Jon Singer on mon 1 nov 04
A couple weeks back, someone here posted a request for a crackle glaze
to be fired "at 1260 celsius". I worked up what I thought was a possible
recipe and sent it off by email, along with the usual caution that they
should test it carefully, because I hadn't tried it.
I also sent the "you can't go by temperature" caution, because 1260 c
can be anything from cone 7 to cone 11 or even 12, depending on how
fast your kiln gets to temperature and how long you keep it there. I
tested a really hefty burner on my tiny gas kiln a few days ago, and
it went up so fast that I had to take it to a little over 1320 celsius
to
reach cone 10. (I use a type S thermocouple and an Omega readout,
so that number is probably fairly accurate.)
I didn't get any reply, but frankly I rarely get replies when I send
email to people in this group whom I don't already know. It peeves
me, but there it is.
Be that as it may, my conscience began to bother me after a while.
I hadn't tried the recipe myself, and I didn't really know whether it
was what it was cracked up to be (sorry). I looked at it, tweaked it
a bit, mixed up a small test batch, and dipped two Helios porcelain
test tiles. Fired one at cone 9 o in the electric kiln, the other at
cone
10 r in the little gas test kiln. While I did get some crazing, it
certainly
wasn't what I would call a crackle glaze. This is why we test things.
I revised it again, and fired a test tile to cone 10 r. Here's a photo,
800 x 600 px:
http://www.jonsinger.org/pictures/crackle.jpg
I haven't done an oxidation firing since I mixed the test batch, so I
don't yet know what it will do, nor what color it will be. OTOH,
I am reasonably sure that this glaze will still melt at cone 9. I should
point out that I double-dipped one corner of the tile, and the glaze
slipped a bit where it was thick. I'd advise against thick application
unless you want it to slide or run.
If anyone cares, here's the recipe:
=====================================
G9 Crackle, cone 9-10
G-200 Feldspar 55
Ferro Frit 3269 7.5 (see note)
Ferro Frit 3110 7.5
Silica 8
Wollastonite 18
Titanium Dioxide 4
Si:Al ratio: 8.8
Seger # for Al2O3: 0.409
Calculated CTE: 8.7 parts per million per degree (celsius)
NOTE: Ferro 3269 is a high-expansion frit.
=====================================
If you don't like the color, you can omit the TiO2 and play with the
other
ingredients to maintain the crackle level. I would guess that an
opacifier
would make it into a pleasant white crackle, but this is a glaze type
that
I've never made before (at least, not on purpose), so don't take my word
for it.
...And, as always, test test test! (This should probably be the TSGG
motto.)
Cheers --
jon
PS: If you answer this and you want to be sure I see your response,
PLEASE send me a copy of your msg. I'm .
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