Craig Martell on mon 28 sep 98
Hello Again:
I've been reading the info on spars and also the Newbie question controversy
that seems to come up on clayart now and again.
I've been making and studying glazes for about 25yrs now. Had a lotta
eyestrain from reading and whatever else comes from doing all this. I still
like it and I don't mind answering elementary questions if I have time. It
sometimes forces me to review some stuff that I need to look at again.
Refreshing ones memory is never a bad thing I think and I still have an
awful lot to learn. I too may need to ask a simple question on of these
days.
Feldspars occur in glaze receipes probably more than any other ingredient.
The reason is, Spars satisfy all the categories found in the Seger Formula
which describes the molecular ratio of a fired glaze. Putting this another
way, Spars provide flux, amphoterics(alumina), and silica to glazes. This
is basically all you need. OK, you need a kiln to fire the stuff in! You
could glaze a pot with Custer Feldspar and it would be pretty durable, but
it would craze from an abundance of potash(potassium). With some additions
of calcium, clay, and possibly some silica to restore balance you'll have a
good functional glaze. There are other oxides you can add, depending on the
type of glaze you are seeking. If you wanted a high fire tenmoku or
celedon, you could make these with potash spar, whiting, kaolin or ball
clay, and silica. Feldspar would be the major percentage in the receipe.
Very useful stuff to potters and glazemakers. I still have two unopened
bags of the old Kingman Potash Spar. Lucky me!
gotta crack my copy of Ceramic Glazes, worlds best cure for insomnia, Craig
Martell-Oregon
Michael McDowell on wed 30 sep 98
Craig Martel wrote a nice explanatory note to the list on why Feldspars are
such a key ingredient in glazes. Craig, I would just also mention that
feldspars are among the few relatively insoluble sources of potash, soda,
and lithia, and that is another reason why they are included in most glaze
recipes.
As an aside, you mention that you have saved a couple of unopened bags of
the old Kingman Feldspar which used to be a real standby on the up until the
late seventies when the mine shut down. I just hope the bags you have saved
are "old enough". Before they shut down the mine, they bagged up and
distributed quite a bit of feldspar that did not perform at all like the
Kingman we all used and loved. Custer is a pretty good substitute for
Kingman, and G-200 for either of those in most glazes, but I will admit that
there are some "special effect" glazes that I've never been able to
recapture. Still, that was twenty years ago, and I'm not particularly
interested in recapturing what I was doing then. I would just caution anyone
who was into finding or using amounts of Kingman Feldspar from long ago that
just because it's in a Kingman Feldspar bag doesn't mean that it is of the
quality that made Kingman a sought after spar. There must be quite a bit of
what still remains from Kingman that came from the mine after the vein had
gone sour.
Michael McDowell
mmpots@memes.com
http://www2.memes.com/mmpots
Craig Martell on thu 1 oct 98
>Craig Martel wrote a nice explanatory note to the list on why >Feldspars
are
>such a key ingredient in glazes. Craig, I would just also mention that
>feldspars are among the few relatively insoluble sources of potash, soda,
and lithia.
Hi:
Absolutely Michael and thanks for pointing that out. I hadn't really
thought about that, but it is very true and important.
>As an aside, you mention that you have saved a couple of unopened bags of
>the old Kingman Feldspar which used to be a real standby on the up until
the
>late seventies when the mine shut down. I just hope the bags you have saved
>are "old enough". Before they shut down the mine, they bagged up and
>distributed quite a bit of feldspar that did not perform at all like the
>Kingman we all used and loved.
Yes, in the latter days of the Kingman mine, the spar they were mining was
lower in potassium and higher in a "dirty" appearing silica. I always do
fusions of spars and compare them with what I've been using. These bags of
Kingman were purchased in the mid seventies and all that I've used and
tested have produced nice white, pearly fusion buttons. The bad variety of
Kingman makes an opaque, grey button that doesn't fuse very well at cone 10.
regards, Craig Martell in Oregon
Michael McDowell on fri 2 oct 98
Craig,
I'm not surprised to hear that you didn't need my caution regarding old bags
of Kingman Feldspar possibly not being true to it's typical analysis. Your
practice of always making fusion buttons to test new batches of feldspar is
yet another example of your thorough craftsmanship which shows so clearly in
your finished work. I'm curious now, whether you make a practice of doing
similar tests on other types of materials. Is it only feldspars that you
take these measures with? Do you then make a practice of purchasing fairly
large amounts of feldspar relative to your needs? And maybe there are some
newbies out there who would benefit by an explanation of what you mean by a
"fusion button" anyway.
Michael McDowell
Whatcom County, WA USA
mmpots@memes.com
http://www2.memes.com/mmpots
Craig Martell on sat 3 oct 98
Hello Again:
Michael McDowell asked if I tested glaze ingredients other than feldspars
and also if maybe I should post something about what fusion buttons are.
I've sent a description to Clayart before regarding these funky little
buttons but I'm glad to do it again. There is a nice photo of some spar
fusion buttons in the Ceramic Spectrum by Robin Hopper.
Since I look at feldspars as the "foundation" on which high fired glazes are
made, I mainly test spars. However, one can certainly test other materials
as well, and I do test other minerals when necessary. I might caution you
guys not to attempt fusion buttons of Lithium carb, Gerstley Borate, or Soda
Ash at cone 10 for obvious reasons. They are REAL runny. You can do spar
fusions and not ruin your shelves because spars contain sufficient alumina
to keep them from heading for the floor of your kiln at an alarming rate.
When testing other minerals, I usually blend them with spars at the normal
percentage and compare with earlier tests to detemine if there has been any
change for the worst, or for the best. Sometime materials actually improve,
instead of causing problems. Whiting (calcium carb.) for example doesn't
melt at cone 10 so you can't really see how it's going to act when fused.
If you do a line blend with a spar, or every spar you can get your hands on,
then you can see how the whiting will affect a feldspathic oriented glaze
and when you determine the optimum melt and effect, you can keep this as a
reference to compare in future tests. I also will test wood ash with spars
to try and keep my ash glazes fairly consisitent. As you guys might have
guessed from my e-mail address, I like to work with woodash. When I'm using
a new source of wood ash, I usually mix glaze tests a couple of firings
ahead so that I can adjust without experiencing too much panic.
In regard to testing materials for glazes, I can't recommend Ian Currie's
method enough. It's the best method I've come across for thoroughly testing
ALL the possibilities. Ian has really done a wonderful job of putting this
together for everyone interested. It's all in his book. Stoneware Glazes A
Systematic Approach. He says that he's going to write another as well.
To make fusion buttons, I use a one half inch porcelain lab crucible. The
designating number assigned to these little pots by the manufacturer, Coors,
is 000. That's easy enough to remember, not like my computer support ID
number which is absolutely insane!! You mix your tests dry, and pack them
tightly into the crucibles and tap the tests out onto a tile that you've
marked so you know what the hell you're looking at later. Fire the tests to
the desired cone and check 'em out. The reason that I use a crucible,
instead of just dumping the mix onto a tile is that I always start with a
known shape and size and this gives me a good reference for comparison in
the future. Some spars don't deviate much from the original crucible shape
and others that fuse more do. Custer and Kingman don't distort much but
g-200, Kona F-4 and Nepheline Syenite(not a true spar) will fuse into a
softer shape, which demonstrates that they are fusing more than custer and
kingman at the target temp.
I buy my crucibles at Nurnberg Scientific in Portland Oregon for $1.25 ea.
You won't need financing to buy three or four of these things. Also, this
isn't something I developed, I was taught to do this kind of test by that
intrepid seeker of knowledge on the molecular level, Jim Robinson of Phoenix
Oregon, who is threatening to go on line this fall when he breaks down and
buys a faster computer. Can't wait!
regards, Craig Martell in Oregon
June Perry on sun 4 oct 98
I also use the small porcelain crucible to do fusion buttons, but a sewing
thimble with a smooth interior should work just fine.
I recently had my students make elbow pots and we put about a level teaspoon
of every material, but clays, in the pots, so they could see the various melts
and refractory qualities of the materials that go into the glazes.
To make elbow pots I just have them roll out clay about 1/4" thick and cut out
circles with a biscuit cutter. Then holding the clay pattie in the palm of
your cupped hand, hit it lightly against your bent elbow a few times. This
will create a nice little shallow dish to hold your raw material. We put holes
just below the rim in case we want to string them on wire and hang for easy
storage, later. Of course you can't hang the more refractory materials, but
you can glue these to a piece of plywood or other board and store flat. I
also have them print the material name on the outside of the pot and just
below the rim.
The first time I made these, years ago, I made them too thin and the soda ash
ate right through the clay, and I had printed the names on the bottoms of the
elbow pots and of course we lost the names on some of those sets.
I like Craigs idea of mixing some flux with the more refractory materials.
Thanks for the idea for another class, Craig!
We did something similar years ago in a glaze course with my late friend
Vivika Heino. She had us use Albany Slip 90, and 10% of any flux, frit, etc.
we could find and we fired large flat tiles with these. Those tiles are long
gone and so is Albany, other than the few pounds I have left and the students
aren't getting that. :-)
Regards,
June
| |
|
