Jeff Lawrence on sat 11 aug 12
Hi,
I've been testing a local volcanic ash (upper Bandelier tuff) that has
great celadon potential and am intrigued by a scaled effect (see attached
image) I'm seeing. I'd like to accentuate this effect with more contrast,
though, because it's pretty subtle now.
The scale outlines remind me of a 'cracked paint' effect I've gotten before
with high-clay glazes over faster-melting fritted glazes, so I'm assuming
the cause is cracks in the drying glaze that heal over (though I don't see
any surface irregularity above the markings, which I expected).
The only way I can think of is what I saw before - i.e. applying the ash
glaze over a more stolid glaze with a contrasting colorant but would prefer
doctoring the glaze recipe for a one-coat solution.
Any thoughts?
This is probably a candidate for a Currie grid, I know, but in the meantime=
?
--
Jeff Lawrence
jefflawr@gmail.com
Craig Edwards on sat 11 aug 12
Jeff:Sorry, no attached image came through....
Make Good Pots
~Craig
New London MN
http://woodfiredpottery.blogspot.com/
On Sat, Aug 11, 2012 at 11:07 AM, Jeff Lawrence wrote:
> Hi,
> I've been testing a local volcanic ash (upper Bandelier tuff) that has
> great celadon potential and am intrigued by a scaled effect (see attached
> image) I'm seeing. I'd like to accentuate this effect with more contrast=
,
> though, because it's pretty subtle now.
> The scale outlines remind me of a 'cracked paint' effect I've gotten befo=
re
> with high-clay glazes over faster-melting fritted glazes, so I'm assuming
> the cause is cracks in the drying glaze that heal over (though I don't se=
e
> any surface irregularity above the markings, which I expected).
> The only way I can think of is what I saw before - i.e. applying the ash
> glaze over a more stolid glaze with a contrasting colorant but would pref=
er
> doctoring the glaze recipe for a one-coat solution.
> Any thoughts?
> This is probably a candidate for a Currie grid, I know, but in the
> meantime?
> --
> Jeff Lawrence
>
> jefflawr@gmail.com
>
--
Steve Mills on sun 12 aug 12
Jeff,=3D20
Clayart doesn't support attachments, but you could put the pic on something=
l=3D
ike Flikr (sp?) and send in the link.=3D20
Steve M
Steve Mills
Bath
UK
www.mudslinger.me.uk
Sent from my iPod
On 11 Aug 2012, at 17:07, Jeff Lawrence wrote:
> Hi,
> I've been testing a local volcanic ash (upper Bandelier tuff) that has
> great celadon potential and am intrigued by a scaled effect (see attached
> image) I'm seeing. I'd like to accentuate this effect with more contrast=
,=3D
>=3D20
Robert Harris on sun 12 aug 12
Jeff,
My totally off the top of my head guess is that there is significant
shrinkage of the raw (or slightly sintered) glaze at a fairly low
temperature. This produces a bunch of fissures that are then filled in with
the first material that melts. (This is basically what you hypothesised I
think). To increase the size of the fissures you could sub out your clay
for ball clay, or use some bentonite. If the fissures occur with loss of
chemical water, then you could use a clay that has a large shrinkage at
this point (don't ask me what that is!).
The first stuff that melts looks to me like it crystallizes more than the
mixture of everything else, producing whiter lines. The reverse may be true
for the reverse situation.
In order to increase the contrast you either want to increase that
crystallization/white effect or make the scales darker. It might be
worthwhile to do button tests of the volcanic rock, and a mixture of
everything else in the glaze, to see which melts first. If it is the rest
of the ingredients that melt first you could increase the crystallisation
by increasing silica slightly (I think). If it is the volcanic rock that is
melting first you really can't affect its make-up, so you'll have to make
the scales darker somehow (increase translucency?).
Ideally you could find a colourant (stains might be useful for once), that
either dissolves better in the material that melts first, or is left behind=
.
How does that sound as an off-the-wall theory?
Robert
On Sat, Aug 11, 2012 at 12:07 PM, Jeff Lawrence wrote:
> Hi,
> I've been testing a local volcanic ash (upper Bandelier tuff) that has
> great celadon potential and am intrigued by a scaled effect (see attached
> image) I'm seeing. I'd like to accentuate this effect with more contrast=
,
> though, because it's pretty subtle now.
> The scale outlines remind me of a 'cracked paint' effect I've gotten befo=
re
> with high-clay glazes over faster-melting fritted glazes, so I'm assuming
> the cause is cracks in the drying glaze that heal over (though I don't se=
e
> any surface irregularity above the markings, which I expected).
> The only way I can think of is what I saw before - i.e. applying the ash
> glaze over a more stolid glaze with a contrasting colorant but would pref=
er
> doctoring the glaze recipe for a one-coat solution.
> Any thoughts?
> This is probably a candidate for a Currie grid, I know, but in the
> meantime?
> --
> Jeff Lawrence
>
> jefflawr@gmail.com
>
--
----------------------------------------------------------
Robert Harris on sun 12 aug 12
Incidentally - just to test some of the ideas, I would stick a glazed piece
in your next bisque to see what it looks like at this temp. I would have
thought fissures would be visible at this temp. You also don't say what
your final glaze temp is - I assume cone 10. If so see if you can get a
friend who does Cone 6 to do button tests and glaze tests at this temp -
might give a good idea of what is happening at different temperatures.
Robert
On Sun, Aug 12, 2012 at 1:17 PM, Robert Harris wro=
te:
> Jeff,
>
> My totally off the top of my head guess is that there is significant
> shrinkage of the raw (or slightly sintered) glaze at a fairly low
> temperature. This produces a bunch of fissures that are then filled in wi=
th
> the first material that melts. (This is basically what you hypothesised I
> think). To increase the size of the fissures you could sub out your clay
> for ball clay, or use some bentonite. If the fissures occur with loss of
> chemical water, then you could use a clay that has a large shrinkage at
> this point (don't ask me what that is!).
>
> The first stuff that melts looks to me like it crystallizes more than the
> mixture of everything else, producing whiter lines. The reverse may be tr=
ue
> for the reverse situation.
>
> In order to increase the contrast you either want to increase that
> crystallization/white effect or make the scales darker. It might be
> worthwhile to do button tests of the volcanic rock, and a mixture of
> everything else in the glaze, to see which melts first. If it is the rest
> of the ingredients that melt first you could increase the crystallisation
> by increasing silica slightly (I think). If it is the volcanic rock that =
is
> melting first you really can't affect its make-up, so you'll have to make
> the scales darker somehow (increase translucency?).
>
> Ideally you could find a colourant (stains might be useful for once), tha=
t
> either dissolves better in the material that melts first, or is left behi=
nd.
>
> How does that sound as an off-the-wall theory?
>
> Robert
>
>
> On Sat, Aug 11, 2012 at 12:07 PM, Jeff Lawrence wrote=
:
>
>> Hi,
>> I've been testing a local volcanic ash (upper Bandelier tuff) that has
>> great celadon potential and am intrigued by a scaled effect (see attache=
d
>> image) I'm seeing. I'd like to accentuate this effect with more contras=
t,
>> though, because it's pretty subtle now.
>> The scale outlines remind me of a 'cracked paint' effect I've gotten
>> before
>> with high-clay glazes over faster-melting fritted glazes, so I'm assumin=
g
>> the cause is cracks in the drying glaze that heal over (though I don't s=
ee
>> any surface irregularity above the markings, which I expected).
>> The only way I can think of is what I saw before - i.e. applying the ash
>> glaze over a more stolid glaze with a contrasting colorant but would
>> prefer
>> doctoring the glaze recipe for a one-coat solution.
>> Any thoughts?
>> This is probably a candidate for a Currie grid, I know, but in the
>> meantime?
>> --
>> Jeff Lawrence
>>
>> jefflawr@gmail.com
>>
>
>
>
> --
> ----------------------------------------------------------
>
>
--
----------------------------------------------------------
Jeff Lawrence on mon 13 aug 12
Hi fellow theorists,
Thanks for helping me feed my glaze jones while away from the studio.
Leonard - you da man for unearthing a fish-scale glaze from the archives.
I'm looking forward to comparing it to what I've been seeing.
Veena, thanks for sharing your experience - Does your celadon have a lot
more clay than the underlying shino? And how does it compare to the
analysis/recipes below?
Robert, I appreciate your methodical approach, and realize I shoulda shared
more data. In particular, I'm remembering that the San Juan Ash (SJA) by
itself shrinks/crawls into tightly spaced little beads of dark gray glass
at cone 6. Maybe it's the primary cracker factor. Just to flex your mental
glaze muscles,
SJA weight % analysis I'm using:
Na2O 4.00
K2O 4.50
CaO .20
Al2O3 12.00
Fe2O3 1.50
SiO2 77.00
TiO2 .10
High silica and alumina for cone 6, but silica's amorphous and alumina's
already fritted with the fluxes.
SJA-Cadycal test - (image 1)
500 San Juan Ash
60 cadycal
35 epk
2 western bentonite
I wonder if the non-ash components would form a boron blue glass bead?
SJA Multiflux test - (image 2)
435 San Juan Ash
45 neph syenite
45 g-200
20 epk
45 cadycal
10 western bentonite
Sentenced to weeks of theory until I get back home...
Jeff Lawrence
jefflawr@gmail.com
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