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
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
10 r in the little gas test kiln. While I did get some crazing, it
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:
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
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
ingredients to maintain the crackle level. I would guess that an
would make it into a pleasant white crackle, but this is a glaze type
I've never made before (at least, not on purpose), so don't take my word
...And, as always, test test test! (This should probably be the TSGG
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