Caroline Jacobs on sat 3 mar 01
i just wondered if you can change a commercial gloss glaze into a matt glaze using a substance around the house. the glaze i have is "duncan gl 612 diamond clear gloss glaze transparent". thanks a lot.
caroline in sta monica
Cindy Strnad wrote:
>
> Hi, Chris.
>
> Here's a nice crystal matte I put together as a kind of synthesis of Ron
> Roy's and Tony Hansen's matte glazes.
>
> Crystal Matte Base ^6
> EPK KAOLIN 30
> WHITING 21
> CUSTER FELDSPAR 20
> FRIT 3134 15
> SILICA 14
>
> If you add 2% Cobalt Oxide, you'll get a nice medium to dark blue. It's
> important to cool this glaze slowly, as you would any crystalline glaze, to
> allow the crystals time to grow. Otherwise, you'll get many shiny spots.
>
> If you have a computer controlled kiln, this is easy. Just program the
> controller to fire down at about 100 degrees F per hour until you get to
> around 1500 or so. If not, I'm afraid you'll need either good thick kiln
> walls, or you'll need to "baby-sit" the kiln until it gets down to a dull
> red glow.
>
> The glaze above is crystalline because of the high calcium content. You can
> also get magnesia mattes, but I don't have a recipe for any of these. Maybe
> I'll ask for one on Clayart soon. I haven't seen any in my books. The
> calcium forms a fine crystalline matrix which mattes the glaze.
>
> The difference between this and the more common alumina mattes is that the
> alumina mattes are generally non-shiny because the high content of alumina
> prevents the glaze from melting fully at the cone level you fire to. Were
> you to fire the glaze higher, it would become glossy. Because these glazes
> are immature and low on silica, they're very susceptible to leaching,
> marking, and absorbing liquids. If they're also used on a clay body which is
> not fully mature, this makes for a very bad situation in functional ware.
> It's fine for artistic only, but not good if you're planning to serve iced
> tea in it.
>
> Crystalline mattes are fully melted (matured) before they become matte. They
> are, therefore, vitrified (glass-like) and better able to withstand wear. If
> they're well-formulated, they can hold coloring oxides without leaching them
> into your food. The only way to know for sure if any glaze is leaching
> oxides is to have it tested in a lab, but a preliminary test would be to
> partially fill a piece with vinegar for a couple of days and then see if you
> can tell a difference in color above and below the tide line.
>
> Another test for copper would be to fill a drinking vessel with an acidic
> beverage (orange juice, for example) and see if you get an off taste. Copper
> isn't going to hurt you unless you get a real load of it, btw, so this test
> is safe enough so long as you don't do it twice a day.
>
> Lots of people use alumina mattes on dinner ware, but I (and most of what
> I've read agrees with this) believe this is really a bad idea. Functional
> ware shouldn't be able to absorb water into the glaze. Maybe it's all right
> to have a purposefully crazed glaze over an impermeable body, but such
> glazes are generally celedons, and quite glossy.
>
> To have the glaze itself (not just the cracks in the glaze) permeable to
> liquid could create some dangerous situations regarding microbial growth,
> leaching of oxides into foods, and overheating in the microwave or
> conventional oven. And even if none of those dire things occur, you still
> have a glaze which will eventually decline in quality with use.
>
> So, that's my very long answer to your very concise question. I think I'll
> post this to Clayart, also, and risk any possible flames if I've said
> anything wrong here. That way we'll both get the benefit of wider experience
> than mine. Hope this is of some help to you. :)
>
> Cindy Strnad
> Earthen Vessels Pottery
> RR 1, Box 51
> Custer, SD 57730
> USA
> earthenv@gwtc.net
> http://www.earthenvesselssd.com
>
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Cindy Strnad on sat 3 mar 01
Hi, Chris.
Here's a nice crystal matte I put together as a kind of synthesis of Ron
Roy's and Tony Hansen's matte glazes.
Crystal Matte Base ^6
EPK KAOLIN 30
WHITING 21
CUSTER FELDSPAR 20
FRIT 3134 15
SILICA 14
If you add 2% Cobalt Oxide, you'll get a nice medium to dark blue. It's
important to cool this glaze slowly, as you would any crystalline glaze, to
allow the crystals time to grow. Otherwise, you'll get many shiny spots.
If you have a computer controlled kiln, this is easy. Just program the
controller to fire down at about 100 degrees F per hour until you get to
around 1500 or so. If not, I'm afraid you'll need either good thick kiln
walls, or you'll need to "baby-sit" the kiln until it gets down to a dull
red glow.
The glaze above is crystalline because of the high calcium content. You can
also get magnesia mattes, but I don't have a recipe for any of these. Maybe
I'll ask for one on Clayart soon. I haven't seen any in my books. The
calcium forms a fine crystalline matrix which mattes the glaze.
The difference between this and the more common alumina mattes is that the
alumina mattes are generally non-shiny because the high content of alumina
prevents the glaze from melting fully at the cone level you fire to. Were
you to fire the glaze higher, it would become glossy. Because these glazes
are immature and low on silica, they're very susceptible to leaching,
marking, and absorbing liquids. If they're also used on a clay body which is
not fully mature, this makes for a very bad situation in functional ware.
It's fine for artistic only, but not good if you're planning to serve iced
tea in it.
Crystalline mattes are fully melted (matured) before they become matte. They
are, therefore, vitrified (glass-like) and better able to withstand wear. If
they're well-formulated, they can hold coloring oxides without leaching them
into your food. The only way to know for sure if any glaze is leaching
oxides is to have it tested in a lab, but a preliminary test would be to
partially fill a piece with vinegar for a couple of days and then see if you
can tell a difference in color above and below the tide line.
Another test for copper would be to fill a drinking vessel with an acidic
beverage (orange juice, for example) and see if you get an off taste. Copper
isn't going to hurt you unless you get a real load of it, btw, so this test
is safe enough so long as you don't do it twice a day.
Lots of people use alumina mattes on dinner ware, but I (and most of what
I've read agrees with this) believe this is really a bad idea. Functional
ware shouldn't be able to absorb water into the glaze. Maybe it's all right
to have a purposefully crazed glaze over an impermeable body, but such
glazes are generally celedons, and quite glossy.
To have the glaze itself (not just the cracks in the glaze) permeable to
liquid could create some dangerous situations regarding microbial growth,
leaching of oxides into foods, and overheating in the microwave or
conventional oven. And even if none of those dire things occur, you still
have a glaze which will eventually decline in quality with use.
So, that's my very long answer to your very concise question. I think I'll
post this to Clayart, also, and risk any possible flames if I've said
anything wrong here. That way we'll both get the benefit of wider experience
than mine. Hope this is of some help to you. :)
Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
USA
earthenv@gwtc.net
http://www.earthenvesselssd.com
Mert & Holly Kilpatrick on sat 3 mar 01
Cindy,
Being a glaze junkie, I added your matte to my pile to try one day. Here
are two mattes I like - someone would really have to test to see if they
suit their purposes. All disclaimers apply!
This one is a matte based on Tony Hansen's 5x20, relying on his commentary
on his website - again a calcium matte, like yours below:
Wollastonite 32.1
Frit 3134 11.3
EPK 40.6
Custer Feldspar 11.3
Flint 4.8
Nice with lots of different colorants. Good to experiment with. ^6 ox. but
very stable over a pretty wide range if your kiln gets a mind of its own.
It is also a glaze I have had outside on tiles over about 3 winters without
crazing or damage (in Penna - zone 6). We hold the kiln about an hour at
about 1650 F on cooling.
This one I think may be called a magnesia matt because the magnesia is at .4
in the unity formula, coming from the dolomite and talc. Again ^6 ox.
Kona F-4 46.4
Flint 12.2
Talc 13.3
Dolomite 13.3
EPK 4.6
Frit 3134 10.2
Again, colorants are up for grabs. I add Zircopax 6 and powdered ilmenite 2
for an offwhite. This glaze interacts well with others. Depending on the
colorants it may not be as matte.
I have not had either tested for food-safeness.
The recent Ceramics Monthly had a tribute to Richard Behrens, and his two CM
handbooks on glazes were the first glaze books I read, and still refer to.
In Ceramic Glazemaking he has a section on high magnesia glazes for Cone 6,
here is one of them, which he says is semi-stony at Cone 6:
Lithium Carb 9.0
Magnesium Carb 10
Whiting 10.3
Kaolin 17.8
Calcined Kaolin 7.6
Flint 45.3
I haven't tried that one. What I really want to find time to do, is to do a
Currie grid with high magnesia in the flux corner. What does John
Hesselberth say, so little time, so much to learn, or something similar. I
am transitioning from my HR Mgr job to database administrator, which my
employer has offered to train me in, and training a new German Shepherd
puppy for the obedience ring, and putting new software in my sister's
veterinary clinic, and wondering why I never get around to doing my Currie
glaze tests....
Holly
Cindy Strnad wrote, in part:
> Here's a nice crystal matte I put together as a kind of synthesis of Ron
> Roy's and Tony Hansen's matte glazes.
>
> Crystal Matte Base ^6
> EPK KAOLIN 30
> WHITING 21
> CUSTER FELDSPAR 20
> FRIT 3134 15
> SILICA 14
>
> If you add 2% Cobalt Oxide, you'll get a nice medium to dark blue. It's
> important to cool this glaze slowly, as you would any crystalline glaze,
to
> allow the crystals time to grow. Otherwise, you'll get many shiny spots.
> The glaze above is crystalline because of the high calcium content. You
can
> also get magnesia mattes, but I don't have a recipe for any of these.
Maybe
> I'll ask for one on Clayart soon. I haven't seen any in my books.
Cindy Strnad on sun 4 mar 01
Hi, Caroline.
It would be nice to be able to change a commercial glossy to a matte glaze.
It's kind of a difficult thing, though, as you don't know the ingredients in
the commercial glaze to begin with.
If you don't need this glaze for functional work, you could try adding
alumina, but you probably don't have that lying about the house. It's really
going to work better for you to just go out and buy a matte glaze.
Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
USA
earthenv@gwtc.net
http://www.earthenvesselssd.com
Richard Aerni on sun 28 mar 04
Just a quick thought to throw out there concerning matte glazes...
Upon reflecting on the posts for roughly twenty seconds, I can come up with
two very distinct types of matte glazes, one I would postulate is food safe
(or more likely to be), the other likely not to be.
They are:
1) Microcrystalline mattes, wherein a glaze is mature, and upon being held
at a temperature (either at peak or when cooling) develops it's matte
surface from crystal development. It would seem that this type of glaze is
more likely to be food safe, unless suffering from devitrification from too
much crystal development.
2) Underfired, or not fully mature glazes, which, not having fully melted
and sintered, are matte, and more likely to leach some of their
ingredients.
And, there may be other types of matte glazes. Perhaps we could engage in
a discussion of this nature, rather than just a blanket condemnation of
matte glazes in general.
Best,
Richard Aerni
Rochester, NY
John Hesselberth on sun 28 mar 04
On Sunday, March 28, 2004, at 11:16 AM, Richard Aerni wrote:
> Upon reflecting on the posts for roughly twenty seconds, I can come up
> with
> two very distinct types of matte glazes, one I would postulate is food
> safe
> (or more likely to be), the other likely not to be.
>
> They are:
> 1) Microcrystalline mattes, wherein a glaze is mature, and upon being
> held
> at a temperature (either at peak or when cooling) develops it's matte
> surface from crystal development. It would seem that this type of
> glaze is
> more likely to be food safe, unless suffering from devitrification
> from too
> much crystal development.
>
> 2) Underfired, or not fully mature glazes, which, not having fully
> melted
> and sintered, are matte, and more likely to leach some of their
Hi Richard,
Just to expand on your thoughts a bit. The matte glazes I have found to
be most stable are those that are alkaline earth mattes--matte because
of high levels of calcium or magnesium (I haven't tested barium mattes)
yet have plenty of silica (over 2.5, preferably over 3) and alumina
(over 0.25) AND are fully melted during firing. Zinc, though not an
alkaline earth, can also yield stable mattes.
The most unstable mattes I have seen are alumina mattes (Si/Al level
<5) where this has been achieved by removing silica. When I see a
dark-colored matte glaze with a silica level <2.0 I reach for my lemon
slices and very frequently can draw the color right out of it in an
hour or two. That's why I am known in some circles as "lemon juice
John".
I have been unable to make a good, stable alumina matte even at higher
Si levels at cone 6 and, frankly, I doubt it can be done at cone 10
either for your second reason--underfiring.
Regards,
John
> ingredients.
http://www.frogpondpottery.com
http://www.masteringglazes.com
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