Paul Borian on fri 13 jun 08
Does anyone know of any specific glaze flaws that tend to result from
the use of bone ash? I believe I read one time that it can cause
pinholes if too much is used but I can't find any more specific info
than that at the moment.
Two of my glazes have about 8% bone ash (one is Ohata red and the other
is a slip glaze of my own design) and they both will sometimes have
large craters if the glaze is applied too thick, does not get hot
enough, or both - and the problem gets worse when other glazes are
layered on top. Sometimes the bare clay body is exposed and it looks
like a large bubble burst through the clay but I don't think this is the
case because it does not happen with my other glazes. I suspect that
some kind of gas is released from the glaze, giving it this bubble look,
possibly from the bone ash.
I plan to mix a batch of the glazes without the bone ash for testing
purposes and do some work with it but it will take several firings
before I have anything conclusive - so in the mean time I thought to ask
here to see if anyone else has had this problem.
Any info appreciated!
Thanks,
Paul
Hank Murrow on fri 13 jun 08
Dear Paul;
I used to avoid bone ash because of such reports. However, one of my
students at Anderson ranch sent me a recipe with 32% bone ash and I
loved it and have used it a lot ever since. Best ware glaze in my
repertoire. i use bone ash in amounts to 50% with no pinholing that I
am aware of. In formulating glazes with bone ash, it is helpful to
recognize that The Phosphorus content is a glass former like silica.
Cheers, Hank
On Jun 12, 2008, at 11:36 PM, Paul Borian wrote:
> Does anyone know of any specific glaze flaws that tend to result from
> the use of bone ash? I believe I read one time that it can cause
> pinholes if too much is used but I can't find any more specific info
> than that at the moment.
>
> Two of my glazes have about 8% bone ash (one is Ohata red and the
> other
> is a slip glaze of my own design) and they both will sometimes have
> large craters if the glaze is applied too thick, does not get hot
> enough, or both - and the problem gets worse when other glazes are
> layered on top. Sometimes the bare clay body is exposed and it looks
> like a large bubble burst through the clay but I don't think this
> is the
> case because it does not happen with my other glazes. I suspect that
> some kind of gas is released from the glaze, giving it this bubble
> look,
> possibly from the bone ash.
>
> I plan to mix a batch of the glazes without the bone ash for testing
> purposes and do some work with it but it will take several firings
> before I have anything conclusive - so in the mean time I thought
> to ask
> here to see if anyone else has had this problem.
John Post on fri 13 jun 08
Hi Paul,
I don't think your specific problem is related directly to the bone
ash though it is a contributing factor. If you remove the bone ash
from your glazes you will lose the red color. The glazes you describe
get their color from the iron oxide and bone ash combination.
You are exactly right in that if the glaze is too thick or does not
get hot enough that you will see what looks like craters. On my glaze
website below there are two iron glaze grids. In the upper right
corner of the grids you will see glazes that look like the iron is
bubbling or boiling. It is this bubbling boiling action that creates
oil spot glazes. The glaze tends to bubble up as the iron is
precipitating to the surface. If the bubbles and craters smooth out
then you have an oil spot glaze.
I think the way around your problem is to carefully control glaze
application thickness. Bone ash is light and fluffy and this along
with large amounts of iron in a glaze tend to flocculate the slurry
and make it thick like pudding. Having a glaze that acts like pudding
and thickens over time in the bucket can lead to glaze application
problems.
The way that bone ash and iron oxide behave in the slurry means that
you will have to either mix up just enough glaze to use in one session
or adjust the thickness of the slurry with water. Adding too much
water can also lead to problems so you will just have to see which
method works better. There are glaze additives you can use to control
the properties of the glaze slurry, but someone else will have to
offer advice here, since I don't have any direct experiences with
those products.
John Post
Sterling Heights, Michigan
:: cone 6 glaze website :: http://www.johnpost.us
:: elementary art website :: http://www.wemakeart.org
> Does anyone know of any specific glaze flaws that tend to result from
> the use of bone ash?
David Martin Hershey on fri 13 jun 08
John Post wrote:
> There are glaze additives you can use to control
> the properties of the glaze slurry, but someone else will have to
> offer advice here, since I don't have any direct experiences with
> those products.
>
Hi John and Paul,
I use Darvan 811 to deflocculate my glazes. This allows me to control
the viscosity of most of my glazes without adding extra water. This is
especially helpful when spraying. (Thanks Jonathan K!)
It is a very powerful additive! Usually 4 drops will deflocculate a
thixotropic 200 gram test batch.
It won't work on glazes that are mostly fritt, (very low clay) or if a
lot of suspension agent is used- but those glazes are rarely too thick
anyhow.
Best, DMH
David Martin Hershey
DMH Studio + Design
2629 Manhattan Ave # 137
Hermosa Beach CA USA
90254-2447
310.379.6890
dmh at dmhstudio dot com
http://www.dmhstudio.com
http://www.dmhstudio.etsy.com
James and Sherron Bowen on fri 13 jun 08
Ohata Kaki has been our most reliable glaze and has exhibited none of the
problems you describe. We haven't had problems with flocculation in that
glaze when stored for extended periods (years). I don't know if it's the
same as your Ohata Red but it has 11% bone ash.
Jim
----- Original Message -----
From: "Hank Murrow"
To:
Sent: Friday, June 13, 2008 8:32 AM
Subject: Re: Glaze flaws associated with bone ash?
> Dear Paul;
>
> I used to avoid bone ash because of such reports. However, one of my
> students at Anderson ranch sent me a recipe with 32% bone ash and I
> loved it and have used it a lot ever since. Best ware glaze in my
> repertoire. i use bone ash in amounts to 50% with no pinholing that I
> am aware of. In formulating glazes with bone ash, it is helpful to
> recognize that The Phosphorus content is a glass former like silica.
>
> Cheers, Hank
>
>
> On Jun 12, 2008, at 11:36 PM, Paul Borian wrote:
>
>> Does anyone know of any specific glaze flaws that tend to result from
>> the use of bone ash? I believe I read one time that it can cause
>> pinholes if too much is used but I can't find any more specific info
>> than that at the moment.
>>
>> Two of my glazes have about 8% bone ash (one is Ohata red and the
>> other
>> is a slip glaze of my own design)
Bill Merrill on fri 13 jun 08
When you say you are using bone ash in your glazes, are you using "real
bone ash"? .
What we would normally get is TCP (tri calcium phosphate) a commercial
replacement.
Bill =20
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG] On Behalf Of Hank Murrow
Sent: Friday, June 13, 2008 7:33 AM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re: Glaze flaws associated with bone ash?
Dear Paul;
I used to avoid bone ash because of such reports. However, one of my
students at Anderson ranch sent me a recipe with 32% bone ash and I
loved it and have used it a lot ever since. Best ware glaze in my
repertoire. i use bone ash in amounts to 50% with no pinholing that I
am aware of. In formulating glazes with bone ash, it is helpful to
recognize that The Phosphorus content is a glass former like silica.
Cheers, Hank
On Jun 12, 2008, at 11:36 PM, Paul Borian wrote:
> Does anyone know of any specific glaze flaws that tend to result from
> the use of bone ash? I believe I read one time that it can cause
> pinholes if too much is used but I can't find any more specific info
> than that at the moment.
>
> Two of my glazes have about 8% bone ash (one is Ohata red and the
> other
> is a slip glaze of my own design) and they both will sometimes have
> large craters if the glaze is applied too thick, does not get hot
> enough, or both - and the problem gets worse when other glazes are
> layered on top. Sometimes the bare clay body is exposed and it looks
> like a large bubble burst through the clay but I don't think this
> is the
> case because it does not happen with my other glazes. I suspect that
> some kind of gas is released from the glaze, giving it this bubble
> look,
> possibly from the bone ash.
>
> I plan to mix a batch of the glazes without the bone ash for testing
> purposes and do some work with it but it will take several firings
> before I have anything conclusive - so in the mean time I thought
> to ask
> here to see if anyone else has had this problem.
Hank Murrow on fri 13 jun 08
On Jun 13, 2008, at 10:46 AM, Bill Merrill wrote:
> When you say you are using bone ash in your glazes, are you using
> "real
> bone ash"? .
>
> What we would normally get is TCP (tri calcium phosphate) a commercial
> replacement.
>
Dear Bill;
Yes, I use 'real' bone ash, sourced from Kodak, which still uses it
in film production. What will happen when digital finally rules is
anybody's guess. if someone knows what is likely to happen, please
inform us all. I have a couple of hundred #s, but might get more if
the supply is closing down.
My real bone ash glazes tend to grow things in the bucket, but a bit
of bleach cures that in a hurry while leaving the glaze unchanged. I
have high(30%+) bone ash glazes sitting in big buckets for years
whithout any difficulty in use and application.
I have pics of such glazes if interested. Can't wait until Clayart
has a pic site!
Cheers, Hank
PS; Tonight I give the free slide show, "50 years of Clay.....so far,
anyway!" at Clay Space in Eugene. Wish y'all could be here.
Ivor and Olive Lewis on sat 14 jun 08
Dear Paul Borian ,
It is possible that we do not understand the chemistries involved in
ceramic systems that incorporate Calcium Phosphate.
Phosphorus pentoxide is a well known body flux and has a reputation as
being a glass former. Since you are using it as a glaze ingredient
then the amount you use may create two immiscible phases in your
glaze. It is possible the properties of the phosphate and silicate
fractions are incompatible and antagonistic. You might consult the
Hamer Dictionary. This warns against the occurrence of opacity, dull
surfaces and blisters when Calcium Phosphate is used in significant
amounts as a glaze ingredient.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Karin Givon on sat 14 jun 08
I LOVE THAT ( whoops!) the letter from Hank had "Glaze FLaws Ass"
in the subject line on my computer. Thanks for that.
And Hank, do your glazes freeze in the winter? And if so does it
change them?
Thanks
Karin
and Yes, wouldn't it be fun to be seeing "50 years so far"--
Hank Murrow on sun 15 jun 08
On Jun 14, 2008, at 8:27 PM, Karin Givon wrote:
> I LOVE THAT ( whoops!) the letter from Hank had "Glaze FLaws Ass"
> in the subject line on my computer. Thanks for that.
Well, gee Karin...... maybe that's my trouble! I am running off to
the bathroom mirror to see.
>
> And Hank, do your glazes freeze in the winter? And if so does it
> change them?
Yes, some are kept out in the kiln shed and do freeze in bad winters.
I can find no effect, good or bad.
>
> and Yes, wouldn't it be fun to be seeing "50 years so far"--
There were around 40+ people there! The stories the slides provoked
were pretty funny, and several folks came up afterward to tell me how
much they enjoyed it.
The workshop following the slide show went wonderfully yesterday, and
today I show my slide show, ""On the Shino Trail in Japan". I hope
folks are suitably informed and entertained.
Cheers, Hank
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