Dave Finkelnburg on thu 15 jan 04
Wayne,
None of us knows it all...darn it! Good for you for doing so much
research!
If you have good iron oxide, whichever color it is, you can substitute
it about one for one with any other iron oxide in your glaze recipes. All
are made primarily as pigments, for paint or ink but you don't care about
the raw color because you destroy that when you fire.
In THEORY:
Red iron oxide, RIO, is 98% Fe2O3, roughly 69% iron.
Black Iron Oxide, BIO, is 98% Fe3O4, roughly 71% iron.
Yellow Iron Oxide, YIO, is Fe2O3.H2O, roughly 63% iron.
I emphasize these figures are theoretical because potters have found
wide variation in the quality of iron available from suppliers. You really
need to ask for an analysis of the material you are buying. If the seller
has no analysis, I would shop somewhere else. Some Red Iron Oxide on the
market is as low as 50% Fe2O3. :-( You need to know that, and also what
the other 50% is!
Daniel Rhodes, in "Clay and Glazes for the Potter," mentions that RIO is
finer in particle size then BIO. Thus RIO is easier to disperse in a glaze,
and also to keep in suspension, but the coarser bits still settle like a
rock!
Regarding kaolins, they vary mostly in the level of iron and titanium
contaminating them. If you want a very white kaolin, you want something
like Tile 6 (low in iron). If you want very low titanium, you want Grolleg.
EPK is most commonly used in the US because it is affordable and commonly
available and it works. It is not necessarily the best for any particular
glaze. Only you can judge that. In general, though, kaolins are all pretty
much alike chemically, especially in alumina and silica content, whereas
ball clays are much more variable in composition.
Good glazing!
Dave Finkelnburg
----- Original Message -----
From: "wayneinkeywest"
Sent: Wednesday, January 14, 2004 3:41 PM
I've been rummaging through the Archives...
Some of the recipes I would like to try
refer to "iron oxide" Ok, simple enough.
Sometimes RED iron oxide, sometimes
BLACK, sometimes YELLOW.
In a recipe calling for simply "iron oxide"
do I (willy-nilly) select a color based on
what would be closest to the color of the
glaze I'm trying for? Did the colors
come about recently, and there was only
ONE available previously? For all my
(lack of) experience with glaze, I do
recall that oxide of iron is usually BROWN
(it's called rust in other trades.)
Second question:
Same deal with Kaolins. GlazeMaster lists
a plethora of Kaolins; some recipes specify
only "kaolin".
Use whatever is hanging around? Go for a specific
type as is used in most other recipes (EPK)?
Match a kaolin to my particular clay body
(in my case Grolleg)?
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