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raku glaze problem...ruined kiln shelf?

updated sat 6 jan 07


Craig Clark on fri 5 jan 07

Tracey, unless the clay body itself melted down and fused to the shelf
then all may not be lost. A face shield, gloves, chipping hammer, wide
flat blade chisel, a grinder (not absolutely necessary), and some
patient careful and steady chipping work could save your shelf.
Hope this helps
Craig Dunn Clark
619 East 11 1/2 St
Houston, Texas 77008

Tracey Duivestein wrote:
> Dear Wally
> Thank you for taking the time to explain what is happening with my
> raku glazes. You describe the effects so well - just watching it all
> puff off (& ruin the kiln shelf) was something else. I will ditch
> those glazes & start afresh .
> Again, thanks for your advice
> Best wishes
> Tracey
> ----- Original Message ----- From: "Wally"
> To:
> Sent: Tuesday, January 02, 2007 1:44 AM
> Subject: Re: Raku Glaze Problem
> Hello Tracey,
> I would suggest to completely abandon the use of colemanite or
> borocalcite in raku glazes at such high percentage.
> Both minerals contain chemical-bound water that decomposes around
> 500-600 Celcius, and tends to pulverise the glaze, if the balance
> with other glazecomponents gets too high.
> Done a lot of experiments with these minerals in my earlier efforts to
> find an equivalent for gerstley borate, and saw all test-samples
> really crushing up, disintegrating and decomposing, clearly visible
> through the peephole, as from 500 Celcius upwards.
> Even when the colemanite and borocalcite were calcined beforehand, the
> results were about the same.
> One solution for you would be to get some gerstley borate shipped by
> post from the US.....
> I know the freight charges are quite expensive, but it is really worth
> it, considering that you don't really need that much dry glazematerial
> for a single glazelayer, and the cost of materials in regard to the
> cost of one's workhours is allmost neglectable.
> I don't know about South African frits or minerals, but a good
> starting point for experiments to formulate a smooth dependable
> rakuglaze could be 75% hi-alkaline low-firing frit, and 25% of fine
> low-fire feldspar, preferably nepheline syenite.
> Wally.
> Schoten, Belgium
> --- In, Tracey Duivestein wrote:
>> Dear Clayarters
>> I have been reading the Clayart forum for several months now and am
> still amazed at all the valuable information available and all the
> advice fellow potters are willing to dispense.
>> I have done several raku firings in a group setting, and have just
> started on my own, experimenting with new glazes, reduction times,
> methods, etc. Three new glazes have just peeled or powdered off my
> work when the kiln reaches 500 C, despite using different clay bodies,
> changing glaze thickness (from thin cream to water) and firing regimes
> (fast, slow, moderate - in a gas kiln). The three glazes are Revised
> Molly Blanding, Lubbcock Red Black Blue and R yainbow, and the only
> common factor in all 3 is Colemanite (60%, 70% & 40% respectively).
> Is South African Colemanite very different to the US one, and does
> anyone have any suggestions to rescue these glazes.
>> Unfortunately I have not been able to source Gerstly Borate in South
> Africa, so my glaze selection tends to be fairly limited as I am not
> yet very comfortable with substitutions. Also, can someone help me
> the breakdown of South African Frits ( FSB 757, FSB 656, F510 &
> Glazecor 20306G L/L, 1601G L/Bisilicate & 3180).
>> Many thanks in advance from a very hot & humid Durban and may 2007
> be the best yet
>> Tracey Duivestein
>> tracey@...
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