Craig Martell on thu 3 may 01
Hi Ed:
Kingman spar has been unavailable since the '80s. I've gotten what I hav=
e
by going to sales where a potter is selling out the studio. I don't have=
a
lot but there's enough for a lot of glaze. With regard to blue celadons,
my choice of what's currently available would be g-200 potash spar. It's
higher in potassium than custer. The difficulty is that both custer and
g-200 contain sufficient sodium that you will encounter glaze fit problem=
s
when you bring the potassium up to around .3 moles in the seger
formula. Sodium has a higher coefficient of expansion than
potassium. This extra sodium won't negate the blue but it will most like=
ly
cause the glaze to craze. Another feldspar to be on the lookout for is
Kona A-3. This is a high potash spar too but like Kingman, it's history.
As Mia has already said, macaloid and vee gum t, or vee gum cer are
available at most suppliers.
That's it for me until next week. I'm going to be at the Oregon Potter's
Assoc. Ceramic Showcase show and exhibition at the Oregon Convention Cent=
er
in Portland. Friday thru Sunday, open at 10am daily. You guys should al=
l
get bookings and show up.
later, Craig Martell in Oregon
ed on thu 3 may 01
Which potash feldspar do you recommend? If not a readily available one wh=
ere
can it be acquired? Where is macaloid or vee gum T available? Is it
possible Kingman's is available somewhere as is the case for g. borate??
Ed
-----Original Message-----
From: Craig Martell
To: CLAYART@LSV.CERAMICS.ORG
Date: Wednesday, May 02, 2001 3:01 PM
Subject: Re: Light Blue Celadon
>Hi:
>
>I sent this post the other day and it hit the cyber round file like anot=
her
>one I sent last week. So this is attempt #2.
>........................................................................=
Ka-
snip
>
>Hello Jim:
>
>I've been able to make some light blue celadons and here's what I've fou=
nd
out.
>
>If you are going to make a "true" blue celadon with only iron as the
>colorant, the glaze needs to be very high in potassium. If you look at
>these glazes in the seger formula, the flux column for an ideal blue
>celadon is: .3 potassium, .7 calcium. This is impossible to do with the
>spars we have. All potash spars contain some sodium. The best I've bee=
n
>able to do is .3 potassium, .1 sodium, and .6 calcium. This makes a ver=
y
>nice blue celadon as in, sky blue. The alumina is at about .4 and the
>silica is about 4.5 moles. To make these glazes, you need a potash
>feldspar that is at least 10% potassium. I use Kingman feldspar which h=
as
>12% potassium and this is really and ideal feldspar for blue
>celadons. Kingman is no longer available commercially. I have a good
>supply laid away.
>
>Two other things to look at are the silica/alumina ratio which should be
>around 12/1 for most blues and also barium carb for a more intense but
>still soft sky blue. The barium carb percentage for these glazes is
>usually around 4 to 5%.
>
>Here's a suggested recipe for a sky blue. If you don't want to use bari=
um,
>just omit it. Also, you will get a much better blue with this recipe if
>you use a feldspar that contains 12% potassium in the analysis. Sodium
>really screws up iron blues unless the glaze is a chun blue. Then, soda
>works very well.
>
>potash feldspar 55
>whiting 15
>barium 4
>silica 26
>
>add: macaloid or vee gum T 3%
> black iron oxide 1%
>
>Mix the glazes wet and ball mill for one hour to disperse the iron and
>avoid spotting. If you don't have a mill, screen twice with a 200
>mesh. You can also use yellow iron instead of black and it will dispers=
e
>better. I get better color with black iron
>
>These glazes will always look best on a porcelain body that is very low =
in
>titanium. I use a grolleg body for the blue celadons and it makes a big
>difference. My other porcelain is higher in iron and TiO2.
>
>regards, Craig Martell in Oregon
>
>________________________________________________________________________=
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Wade Blocker on thu 3 may 01
Ed,
Kingman spar has not been available for a long time. Custer seems to
replace it without a problem. Laguna and New Mexico clay both sell
macaloid. Mia in ABQ
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