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turquoise raku glaze.

updated sun 31 mar 96


John Termeulen on fri 15 mar 96

>From: (John Termeulen)
>Subject: turquoise raku glaze.
> Hi fellow Clayarters,
> I am looking for help to resolve a long outstanging problem an hopefully I
get this done with your assistance.
>I am doing a fair amount of raku and am able to get most of the glaze
effects I am looking for. The only problem I have have is with Turquoise
glazes. I am not getting consistent results. The clay used is from my
friendly local dealer, Tucker's pottery supplies, located in Richmond Hill.
In the past I used their Thompson raku clay but for the last few years I am
using white sculpture/raku clay. I am using this since I also do sculpting
with this material and this simplifies the clay stock.
>The greenware is bisque fired to 1920F and the glaze is applied mostly by
brush and pouring because they are mostly too large to dip. The pots are
then fired in a fiber kiln and a propane burner is used. Not in a reduction
atmosphere I think! When the required temperature is reached ( about 1900F)
the glaze has becomes shiny, the pot is removed and post firing reduction
takes place. This is accomplished by putting the pot in a metal container
filled with combustables, such as wood shavings and newspaper. This is
roughly the normal process.
>When using turqoise the following four glazes are used:
>1-TURQOISE. Analysis in unit formula.
>5- Gerstely borate 0.24 Cao 0.16 B2o3 2.66 Si02
>2- Zircopax 0.04 K2o 0.1 Al2o3
>10-Flint 0.71 Na2o
>70-Frit 3110
>10-Soda ash Ratio: 35.55 Expansion:11.12
>5- Ball clay
>10-Frit 3110 0.29 Cao 0.1 B203 3.07 Si02
>3- Copper carbonate 0.06 K20 0.1 Al203
>2- Bentonite 0.64 Na2o
>3- Tin Oxide Ratio: 29.36 Expansion:10.28
>80-Gerstely Borate 0.6 Cao 1.51 B2o3 .57 Sio2
>20-Soda feldspar 0.4 Na2 0.1 Al2o3
>1- Copper carbonate Ratio: 6.0 Expansion:9.45
>50- Gerstely Borate 0.27 Cao 0.66 B2o3 .43 Sio2
>21-Lithium carbonate 0.51 Lio2 0.1 Al2o3
>24-Nepheline synite 0.02 K2o
>1.5-Tin oxide 0.20 Na2o
>1- Copper carbonate Ratio:4.36
Expansion: 9.23
>The reason that the analysis is shown because there quired elements for
this type of glaze is:
>1) Must have a low alumina content.
>2) Must high level of alkaline e.g. sodium, lithium and potassium.
>3) Must comtain copper in some form.
>4) Stain is not acceptable.
>Sample pots were fired in the raku kiln with the glazes. When the glazes
matured, the pots were removed and let to cool without post firing
reduction. All showed degrees of the blue colour.
>The next batch was fired as above but were reduced in a metal container.
Different reduction cycles were tried and the result was mostly green or
copper(red). At times the glazes turned out the most beautiful turquoise.
This could have happened when the glaze was freshly mixed?
>I have read various books, such as Raku by Steve Branfman, The ceramic
spectrum by Robin Hopper and Raku by Robert Piepenburg without success. I
have also looked in the Clayart files at the San Diego State University by
means of the Internet. Two articles appeared
>, one indicating short reduction time and the other a long reduction time
depending on the glaze and might not have been the colour I am looking for.
>I am looking for help to resolve this problem so that consistent turquoise
can be produced.
> John Termeulen
> Trenton, Ontario
> Canada
> e-mail