iandol on sat 22 jul 00
Dear Phil
I would like to have a shot at this problem but need to know how you =
deal with your salt. Do you throw it into the fire mouth or has your =
kiln special ports for presenting the salt to the pots? The second thing =
I would like to know is the composition of your refractories, in =
particular the iron content. What else can you say about the kaolin and =
the ball clay in the slip recipe? How did this recipe originate? Have =
you ever tested all of your raw materials individually to see how they =
behave? Why did you choose cone 8. I presume this is Orton and not =
Seger. Do you have a bag wall between the raw flame from the burner?
Salt glaze chemistry, using sodium chloride, is much misunderstood. I =
would have expected your slip to give a good glaze, even reasonable =
orange peel, but would prefer to use a potash felspar rather than Neph =
Syenite. Do you have a computer glaze program which could be manipulated =
to give the Seger unity formula by replacing the potash in the Neph =
Syenite with Na2O. It would be interesting to know where this fits into =
the commonly accepted limits formulae.
Given this information I can take the discussion further.
Regards,
Ivor Lewis.
Craig Martell on sun 23 jul 00
Ivor commented:
>I would have expected your slip to give a good glaze, even reasonable
>orange peel, but would prefer to use a potash felspar rather than Neph
>Syenite. Do you have a computer glaze program which could be manipulated
>to give the Seger unity formula by replacing the potash in the Neph
>Syenite with Na2O. It would be interesting to know where this fits into
>the commonly accepted limits formulae.
Hello Ivor:
The idea behind a slip of this composition is to give a warm orange "flash"
to the pots and not necessarily to produce a rich salt glaze look. The
silica/alumina ratio is something to be considered when going after this
sort of effect. The idea is to get the ratio in the 3 to 1 range. For a
good salt glaze, slips are usually targeted at about 5 to 1 for the
silica/alumina ratio. Nepheline syenite will contribute more alumina than
silica while being a very active flux and is a good choice for this kind of
approach. When I use these types of slips it is on the outside of a pot so
I'm not concerned about limits which are sort of "out the window" anyway
because of the very nature of vapor glazing.
regards, Craig Martell in Oregon
philrogers pottery on sun 23 jul 00
Craig,
Do you think there is any mileage in the theory that Nephaline contributes
to the trapping of carbon as in shino glazes? I am convinced that the carbon
is not there before residual salt begins to rise and is therefore
accumulating from that point on.
Any experience of sulphur in relation to this problem?
Phil.
----- Original Message -----
From: Craig Martell
To:
Sent: Sunday, July 23, 2000 5:57 PM
Subject: Re: : Salt glaze
> Ivor commented:
>
> >I would have expected your slip to give a good glaze, even reasonable
> >orange peel, but would prefer to use a potash felspar rather than Neph
> >Syenite. Do you have a computer glaze program which could be manipulated
> >to give the Seger unity formula by replacing the potash in the Neph
> >Syenite with Na2O. It would be interesting to know where this fits into
> >the commonly accepted limits formulae.
>
> Hello Ivor:
>
> The idea behind a slip of this composition is to give a warm orange
"flash"
> to the pots and not necessarily to produce a rich salt glaze look. The
> silica/alumina ratio is something to be considered when going after this
> sort of effect. The idea is to get the ratio in the 3 to 1 range. For a
> good salt glaze, slips are usually targeted at about 5 to 1 for the
> silica/alumina ratio. Nepheline syenite will contribute more alumina than
> silica while being a very active flux and is a good choice for this kind
of
> approach. When I use these types of slips it is on the outside of a pot
so
> I'm not concerned about limits which are sort of "out the window" anyway
> because of the very nature of vapor glazing.
>
> regards, Craig Martell in Oregon
>
>
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Hank Murrow on sun 23 jul 00
>Craig,
>
>Do you think there is any mileage in the theory that Nephaline contributes
>to the trapping of carbon as in shino glazes? I am convinced that the carbon
>is not there before residual salt begins to rise and is therefore
>accumulating from that point on.
>Any experience of sulphur in relation to this problem?
>
>Phil.
Dear Phil; I look forward to Craig's early post on this; but will throw in
my 'two sous now'. I've done some work with Shinos and have come to believe
that Neph Sye does not particularly 'facilitate' carbon trapping. However,
ANY soluble, like soda ash or wood ash (if still containing solubles) WILL
show some carbon trapping. Just add 4% soda ash to a Neph Sye Shino and see
what happens.
Cordially, Hank in Eugene
Craig Martell on mon 24 jul 00
Phil asked:
>Do you think there is any mileage in the theory that Nephaline contributes
>to the trapping of carbon as in shino glazes?
Yes. Nepheline starts to melt between cone 03 and 02 according to
everything that I've read. Maybe I should do some draw trials of high
nepheline syenite glazes and see what's really happening. Here in the US,
avid carbon trappers will add soda ash to their shinos with the idea that
this stuff precipitates on the surface of the pot and glaze thereby
increasing the chances for some very aggressive carbon trap. These folks
usually start reduction earlier than I do at around cone 06 too. But I've
trapped carbon occasionally and it's usually where I've seen this white
fuzz on the surface of the pots from precipitated soda ash. But, back to
nepheline. I knew a guy who used a very high Nepheline Syenite copper red
glaze and he insisted on using pure carbon for early reduction. His copper
red would trap so much carbon that he had areas that were very black and he
refused to lighten up on reduction. So this led me to think that one could
trap carbon with a glaze containing just Neph Sy and no soda ash. Ian
Currie says that he sees this happen a lot in high flux mixes in his 35
glaze biaxial grid tests. I say some carbon trap in the .6 limestone
biaxial of Ian's that I did here and the trapping occured in the higher
flux areas. Sorry for the long winded account.
>Any experience of sulphur in relation to this problem?
Absolutely zip!! I fire with propane and my clay is porcelain so I don't
have any experience with sulphur at all. Perhaps someone on the list that
does know about this will comment. If not, I can talk to some folks here
that oil fire and see what gives.
later, Craig Martell in Oregon
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