Tom Buck on mon 26 apr 99
VR:
It is difficult to speak and be sure with Iron Oxide Red ...there
are several makers of synthetic RIO, each with different purity and
unwanted contaminants, and at least one good natural producer, the one in
Spain (via bacterial leaching of ore). Spanish RIO is 99+% pure and is
consistent bag to bag. Perhaps this is the "Special" you obtained
previously.
Your favourite glaze (Randy's Red, yes?) is an iron oxide
saturated one, ie, RIO crystals both dissolve in the glaze melt (5% max),
yielding the brown colour, and also when in surplus disperse throughout
the melt and be on the surface as the glaze freezes, yielding the Iron
Oxide Red colour. With such a glaze the original particle size will be
important, as the RIO crystals themselves, if not fluxed, do not melt
until the temperature goes above 1500 oC. This surface effect is subject
to vagaries in glaze placement and firing conditions, in variations in the
other glaze ingredients. EG, RRed shows well on a horizontal surface
(plate, etc.) but less so on a vertical surface.
What to do? Perhaps firstly check the particle of the new RIO; my
guess is that 200 mesh probably would work best, 100-200 would like serve
but anything on a 80 mesh screen should be discarded (or milled finer).
And then, if you now get brown exclusively, there is not enough RIO being
added to provide the surplus which creates the RR effect. So,after
screening, use perhap 20% more of the new RIO.
And next time you buy RIO, ask for Spanish Red; it would be right
for your needs, with the appropriate particle size and purity.
bfn. Tom.
Tom Buck )
tel: 905-389-2339 (westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street,
Hamilton ON L8T 3E1 Canada
Lois Ruben Aronow on tue 27 apr 99
------------------
On Mon, 26 Apr 1999 09:45:08 EDT, Tom Buck wrote:
=3E And next time you buy RIO, ask for Spanish Red=3B it would be =
right
=3Efor your needs, with the appropriate particle size and purity.
=3E bfn. Tom.
=3E
I was under the impression that the Spanish red is for clay bodies,
and the Special red is for glaze. Is this not the case?
Tom Buck on wed 28 apr 99
Lois RA:
Why should Spanish Red be limited to claybodies? Does it contain
something that would interfere with its use as a glaze ingredient? Or is
it a question of cost? Or??? Perhaps one of the geo-specialists knows.
Chemically, Spanish Red is Fe2O3 of approximately 90+ wt% purity
(I cannot find the precise level in my reference books).
In my chem dictionary there is this listing for iron oxide red:
"Iron oxide red (burnt sienna, caput mortuum, chemical red, crocus
martis, Indian red, jeweler's rouge, Mars red, metallic red, mineral red,
natural red oxide, Persian Guld oxide, purple oxide, raddle, red bole, red
earth, red iron oxide, red ocher, red oxide, Spanish oxide, Turkey red).
Pigments composed mainly of Iron(III) Oxide, Fe2O3."
Others have said Randy's Red is troublesome, but elegant when it
works. I have seen a small platter with RR on it that showed beautifully.
The same potter had another one, fired in the same kiln load with the
same RR on it, that failed badly. Why? Perhaps because there was a
difference in glaze coat thickness, or equally likely a difference of a
cone or two in heat work, C4 vs C6 or C8 vs C6.
RR is tricky because it is a dispersion colour, not a solution
colour (which most are). The red pigment behaves somewhat like Zircon (or
Tin oxide) and these opacifiers work best when the particle sizes are
sub-micron (ie, less than one-millionth of a metre in diametre or
smaller than one thousandth of a milli-metre). Will this be so for RR? My
guess is yes, but I have no experimental proof. Perhaps a curious
Clayarter will obtain some Iron Oxide Red, screen it to different size
ranges, then test these ranges in separate RR mixes. And let us all know
what happened.
Tom Buck ) tel: 905-389-2339
(westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street,
Hamilton ON L8T 3E1 Canada
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