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ash glaze problems

updated fri 19 jul 02


Wesley Rolley on thu 13 dec 01

I recently started to use Ash from a different batch (same neighbor's=20=

fireplace, but probably had a different mix of oak/pine). In general,=20=

this neighbor burns several cords of local California Coastal Live Oak. =
I have been successful using this ash without washing. With the new=20=

batch, I have some blistering problems that I did not have before and=20=

would like to have some guidance in adjusting the glazes to fix the=20
problems. Without an analysis of the ash, I find it difficult to use=20=

Insight (which I have) to plan a solution.

First, some background:

Two different clays had the same problems. One was a ^10 white stonewar=
and the other a ^10 Stoneware with significant iron. Problems occurred =

on both bodies

Pots were all raw glazed when bone dry. Some were dipped. Others were =

sprayed. Problems occurred with both glazing practices.

Pieces were single fired to ^8 with the computer controls set as follows=

Temp (F) Rate Hold
200 150/hr 2.00
900 150/hr 0
1100 100/hr 0
2100 200/hr 0
2272 108/hr 30
1960 9999 0=09
1560 150/hr 0

Cone 8 was at 6:00 on middle shelf of 3.

Glaze 1 Normaly a semimatte tan that is more red where thinner. Has som=
typical iron glassy pooling in the bottom of bowls, throwing marks, etc.=

Redart 50
Oak Ash 40
Flint 10
+ Rutile 5

Glaze 2 (Testing for a new glaze). Was grey breaking pinkish where=20

Oak Ash 50
Ball Clay 25
Flint 25
+ Rutile 5

The basic problem is that the glaze blistered where thickest. This=20
happened on both bodies and with both glazes. Some standard answers for =

this are:
to bisque hotter to prevent off-gassing (I single fire fairly slowly)
to soak longer to allow blistering and pinholing to heal (I did soak for=
30 min. and cooled fairly slowly). =20

In the same firing a third glaze did not have the blistering effect.

Glaze 3 (Similar to my expected results with Glaze 1, but with no=20
pooling and no blistering. A bit more redish tones.)
Redart 50
Oak Ash 40
Custer Feldspar 10
+RIO 5

Would a longer soak have helped? A slower temp rise?
What would happen if I refire at the same cone?
What would happen if I refire at ^10?
What would happen if I fired new pieces a little cooler? ^6?
What adjustments can be made to the glazes to prevent the blistering (I =

would like to keep on firing at ^8.)

So, here are all of the problems of using ash based glazes and=20
singlefiring. Maybe I am just a masochist.

Wes Rolley

Waiting for the first "cold" storm of the year so that I can see the sno=
on our low (4000 ft.) mountains. Had the first bloom on our Japanese plu=
(ume) tree this week. Last year, the first blossom opened on New Year's=
Eve and the tree bloomed slowly for 1. months. Wonder what I can expect=
this year. =20

Holly Keeney on wed 17 jul 02

Hi all,
I am having a problem with an ash glaze that once worked beautifully. For
some reason it no longer will stay on the pot. I dip it and it falls right
off. Any suggestions?

Ababi on wed 17 jul 02

Hello Holly
Please send the recipe.
God knows what happened.
Perhaps you changed the would ash?
Ababi Sharon
Glaze addict
Kibbutz Shoval Israel

---------- Original Message ----------

>Hi all,
>I am having a problem with an ash glaze that once worked beautifully. For
>some reason it no longer will stay on the pot. I dip it and it falls right
>off. Any suggestions?

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BVCuma on thu 18 jul 02

I am having a problem with an ash glaze that once worked beautifully. =
some reason it no longer will stay on the pot. I dip it and it falls =
off. Any suggestions?
Dear Holly,
I would assume that when you say you "dip it in=20
and it falls right off"...
that presumably as it dries it cracks and peels back
and then falls off.
I gather that you are experiencing changes
within the same glaze batch that once previously gave=20
good adherence to the bisque ware?...
therefore the bafflement.
My assumption is that you are using unwashed ashes
and that the soluble sodium/potassium etc. are=20
crystallizing out of solution...
Recalling that Vince mentioned borax at 5% in all slip/engobe=20
recipes for raw, leatherhard, greenware and bisque ware
as a soluble which upon dehydration crystallizes into a matrix
that forms a bond aiding in adhesion to the pot.
Also in creating my own multi-stage applicable slip..
I included 10 % of the nonfritted meta-borate compound...
which I had assumed was non soluble,
but on observation of the dried residue left
over after a mortar pedestal grinding of said slip.
A beautiful snow flake type of crystal matrix had formed...
I believe this is what gives the slip and stick
properties that allow me to put it on at any stage
without any troubles.

I wonder about the nature of crystallization..
How fast would crystals form in the bucket
Temperatures obviously plays a role.
What else could initiate this process..seeding of some sort?
which seem to have in your case developed "overnight".
If the crystals formed are small and square=20
without an interlocking matrix formed while
drying on the pot but in the bucket this would
create an inability to adhere...
what about plasticity..this would be decreased?
shrinkage rates would change?
complex problem
creates interesting possibilities in solution
I'm always hesitate to dump such opportunities
down the drain....
could lead to better things.

Crystals are cool..
I grew sweet ones on string as a kid..
fun to munch on them afterwards,
these are damn tasty two!

best wishes