Fred Hagen on sun 3 apr 05
As a relative beginner in pottery I'm confused by the use of the above ingredients --when to use them. Can CMC be added to a glaze that is already mixed or must it go in at the dry stage? Bentonite is often used in the dry state at 1 to 2% it seems --I know that it helps to keep the glaze ingredients in suspension but can CMC be used in the same way. I'm told that CMC is a brushing medium ? The use of Epson Salt is a little confusing as well. I see people put it into a glaze in the liquid state but how much for say 1500grms of glaze ? (and I know that if too much is added it turns the glaze into something like paste) and can it be used as an ingredient in the dry stage --at what level? Then there is Veegum--same sort of thing? I like to spray the glazes so which would be best for that method?
Lots of questions --perhaps someone can enlighten me
Thanks Fred Hagen
william schran on sun 3 apr 05
Fred wrote:>As a relative beginner in pottery I'm confused by the use
of the above ingredients --when to use them. Can CMC be added to a
glaze that is already mixed or must it go in at the dry stage?
Bentonite is often used in the dry state at 1 to 2% it seems --I
know that it helps to keep the glaze ingredients in suspension but
can CMC be used in the same way. I'm told that CMC is a brushing
medium ?<
I will let others explain the whys and wherefores of each material
and simply tell you how I use CMC and bentonite.
I use a CMC solution to wet my crystalline glazes. The glazes contain
no clay or other materials that will suspend the glaze or harden it
on the pot as the glaze dries. As I apply the crystalline glaze very
thick, if I just used water, it would all just crack off. I add a few
tablespoons of dry CMC to a couple quarts or so of hot water and let
it sit a couple of days without mixing (let the CMC just soak up the
water), then stir it up to a honey/syrup consistency - this is what
I mix with the glaze.
I use bentonite to suspend glazes that are short of clay. I weigh
glaze materials and add to water, allow to soak for a while, stir
with electric drill/stir paddle, then sieve twice.
Bill
David Beumee on sun 3 apr 05
Hi Fred,
I use CMC for sizing the surfaces of pots that have been sprayed with glaze, and for helping with the brushability of stains. I start with a thousand ml hot water in my studio blender and put in 5 teaspoons dry CMC as the blender is running, then run the mixture through a strainer into a gallon jug and fill with water.
If a glaze recipe has no clay content or if its one that settles badly, using 2% bentonite added dry to the mixture will help with keeping the glaze in suspension. Many bentonites are loaded with iron, so as a porcelain guy I sometimes use 2% VeegumT instead, especially for white, clear, and celadon glazes. It works great for keeping a glaze from hardpanning in the bucket and adds no iron.
Epsom Salts, magnesium sulphate, also helps to keep glazes in suspension. Epsom Salts are a flocculant, so it's action is helped if there is some clay or bentonite content to the glaze. Again, I start with 1000 ml warm water in my studio blender and make a saturated solution of Epsom Salts that I keep in a small covered plastic bucket. Begin by adding a scoop of Salts to the water, mix it up in the blender and wait until the water becomes clear before adding another scoop. Each time you add a scoop of Salts, wait until the mixture clears before adding more Salts. You end up with a very saturated solution, and may need only a couple of tablespoons of the solution to float a 10,000 gram batch of glaze. Works great.
David Beumee
Earth Alchemy Pottery
Lafayette, CO
www.davidbeumee.com
-------------- Original message ----------------------
From: Fred Hagen
> As a relative beginner in pottery I'm confused by the use of the above
> ingredients --when to use them. Can CMC be added to a glaze that is already
> mixed or must it go in at the dry stage? Bentonite is often used in the dry
> state at 1 to 2% it seems --I know that it helps to keep the glaze ingredients
> in suspension but can CMC be used in the same way. I'm told that CMC is a
> brushing medium ? The use of Epson Salt is a little confusing as well. I see
> people put it into a glaze in the liquid state but how much for say 1500grms of
> glaze ? (and I know that if too much is added it turns the glaze into something
> like paste) and can it be used as an ingredient in the dry stage --at what
> level? Then there is Veegum--same sort of thing? I like to spray the glazes so
> which would be best for that method?
> Lots of questions --perhaps someone can enlighten me
> Thanks Fred Hagen
>
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Vince Pitelka on sun 3 apr 05
Here's some information that I hope will be helpful.
CMC gum is carboxy-methyl-cellulose (or something close to that). It is a
very-long-chain organic molecule that simply interferes with the movement of
particles in a solution. In other words, it is a thickener. But as an
organic molecule, in its wet form it will eventually succumb to mold or
other organic decay. In use, the normal approach is to add a few teaspoons
to a quart of hot water and let it sit overnight, and then add small amounts
of the resulting thick liquid to glazes, underglazes, or slips, to use as a
brushing medium or a suspension (anti-settling) agent.
Bentonite is an extremely-fine-particle-size clay, and thus imparts extreme
plasticity (the more particles, the more water layers and contact points,
the more plasticity). It is added to claybodies as a plasticizer, and to
glazes as a suspension agent. In a very different way, it has an effect
similar to CMC. The ultra-fine particles take much longer to settle out,
and thus interfere with gravity and help all of the materials to remain in
suspension. Because it is so fine in size, it gives more contact points
between particles in the dry state, improving dry strength and adhesion of
glazes. But because it is so fine, it is rarely used in amounts greater
than 3%, because larger amounts would induce high shrinkage and cause a host
of other problems. Dry bentonite can be very resistant to mixing in an
existing suspension, floating immediately back to the surface. When
possible, mix it with dry ingredients before adding water to the initial
batch recipe. If you must add bentonite to an exisiting wet batch, use a
blender or hand blender to thoroughly mix it with a small amount of the
glaze or slip.
Epsom Salts, or magnesium sulfate, is a soluble material which produces an
acidic nature in a water suspension. In glazes or slips it behaves as a
flocculant, causing the particles to attract one-another. In a very thin
suspension, like a terra sigillata, that would cause everything to settle
out of suspension (a de-flocculant like soda ash or sodium silicate is what
is needed in that case). In a thicker suspension, like a glaze or slip, it
does the opposite, causing the solution to thicken, reducing settling and
improving application properties. Dissolve dry Epsom salts in quart of hot
water until no more will dissolve, and keep that handy in a closed bottle.
With a troublesome glaze, add a little at a time, stirring frequently, until
the glaze thickens slightly.
Good luck -
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
Vince Pitelka on mon 4 apr 05
I hope I am not duplicating a previous post, but I never saw this one =
appear on Clayart. Seems to be quite a few messages slipping into the =
ether these days. Wonder why . . . .=20
Original Message =
-------------------------------------------------------------------------=
----
Here's some information that I hope will be helpful.
CMC gum is carboxy-methyl-cellulose (or something close to that). It is =
a=20
very-long-chain organic molecule that simply interferes with the =
movement of=20
particles in a solution. In other words, it is a thickener. But as an=20
organic molecule, in its wet form it will eventually succumb to mold or=20
other organic decay. In use, the normal approach is to add a few =
teaspoons=20
to a quart of hot water and let it sit overnight, and then add small =
amounts=20
of the resulting thick liquid to glazes, underglazes, or slips, to use =
as a=20
brushing medium or a suspension (anti-settling) agent.
Bentonite is an extremely-fine-particle-size clay, and thus imparts =
extreme=20
plasticity (the more particles, the more water layers and contact =
points,=20
the more plasticity). It is added to claybodies as a plasticizer, and =
to=20
glazes as a suspension agent. In a very different way, it has an effect =
similar to CMC. The ultra-fine particles take much longer to settle =
out,=20
and thus interfere with gravity and help all of the materials to remain =
in=20
suspension. Because it is so fine in size, it gives more contact points =
between particles in the dry state, improving dry strength and adhesion =
of=20
glazes. But because it is so fine, it is rarely used in amounts greater =
than 3%, because larger amounts would induce high shrinkage and cause a =
host=20
of other problems. Dry bentonite can be very resistant to mixing in an=20
existing suspension, floating immediately back to the surface. When=20
possible, mix it with dry ingredients before adding water to the initial =
batch recipe. If you must add bentonite to an exisiting wet batch, use =
a=20
blender or hand blender to thoroughly mix it with a small amount of the=20
glaze or slip.
Epsom Salts, or magnesium sulfate, is a soluble material which produces =
an=20
acidic nature in a water suspension. In glazes or slips it behaves as a =
flocculant, causing the particles to attract one-another. In a very =
thin=20
suspension, like a terra sigillata, that would cause everything to =
settle=20
out of suspension (a de-flocculant like soda ash or sodium silicate is =
what=20
is needed in that case). In a thicker suspension, like a glaze or slip, =
it=20
does the opposite, causing the solution to thicken, reducing settling =
and=20
improving application properties. Dissolve dry Epsom salts in quart of =
hot=20
water until no more will dissolve, and keep that handy in a closed =
bottle.=20
With a troublesome glaze, add a little at a time, stirring frequently, =
until=20
the glaze thickens slightly.
Good luck -
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/=20
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