Gail Dapogny on mon 10 may 04
Hi David and Taylor,
I'm asking/speculating about this subject rather ignorantly but
wondered what you both (and anyone else) think: I recall Tom Turner
saying that he was adding some small percentage of talc--I forget
what-- in clay bodies for every cone higher he wanted it to be able to
fire to. Could that have anything to do with why Rhodes was adding it
to engobes?
---Gail Dapogny
gdapogny@umich.edu
Howdy Taylor,
Besides adding flux, borax aids in "physically" keeping the engobe
on the pot. Before firing.
Since it is water soluble, it migrates to the surface and dries in
a hard coating which, when totally dry is tougher than the powered
surface of only insoluble materials.
I don't know why Rhodes liked to add talc to his engobe recipes.
It is generally better to have more fluxes rather than fewer, but
in engobes it doesn't seem too important to me.
Don't be spoonin' no EPK without wearing a respirator.
David Hendley
david@farmpots.com
http://www.farmpots.com
Gail Dapogny on wed 12 may 04
OOPS, SORRY, AND THANKS, DAVID. Glad to be se straight on that. Now
another question -- really wading in over my head here but I'm curious.
Would the high loss on ignition rate of whiting possible have affected
his choice? That is, would that have caused more shrinkage and
therefore fit problems?
Gail Dapogny in Ann Arbor
On May 12, 2004, at 11:56 AM, David Hendley wrote:
> Gail, Tom Turner may have, indeed, told you he added talc to
> change the maturing temperature of claybodies. However,
> you have it backwards - talc is a flux, and adding talc lowers,
> not raises, the maturing temperature.
> Low-fire whiteware bodies, by the way, are traditionally half
> ball clay and half talc.
>
> Anyway, I'm sure Rhodes added talc to his engobe recipes
> as a flux. I just don't know why he wanted to add the
> magnesium from the talc rather than just using more calcium
> from whiting, since they are similar fluxes (talc also adds
> silica, but that would not be a reason to use it).
>
> Here's another "by-the-way":
> Low fire casting slip (half talc, half ball clay, and deflocculant)
> makes a good engobe at cone 10. Ron Roy will tell you that
> theoretically this will not work, but it will.
> The neat thing is that, because of all that magnesium, the addition
> of cobalt yields a nice rich purple, nothing like the typical cobalt
> blue color.
>
> David Hendley
> david@farmpots.com
> http://www.farmpots.com
>
>
> ----- Original Message -----
>> Hi David and Taylor,
>> I'm asking/speculating about this subject rather ignorantly but
>> wondered what you both (and anyone else) think: I recall Tom Turner
>> saying that he was adding some small percentage of talc--I forget
>> what-- in clay bodies for every cone higher he wanted it to be able to
>> fire to. Could that have anything to do with why Rhodes was adding it
>> to engobes?
>> ---Gail Dapogny
>
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David Hendley on wed 12 may 04
Gail, Tom Turner may have, indeed, told you he added talc to
change the maturing temperature of claybodies. However,
you have it backwards - talc is a flux, and adding talc lowers,
not raises, the maturing temperature.
Low-fire whiteware bodies, by the way, are traditionally half
ball clay and half talc.
Anyway, I'm sure Rhodes added talc to his engobe recipes
as a flux. I just don't know why he wanted to add the
magnesium from the talc rather than just using more calcium
from whiting, since they are similar fluxes (talc also adds
silica, but that would not be a reason to use it).
Here's another "by-the-way":
Low fire casting slip (half talc, half ball clay, and deflocculant)
makes a good engobe at cone 10. Ron Roy will tell you that
theoretically this will not work, but it will.
The neat thing is that, because of all that magnesium, the addition
of cobalt yields a nice rich purple, nothing like the typical cobalt
blue color.
David Hendley
david@farmpots.com
http://www.farmpots.com
----- Original Message -----
> Hi David and Taylor,
> I'm asking/speculating about this subject rather ignorantly but
> wondered what you both (and anyone else) think: I recall Tom Turner
> saying that he was adding some small percentage of talc--I forget
> what-- in clay bodies for every cone higher he wanted it to be able to
> fire to. Could that have anything to do with why Rhodes was adding it
> to engobes?
> ---Gail Dapogny
Kathy McDonald on wed 12 may 04
David,
I have tried the low fire casting slip
and i guess it must depend on the clay slip fit
mine flaked off before i could even get it to bisque.
I have tried both o6 and 6 Seeleys slip...applied both at freshly thrown
and at leather hard. it does work ,,,but only sometimes.
Any further info to share about making it work better i'd appreciate
-----Original Message-----
From: Clayart [mailto:CLAYART@LSV.CERAMICS.ORG]On Behalf Of David
Hendley
Sent: Wednesday, May 12, 2004 10:57 AM
To: CLAYART@LSV.CERAMICS.ORG
Subject: Re. Engobe formation
Gail, Tom Turner may have, indeed, told you he added talc to
change the maturing temperature of claybodies. However,
you have it backwards - talc is a flux, and adding talc lowers,
not raises, the maturing temperature.
Low-fire whiteware bodies, by the way, are traditionally half
ball clay and half talc.
Anyway, I'm sure Rhodes added talc to his engobe recipes
as a flux. I just don't know why he wanted to add the
magnesium from the talc rather than just using more calcium
from whiting, since they are similar fluxes (talc also adds
silica, but that would not be a reason to use it).
Here's another "by-the-way":
Low fire casting slip (half talc, half ball clay, and deflocculant)
makes a good engobe at cone 10. Ron Roy will tell you that
theoretically this will not work, but it will.
The neat thing is that, because of all that magnesium, the addition
of cobalt yields a nice rich purple, nothing like the typical cobalt
blue color.
David Hendley
david@farmpots.com
http://www.farmpots.com
----- Original Message -----
> Hi David and Taylor,
> I'm asking/speculating about this subject rather ignorantly but
> wondered what you both (and anyone else) think: I recall Tom Turner
> saying that he was adding some small percentage of talc--I forget
> what-- in clay bodies for every cone higher he wanted it to be able to
> fire to. Could that have anything to do with why Rhodes was adding it
> to engobes?
> ---Gail Dapogny
____________________________________________________________________________
__
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Ron Roy on wed 12 may 04
Hi David,
It's not a slip - but a high magnesia slip glaze - better not put it on too
thick - the expansion is way low - if you were to test a sample in a
dilatometer you are probably going to get a lot of cristobalite - not
something I would incorporate into any pottery to be used in the oven.
Just so everyone else knows - this is not something I would recommend -
I"ve seen what the expansion profile of a high talc body looks when fired
to cone 6.
RR
>Here's another "by-the-way":
>Low fire casting slip (half talc, half ball clay, and deflocculant)
>makes a good engobe at cone 10. Ron Roy will tell you that
>theoretically this will not work, but it will.
>The neat thing is that, because of all that magnesium, the addition
>of cobalt yields a nice rich purple, nothing like the typical cobalt
>blue color.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
David Hendley on fri 14 may 04
I hate to strike out twice in one day, but I can't help either one of
you - I don't know why the low fire casting slip flakes off your clay,
or if Rhodes avoided the use of whiting in engobes because
of excessive shrinkage. I can make some guesses:
The slip flaking off suggests that perhaps the clay content is too low.
Since you don't know the recipe for the slip, I would suggest
making your own slip (engobe) of half talc and half ball clay, and
give it a try.
As for the whiting - the fit problems with engobes are usually ones
of drying - before anything is fired. The high LOI rate of whiting
takes place in the firing, so I don't think that is a factor.
David Hendley
david@farmpots.com
http://www.farmpots.com
----- Original Message -----
> David,
> I have tried the low fire casting slip
> and i guess it must depend on the clay slip fit
> mine flaked off before i could even get it to bisque.
> I have tried both o6 and 6 Seeleys slip...applied both at freshly thrown
> and at leather hard. it does work ,,,but only sometimes.
> Any further info to share about making it work better i'd appreciate
>
> another question -- really wading in over my head here but I'm curious.
> Would the high loss on ignition rate of whiting possible have affected
> his choice? That is, would that have caused more shrinkage and
> therefore fit problems?
> Gail Dapogny in Ann Arbor
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