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my first glaze test - fell off in sheets - advice please!

updated mon 7 jun 04


lewis ramage on sat 5 jun 04

This was my first batch of 'home made glaze' test times ,and all were =
or less as anticipated, except for one. =20

I was trying to make a glaze based on RR's Raspberry Glaze but using the
ingredients I had to hand. Partly to try out Insight, partly to see =

I was not expecting what did happen: the glaze had literally fallen off =
vertical test tiles in sheets - as if it had at some stage formed a =
but then crazed and crackled like mad. No sign of flowing or running
particularly, just as if it had never bonded with the clay surface, and =
some stage fractured spit and spattered itself around but none of the
spatters had stuck to the bat washed shelf or other tiles. Weird. The
kiln was over-fired to Cone 8 (I'm having a real problem calibrating my
controller: with a 60C/hr rise I fired to 1205C, and still got Cone 8).
Electric, Oxidation, on a buff stoneware body.=20

Here's what I ended up with compared to what I was matching with (which =
or may not be what the original glaze was, because I don't know if the
Insight database for the demo version would have the same material
composition as the RR original. =20

I would really appreciate some pointers on where to research the =



My recipe:

20.78 Whiting
27.09 SILICA
7.02 Colemanite
9.60 Hymod AT
35.52 Potash Feldspar

0.24 CaO
0.01 MgO
0.05 K2O
0.02 Na2O
0.00 TiO2
0.09 Al2O3
0.04 B2O3
0.93 SiO2
0.00 Fe2O3

What I was aiming for:

20.00 Whiting
30.00 Silica
18.00 Nepheline Syenite
14.00 FRIT 3134
18.00 OM #4 Ball Clay

0.25 CaO
0.00 MgO
0.01 K2O
0.06 Na2O
0.00 TiO2
0.09 Al2O3
0.05 B2O3
0.95 SiO2
0.00 Fe2O3

Carol Tripp on sun 6 jun 04

Only part of what Dave wrote
. The
>kiln was over-fired to Cone 8 (I'm having a real problem calibrating my
>controller: with a 60C/hr rise I fired to 1205C, and still got Cone 8).
>Electric, Oxidation, on a buff stoneware body.

Hi Dave,
The slower you fire near the top temp for the cone you are aiming for, the
lower the actual top temp you want to program in. Slower firing means more
heatwork done. I fire to cone 6 and I program in 1188C for my top temp
because I fire at 50C/hour from 1050C, with a soak part of the way going up.
I don't use any cone 8s. I have sets of three cones to look at during the
firing; cone 5, cone 6 and cone 7. The cones are the self-supporting kind
because I have never seen a cone pack in real life and have no idea how to
do one right. Pay a bit more for the self supporters and save in the end.

To find your top ideal top temp, you have to experiment, keep notes and be
flexible. Temp guides in books don't really give more than an very general
idea, as you have already discovered. It took me lots of firings to arrive
at my current schedule and it will change some more I am sure. Use cones
and observe them during firings.

Program in a long soak at the end (say, 40 minutes) when you try out lower
top temps. This way, you can watch the cones as your kiln reaches the top.
My controller has a skip forward feature. When the soak has gone on long
enough (10 to 20 minutes depending) and cone 6 falls, I can stop the soak
and skip forward to the next segment, which is the fire down segment. You
have to be around the kiln for all this - no sleeping allowed at the end. I
hope this is clear.

Good luck on your next firing.
Best regards,
Dubai, UAE

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