Bill Edwards on wed 19 mar 03
Hi,
Its fun reading all the pro's and con's of glaze
chemistry but often many people rely on others to
formulate while learning some basic principles can
help most beginners gain a working knowledge that can
produce viable and stable glazes.
There are 3 categories of concern. Flux, Stabilizers
and the glass formers. I provided recently a copy of
my RED made from chromium oxide and Tin. As I stated
earlier I would be happy to help those interested in
working this glaze so as to fit materials that exist
or used by you. I would need a chemical analysis if I
calculate or use your chemical as a replacement. I
mentioned a good while back about the effects of AL203
being shifted could create chrome tin pinking. This is
also established in Hamners. A chrome tin glaze then
would seem to work best with the lowest possible AL203
or stablizer. but if you fire in a with a kiln loaded
with tin white and the calcium is present you can
expect to see some pinking based on how high or low
the AL203 is in the other glazes. If you are used to
doing line grids you can see this change take place by
keeping the calica about the same while going up or
down on Al203. But then we can take the next step and
see that they work even better with silica that is
almost to the point of running based on the flux
oxides available. Silica alone would be stiff and
matte but once fluxed it can become very fluid. You
can use other flux oxides other than Neph Syen. There
is sometimes a eutectic point in the glaze chemistry
that may not show in simple calculations. Again, the
grid method is worth mentioning when you want to move
your paper work through the quickest visual way of
finding out. Increasing silica, providing calcium and
having just enough AL203 to stiffin the glaze where a
good melt is achieved is the balance we are looking
for. Calcium/sodium can be considered part of the flux
oxides but we also need to include the K2O that is
involved via Neph. Syen, so we have the potential for
a glossy red. A base that usually isn't clear or
approaching clear without coloring oxides is leaning
towards an AL203 matting effect or SiO2 matting
effect. Also slow cooling as mentioned in other
reading where crystal growth occurs can be mentioned.
Fire your chrome Tin reds without slow ramps for best
results. Also full oxidation. These reds will form at
higher/lower than ^6 temperatures also. Components
from many chemicals in the mix such as magnesia and
excesses can produce these buttery looking glazes, I
believe you have seen dolomite mattes and Alumina
matts. Many are stable even with ample additions of
soda spars based on the lab work I have recieved and
had done over the years. But because of solubles of
sodium verses potash being discussed at length, no
need in covering that here. High potash glazes are not
the answer either in some cases due to crazing issues.
Color verses function comes into play with safety
being key in all of it if and where used on food
service wares.
How to adjust your Chrome Tin red?
(1) - Neph Syen - 25.00 (Flux - *Na2O/K2O)*Primary
(2) - Wollastonite - 25.00 (Calcium, SiO2)
(3) - Ferro Frit 3134 - 15.00 (B203, Al203, SiO2)
(4) - EPK - 15.00 (Stabilzer)
(5) - Silica - 20.00 (glass former)
Tin Oxide - 5.00
Chromium Oxide - 0.15 to 0.30% I use about 0.30
myself.
I believe that 0.15 is a tad too little to fully get
the brightness expected. Its diluted and also this
glaze does better after setting up for a week or so. I
would compare this alot to turquoise glazes using
copper in regards to certain chemical balances to
obtain the best color with fewest flaws in the fired
piece. I have a soda based copper/tin that can be a
beauty when mixed correctly and fired correctly. Low
in copper. Stable too!!!
The replies I have seen regarding many chrome tin
glaze were they either run, they were pinkish with
white, they pool red. These are all good examples of
getting the chemistry as close as possible to
stablilize against pooling, running or chemistry too
low in calcium or too high in Al203 for good reds.
Another note is to remember these colors usually look
much redder on white or light bearing clay. Clays with
iron content in some cases can help give the illusion
of a deeper red but generally not brighter. Go for
light or white clay or porcelain for the brightest
effects. Calcium, boron and Soda content as a flux
with SiO2 and low AL203 can also lead to crazing on
some clays given the expansion of the above glaze is a
little high. The above recipe has yet to craze that I
can see but I haven't done anything extraordinary at
this point since I have pieces several years old that
have not crazed yet. Trivalent chromium is the kind
that occurs naturally in the earth. It is insoluble
except with specific strong acids. (Notation at the
end of this page).
The above glaze did get the lemon test and acidic
vinegar set for several days. NO visable change! But
to answer what it takes for making the best Chrome Tin
red is to have a starting point, a good light clay,
Stick close to selection of materials or mols if
exchanging one material for another. High calcia but
the magic number seems to be 17-18% Mol's % or 15% by
weight. I try to keep boron under 4.00% weight or
less. The above base has more AL203 than many I have
tested but you may lower it some. It would depend on
your needs and which clay you would use. Raising it
may result in loss of red and get you back to more
streaks of white where the red pools in indices. We're
also not looking for a food base glaze just yet. We
are looking to achieve the best possible red and
without or as few flaws as possible. Once this is
settled then FDA/FSIS testing could continue. The
potential of losing more chrome than allowable from
this glaze would be at a minimum if I hazard a guess.
Regardless of LOI. Chromium that we use in glaze
chemistry is not the toxic brew some have made it out
to be. NO more so than cobalt. (I await the usual
replies to that) Enjoy the process of discovery. Do
some grid tests here and see how you can manipulate
materials to your advantage. A good glaze calculations
system I have used most often is called MATRIX. There
are other software programs available as well. Have
lots of fun and lets see some pictures of this. I
will, as stated before, send anyone a picture of this
glaze upon request.
Chrome - Chromium compounds vary greatly in their
toxic and carcinogenic effects. Trivalent chromium
compounds are considerably less toxic than the
hexavalent compounds and are neither irritating nor
corrosive. There is inadequate evidence for
carcinogenicity of chromium oxide, and chromium
acetate. There is sufficient evidence for
carcinogenicity of barium chromate, calcium chromate,
chromium trioxide, lead chromate, sodium dichromate,
and strontium chromate. These compounds have not been
evaluated for their carcinogenicity: chromium
carbonate, chromium phosphate, cobalt chromium alloy,
lead chromate oxide, potassium chromate, potassium
dichromate, sodium chromate, and zinc chromate.
I would advise anyone to consult someone like Ed
Basterache for advice on chemical toxicology since
that is what he does for a living. I am simply looking
to achieve the best possible glaze be it for food
service or simply for decoration. I want pretty, I
want it to work, Simplicity, affordable, and for food
service I must have safety being my number one
priority. For pretty, I am open minded enough to know
that some things are not going to be mastered at ^6
oxidation that will remain stable. But I wouldn't want
to use barium, lithium or cadmium and lead even for
that.
Bill Edwards
=====
http://www.tallapoosariverpottery.com/
Bill Edwards
PO Box 267
Lafayette, AL, 36862
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