Brian Dean on wed 15 feb 12
Hi Again:
I've been asked what type of cobalt was being used, cobalt
carbonate or Cobalt Oxide, and what percentages in the glazes.
Here are the glaze recipe specifics:
1. Hannah's Fake Ash Glaze (Steven Hill's Revised Version for Cone 6)
Stontium Carb: 10.1
Whiting 29.0
Silica 22.7
Redart Clay 56.1
Frit 3195 4.8
*************************
100
Additives:
Bentonite 10.0
Cobalt Oxide 1.3
2. Val Cushing's G4 Ash Glaze (Cone 5 - 6)
Washed Wood Ash 50.0
Gerstley Borate 20.0
Whiting 12.0
EPK 8.0
Silica 10.0
*************************
100
Additives:
Cobalt Carbonate 2.0
Both Glazes when sprayed to a reasonable thickness to
enable them to run and when not layered with a number
of other glazes exhibit pin holing. Not really getting much pin holing
from any other glazes using the below firing schedules with either
010 or 04 bisque firing. Just the cobalt glazes seem to have the
pin holing. There is also a third ash glaze that has had pin holing
but I think my above two most used cobalt based glazes should
be sufficient for this discussion.
I have been firing to Steven Hills electric firing schedule which is as
follows:
Segment Ramp End point Hold
1 200o F / hour 220o 30 minutes
2 400o / hour 2100o 0
3 100o / hour 2170o 30 - 60 Minute=
s
4 9999o / hour 1700o 0
5 50o / hour 1600o 60 Minutes
6 500 / hour 15000 Kiln OFF!
What, if any, effect would the firing schedule have on
pinholing in your opinion as it relates to pinholes showing up
in cobalt glazes?
Regards,
Brian Dean
info@the-five-elements.com
http://www.the-five-elements.com
Brian Dean on wed 15 feb 12
Hi Gang:
I have been doing a lot of glaze firing at cone 6 lately
and have had some pin holing occur. Have read the recent
thread regarding bisquing to 04 and not lower to burn out
organics, etc. I have bisqued some of my pots to 010 and
some to 040 and some I have been single firing to test that
out as well. No matter what I have been bisquing to, however,
the pots that exhibit pin hole are 99% exclusively pots that
have colbalt prominent in the glazes and the pin holing only
occurs on those pots specifically within the cobalt glazes.
Any thoughts out there about the contribution of cobalt
to glaze pin holing and what to do about it?
Also, recently purchased Steven Hills new DVD "The Surface
Techniques of Steven Hill." Great, great DVD. If you are
doing cone six electric firing and spray glazes it is a "Must Have"
learning tool. The point is that I have been firing to Steven
Hills electric firing schedule which is as follows:
Segment Ramp End point Hold
1 200o F / hour 220o 30 minutes
2 400o / hour 2100o 0
3 100o / hour 2170o 30 - 60 Minute=
s
4 9999o / hour 1700o 0
5 50o / hour 1600o 60 Minutes
6 500 / hour 15000 Kiln OFF!
What, if any, effect would the firing schedule have on
pinholing in your opinion as it relates to pinholes showing up
in cobalt glazes?
Your most wonderfully educated and experienced opinions
would be most appreciated!
Regards,
Brian Dean
info@the-five-elements.com
http://www.the-five-elements.com
Dick White on thu 16 feb 12
Brian, you posit that cobalt is causing pinholing in your glazes. Can you=
=3D
tell us more about the glaze, specifically, the recipe of the glaze. Have=
=3D
you tried the same base glaze without cobalt or with other colorants and
found there were no pinholes? Do the offending glazes contain any Custer
feldspar?
dw
Eva Gallagher on thu 16 feb 12
Hi - I've used the hannah Fake Ash cone 10 version and it would pinhole in
redux until I fired the last two hours from cone 8 to 10 in oxidation. I
also switched clays from a meduim dark stoneware to either a very light
stoneware or porcelain. The Ochre ash as well as the blue ash would
pinhole - as well as steven Hill blue ash and his cone 6 green ash - never
could get that to work - but otherwise his system of layering and spraying
glazes gives amazing results - if you can, take one of his workshops.
It may have to do with off gasing from all whiting and/or red art /Alberta
slip?
Eva Gallagher
Deep River, Ontario
http://newfoundoutpotter.blogspot.com/
----- Original Message -----
From: "Brian Dean"
To:
Sent: Wednesday, February 15, 2012 3:26 PM
Subject: Re: Cobalt and its Effects on Cone 6 Glaze Pin Hole Occurance
> Hi Again:
>
>
>
> I've been asked what type of cobalt was being used, cobalt
>
> carbonate or Cobalt Oxide, and what percentages in the glazes.
>
>
>
> Here are the glaze recipe specifics:
>
>
>
> 1. Hannah's Fake Ash Glaze (Steven Hill's Revised Version for Cone
> 6)
>
>
>
> Stontium Carb: 10.1
>
> Whiting 29.0
>
> Silica 22.7
>
> Redart Clay 56.1
>
> Frit 3195 4.8
>
> *************************
>
> 100
>
> Additives:
>
> Bentonite 10.0
>
> Cobalt Oxide 1.3
>
>
>
> 2. Val Cushing's G4 Ash Glaze (Cone 5 - 6)
>
>
>
> Washed Wood Ash 50.0
>
> Gerstley Borate 20.0
>
> Whiting 12.0
>
> EPK 8.0
>
> Silica 10.0
>
> *************************
>
> 100
>
> Additives:
>
> Cobalt Carbonate 2.0
>
>
>
> Both Glazes when sprayed to a reasonable thickness to
>
> enable them to run and when not layered with a number
>
> of other glazes exhibit pin holing. Not really getting much pin holing
>
> from any other glazes using the below firing schedules with either
>
> 010 or 04 bisque firing. Just the cobalt glazes seem to have the
>
> pin holing. There is also a third ash glaze that has had pin holing
>
> but I think my above two most used cobalt based glazes should
>
> be sufficient for this discussion.
>
>
>
> I have been firing to Steven Hills electric firing schedule which is as
> follows:
>
>
>
> Segment Ramp End point Hold
>
> 1 200o F / hour 220o 30 minutes
>
> 2 400o / hour 2100o 0
>
> 3 100o / hour 2170o 30 - 60
> Minutes
>
>
> 4 9999o / hour 1700o 0
>
> 5 50o / hour 1600o 60 Minutes
>
> 6 500 / hour 15000 Kiln OFF!
>
>
>
> What, if any, effect would the firing schedule have on
>
> pinholing in your opinion as it relates to pinholes showing up
>
> in cobalt glazes?
>
>
>
> Regards,
>
> Brian Dean
>
>
>
> info@the-five-elements.com
>
> http://www.the-five-elements.com
>
>
ivor and olive lewis on fri 17 feb 12
Dear Brian Dean
Greenwood and Earnshaw, in "Chemistry of the Elements", ISBN 0-7506-3365-4,
p. 1118, write that Cobalt Monoxide (CoO) is stable in air at ambient
temperatures and above 900 deg C but if heated to 600-700 deg C is converte=
d
to black cobalt oxide, Co3O4. This may explain the release of Oxygen as a
cause of pinholing.
Regards,
Ivor Lewis,
REDHILL,
South Australia.
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