Janet Haugen on tue 24 oct 06
have a green matt glaze I found in the archives that I love and have been using on vases and planters. I'm wondering if it would be safe and durable enough to use on table or ovenware.
This originally came from Bill Seeley, who identified it as a cone 9-10 glaze for use in either oxidation or reduction. I use it in cone 9 reduction.
I of Toad Green Matt Glaze
Kona F-4 (soda) Feldspar 40
EPK (kaolin) 20
Talc 20
Dolomite 20
Cobalt oxide 1
Chrome oxide 1
Granular rutile 20
I'm hoping that Ron R. or John H. or any others of you with glaze expertise will comment. I would also like to experiment with different oxides/amounts and wonder if that would affect the safety of the glaze.
Thank you for any help you can offer.
Janet
John Hesselberth on tue 24 oct 06
On Oct 24, 2006, at 10:46 AM, Janet Haugen wrote:
> have a green matt glaze I found in the archives that I love and =20
> have been using on vases and planters. I'm wondering if it would =20
> be safe and durable enough to use on table or ovenware.
> This originally came from Bill Seeley, who identified it as a =20
> cone 9-10 glaze for use in either oxidation or reduction. I use it =20=
> in cone 9 reduction.
>
> I of Toad Green Matt Glaze
>
> Kona F-4 (soda) Feldspar 40
> EPK (kaolin) 20
> Talc 20
> Dolomite 20
>
> Cobalt oxide 1
> Chrome oxide 1
> Granular rutile 20
>
>
> I'm hoping that Ron R. or John H. or any others of you with glaze =20=
> expertise will comment. I would also like to experiment with =20
> different oxides/amounts and wonder if that would affect the safety =20=
> of the glaze.
>
> Thank you for any help you can offer.
>
> Janet
Hi Janet,
I could not consider this glaze durable. It is way low in silica =20
level (see the unity formula below). I have found that silica levels =20
above 2.5, and preferably above 3.0, are necessary for good =20
stability. Levels below two are almost always a problem. This one has =20=
a silica level of only 1.7. If I were rating this for Clay Times =20
(which I do in each issue) it would definitely be rated as suitable =20
only for decorative or sculptural work. Even then I wouldn't want to =20
see it exposed to acid rain or bird poo in an outdoor setting.
And yes other oxides and the levels of them can definitely affect =20
stability. Copper is the most difficult to keep in a glaze. Iron is =20
probably the easiest.
One good screening test is to soak half of a small test tile in =20
vinegar for three days, wash it in plain water, dry it, and examine =20
for change in color or sheen. If you see either the glaze is not very =20=
durable. We explain the details of how to do this in MC6Gs or it is =20
probably in the archives in several places over the years.
Regards,
John
-------
Recipe Name: I of Toad Green Matte
Cone: 9 Color:
Firing: Ox. or Red. Surface: Matte
Amount Ingredient
40 Feldspar--Kona F4
20 Talc
20 Dolomite
20 Kaolin--EPK
100 Total
Additives
1 Cobalt Oxide
1 Chromium Oxide
20 Rutile--Granular
Unity Oxide
.097 Na2O
.045 K2O
.532 MgO
.326 CaO
1.000 Total
.321 Al2O3
.004 Fe2O3
1.655 SiO2
.002 TiO2
0 P2O5
5.2 Ratio
69 Exp
Comments:
-----------------------------------
Calculations by GlazeMaster=99
www.masteringglazes.com
------------------------------------
Craig Martell on tue 24 oct 06
Janet was asking:
> have a green matt glaze I found in the archives that I love and have
> been using on vases and planters. I'm wondering if it would be safe and
> durable enough to use on table or ovenware.
Hello Janet:
Your glaze is a very high magnesia matt with a shortage of silica which
will give you a "no" with regard to the question on durability. The low
limit for silica at cone 9 and 10 is 3.0 moles and your glaze has about
1.7. The Si/Al ratio is pretty low for a functional, durable glaze too.
Magnesia, especially talc are very low on the Moh's scale of hardness. I
think talc is 1. The thing that gives a glaze a hard, durable character is
quartz(silica) which is number 7 out of 10 on the Moh's scale.
regards, Craig Martell Hopewell, Oregon
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