Alistair Gillies on sat 21 oct 06
Hi,
Any frit experts want to help me?
For the last month I have been developing a glaze to fit my red earthenware
body [I thought],and being fairly sure that I had a glaze that fitted I
have a kiln full of pots on as I write.
Looking at my clear test today after 10 days out of the kiln, and after
doing hot/cold water tests on the samples it has started to craze - it
certainly had not two days ago. The versions coloured with copper and iron
have not crazed.........yet.
The recipe is below - do we think that I might save this kiln load by firing
higher? or is it a write off.
Ferro 3195 - 62
Ferro 3134 - 9.5
Flint - 9.5
China Clay - 19
Firing at 1075c, on Spencroft clay over white slip.
I think my next step is to take out the 3134 altogether.
Thanks for your thoughts in advance,
Alistair
Ironbridge, England
Steve Slatin on sat 21 oct 06
Alistair --
I have about zero experience at low-temp firings (one experiment,
mixed in with a bisque firing) but I've started to get a feel for the
crazing problem.
First, be encouraged by the fact that things didn't craze immediately --
that means you're probably not too far off on expansion. Frit 3195
has calculated COE of 71.49; Frit 3134 is 93.05. So yes, the first
step if you're crazing is to lower the 3134. You might not need to
remove the 3134 altogether, though.
I'm willing to bet that Lili Krakowski would tell you to do a line blend
with one batch mixed all 3195 and the other with the batch you've
already mixed. This is one good way to experimentally test your
way to a craze-free glaze.
Your glaze, as written (using grolleg in the calculation for
the china clay) gives 67.49 as a COE. I don't know if this works
for your clay body or not. If you remove the 3134 altogether,
your calculated COE comes to 64.73. That's substantially lower
than you have now, and again, as a non-low-fire kind of guy, I
have no idea if that'll work or not. If you don't want to do a line
blend, you might find that cutting the 3134 down to 2% (COE
calculated to 65.36) is a good bet, given that you're already
close.
I used to prefer the 'direct reckoning' approach, but with time
I more and more am attracted to the wisdom of the line blend.
There's a reason why well-established experts in this business
like Mrs. K do line blends. Some chemical reactions are
unpredictible to us, eutectics appear where we don't expect
them, and trawling with a line blend makes more sense than
fly fishing with direct reckoning.
Please let us know what you try, and whether it comes out
well --
Best wishes -- Steve Slatin
Alistair Gillies wrote:
Hi,
Any frit experts want to help me?
For the last month I have been developing a glaze to fit my red earthenware
body [I thought],and being fairly sure that I had a glaze that fitted I
have a kiln full of pots on as I write.
Looking at my clear test today after 10 days out of the kiln, and after
doing hot/cold water tests on the samples it has started to craze - it
certainly had not two days ago. The versions coloured with copper and iron
have not crazed.........yet.
The recipe is below - do we think that I might save this kiln load by firing
higher? or is it a write off.
Ferro 3195 - 62
Ferro 3134 - 9.5
Flint - 9.5
China Clay - 19
Firing at 1075c, on Spencroft clay over white slip.
I think my next step is to take out the 3134 altogether.
Thanks for your thoughts in advance,
Alistair
Steve Slatin --
They grew and grew up the old church wall
Till they could growe no higher
They lapped and tyed in a true love knot
The rose wrapped ‘round the briar”
---------------------------------
All-new Yahoo! Mail - Fire up a more powerful email and get things done faster.
Alistair Gillies on sun 22 oct 06
Steve and Vantuil, thank you for your replies. Both very useful.
I decided to increase the firing temp of the kiln load that I had on-
thinking that I don't really have anything to lose if it is going to craze
anyway.
I fired it to 1090 instead of 1075 - this is well within the capability of
the clay and I hoped would not over fire the glaze. I understand that firing
higher tends to reduce the amount of free alumina in the clay body and thus
reduce the expansion of the clay through absorbing moisture?? - anyone want
to expand on that?
The result - whilst I do not know if it has stopped the crazing it has
increased the gloss [a good thing] and the clay has matured more and
probably more chip resistant.
Alistair
Ironbridge Gorge, England
Steve - I recognise your quote as from 'Fair Margaret and Sweet William'.
>
> They grew and grew up the old church wall
> Till they could growe no higher
> They lapped and tyed in a true love knot
> The rose wrapped 'round the briar"
Ron Roy on mon 23 oct 06
Hi Alistair,
Firing higher also reduces the silica in the body - which reduces the beta
to alpha inversion of quartz at 573C - that makes for less glaze
compression (more crazing.)
Adding more silica to a low fire body increases glaze compression and helps
to stop crazing - in low fire glazes. Low fire glazes may not be "set" (may
still be molten) at 573C and will adjust to the body so that they will not
be put into compression.
The factors used for calculating glaze expansion indicate that adding
alumina will raise the expansion of almost all glazes - adding silica will
lower expansion.
You should also consider increasing the time at top temperature - in low
fire glazed ware there is less interaction between glaze and clay than at
stoneware temperatures - this can affect fit - I'm suggesting a soak of at
lest 30 minutes at top temperature to help with the clay/glaze bonding.
Moisture crazing is still a problem over time unless you are using a high
talc body.
RR
>Steve and Vantuil, thank you for your replies. Both very useful.
>
>I decided to increase the firing temp of the kiln load that I had on-
>thinking that I don't really have anything to lose if it is going to craze
>anyway.
>
>I fired it to 1090 instead of 1075 - this is well within the capability of
>the clay and I hoped would not over fire the glaze. I understand that firing
>higher tends to reduce the amount of free alumina in the clay body and thus
>reduce the expansion of the clay through absorbing moisture?? - anyone want
>to expand on that?
>
>The result - whilst I do not know if it has stopped the crazing it has
>increased the gloss [a good thing] and the clay has matured more and
>probably more chip resistant.
>
>Alistair
>Ironbridge Gorge, England
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Curtis M. Kularski on sun 29 oct 06
For low-fire clear I use a very simple recipe, which tends to not craze (comes out very smooth actually) and is fairly hard.
Ferro Frit #3195 88.24%
EPK 11.76%
It was originally published in \'Clay Times\' as "Base Glaze #2", but I\'ve found using it without colorants tends to make it a fairly good clear dipping/pouring glaze. It doesn\'t seem to run, craze, or bubble easily. I fire it to 06 or 05, depending on if it is over an underglaze or not (typically 06 by itself, 05 over an underglaze). From my experience this glaze comes out much smoother to the touch than some of the common commericial clear glazes (LG-10, etc).
Ron Roy on sun 29 oct 06
Hi Curtis,
Looks like a stable glaze as well - calculated expansion shows up very low
but it is not a reliable indicator.
Boron has a very low expansion rate - if keep under 15% in a glaze - this
glaze has over 21% brorn so the calculated expansion cannot be relied on -
it will be higher because of the extra boron.
Normally I would cauction if the calculated expansion is this low.
I would be very interested in knowing if lab testing proved this to be a
stable low fire glaze - do you have any intention of finding out just how
good it is?
RR
>For low-fire clear I use a very simple recipe, which tends to not craze
>(comes out very smooth actually) and is fairly hard.
>
>Ferro Frit #3195 88.24%
>EPK 11.76%
>
>It was originally published in \'Clay Times\' as "Base Glaze #2", but
>I\'ve found using it without colorants tends to make it a fairly good
>clear dipping/pouring glaze. It doesn\'t seem to run, craze, or bubble
>easily. I fire it to 06 or 05, depending on if it is over an underglaze or
>not (typically 06 by itself, 05 over an underglaze). From my experience
>this glaze comes out much smoother to the touch than some of the common
>commericial clear glazes (LG-10, etc).
>
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
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