Janet H Walker on mon 23 feb 98
Someone asked the other day about the billions of tiny bubbles that
formed the clouds in a glaze that was supposed to be clear.
I've been wondering about a similar phenomenon in a glaze that is
clear to start with. I added 1% TiO2 which gives a nice blush of
crystalline effects, without the yellowish orange you get from
rutile. Where this glaze is thicker though and right together with
the crystals/bubbles, there's a foamy texture inside the glaze. I
can tell because I fire on stilts and when the stilts come off, a
little circular crust of glass stays on the stilt pin and the
interior of the glaze where the stilt pulled off the surface looks
like froth. Foam. (P.S. is this behavior an indicator of a fit
problem?)
Here's the recipe, fired to about cone 5.4 or 6.2 (depending on
which cone you want to read). Or fired to 1200C at 50C/hr.
Powder Box Pink (Mimi Obstler's Out of the Earth Into the Fire p147)
50 Ferro frit 3134
15 Ball Clay (OM #4)
5 Whiting
25 Silica (300 mesh)
-----
2 Bentonite
1 Titanium dioxide
So the glaze goes onto greenware and gets put in the kiln on
stilts. Fired. Tons of what I thought were crystals but maybe they
really are bubbles, as revealed by the froth. The presence of some
sharp-edged pinholes argues that my crystals are bubbles maybe.
Conventional wisdom I: soak more or cool more slowly.
Done that I: Goes up to temp at 60C/hr, soaks 25 minutes, cools
at 150C/hr to 900, then at 60C/hr to 700. If it gets any slower
than that, won't I be a laughing stock?
Conventional wisdom II: try refiring to convince yourself that it
needs to soak some more.
Done that II: Refired a set of pieces. Foam is still there. Some
of the sharp edged pinholes have closed up.
I have used this same recipe with 8% rutile (lovely effect by the
way if your rutile is finely enough ground). It doesn't exhibit the
same problem as far as I can see. A few pinholes where quite thick
but nothing like this.
So. Is this because titanium actually makes a glaze more
refractory? I'm kinda puzzled about calculating how to make
something melt more smoothly. Especially since it already seems
totally loaded up with melter, all that boron.
Thanks for any pointers to enlightenment
Jan Walker
Cambridge MA USA
Ron Roy on thu 26 feb 98
Titaniaum does provide a seed for crystals to start.
When cones go down together it is usually an indicatin that firing is too
fast. Cone 5 should be - tip touching the shelf just as cone 6 starts. At
least this is how it works at high fire. I realize there are old style
Orton cones and a newer formulation and they should not be used together. I
think it is also commom now for cone 6 and 7 to go down at almost the same
time.
"foamy texture" is not an indication of fit problems - I would think
melting or even cooling would be the cause.
>----------------------------Original message----------------------------
>Someone asked the other day about the billions of tiny bubbles that
>formed the clouds in a glaze that was supposed to be clear.
>
>I've been wondering about a similar phenomenon in a glaze that is
>clear to start with. I added 1% TiO2 which gives a nice blush of
>crystalline effects, without the yellowish orange you get from
>rutile. Where this glaze is thicker though and right together with
>the crystals/bubbles, there's a foamy texture inside the glaze. I
>can tell because I fire on stilts and when the stilts come off, a
>little circular crust of glass stays on the stilt pin and the
>interior of the glaze where the stilt pulled off the surface looks
>like froth. Foam. (P.S. is this behavior an indicator of a fit
>problem?)
>
>Here's the recipe, fired to about cone 5.4 or 6.2 (depending on
>which cone you want to read). Or fired to 1200C at 50C/hr.
Ron Roy
93 Pegasus trail
Scarborough Otario
Canada M1G 3N8
Phone: 416-439-2621
Fax: 416-438-7849
Web page: Home page http://digitalfire.com/education/people/ronroy.htm
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