANSION RATES OF THESE GLAZES WITH THAT OF THE
>SHIVERING GLAZE,
> AND
>SIMPLY MIMIC THAT NON-SHIVERING RATE?
>
>any ideas will help
>
>Barry
>
>First snow and "killing frost" this week,
>But snatched a last bouquet of red and white dahlias from its murderous grasp.
>
>Barry Salaberry
>3642 Glencoe Rd.
>Westbank, B.C. V4T 1L8
>250-768-3268
>
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>
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Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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