"Karl P. Platt (by way of Richard Gralnik on sat 3 oct 98
Here's another post on Titanium blues from Karl Platt. I thought I'd
add it to the one Richard Aerni posted.
Richard
Date: Fri, 29 Apr 1994 13:14:13 EDT
Reply-To: Ceramic Arts Discussion List
Sender: Ceramic Arts Discussion List
From: "Karl P. Platt" <71562.1555@COMPUSERVE.COM>
Subject: Titania in Glazes
X-To: Clayart
To: Multiple recipients of list CLAYART
X-Lines: 58
In response to the Ti "blue" query:
The nature of the color(s) produced by the addition of titania to ceramic
glazes
is severalfold.
First, it should be noted that the solubility of titania in glazes or
glasses is
rather low. What does get dissolved will insinuate itself into the glassy
network. For the most part TiO2 will either never dissolve or will come out of
solution in ceramic glazes, which are usually crowded, fairly fast. In so
doing
it serves to nucleate crystals -- it acts as a "seed" if you will. This lends,
in part, to the milky habit of the glaze. Titania (TiO2) itself in solution
will
produce a blue cast, but only in highly oxidized glazes made from frit in
which
niter figured heavily in the batch and when fired under consistently oxidizing
conditions. This blue is very difficult to stabilize and, in the end, not the
source of the color being sought here.
B2O3 and SiO2 glasses, while they do have differing indices of refraction, are
notably insoluble in one and other. The effect may be likened to attempting to
mix water and glycerine. Placing titania into the mix would serve to encourage
them to separate into their own little pools, if you will. This phenomina is
known as phase separation and it is the bane of any glassmaker making
borosilicate type or phosphate rich glasses/glazes.
TiO2 has the effect of strongly modifying the colors given by the transition
elements in a glaze. In the presence of iron (Fe) TiO2 pushes the Fe3+ into
the
glassy network, where it would rather not be, to exist as FeO4, thus producing
strong brown tones. The Fe-Ti color is commonly used to color pharmacopia (
pill
bottles) to preclude degradation of the contents from UV. Similarly, Ti forces
Cu2+ (CuO) into network forming positions lending to green rather than blue
shades. It does the same thing to Cerium (Ce) lending to yellows.
The purple-blue frequently seen in ceramic glazes can form only in those which
are fairly acid and containing B2O3 (high silica, low alkali and/or with K2O
predominating). The color is caused is caused by the presence of Ti3+,
which is
to say that Ti must be reduced. Whether or not the Ti4+ can be reduced to
Ti3+
depends on its ratio to the basic oxides as a lot to Ti and a sufficient
quantity of B2O3. Adding a small bit (0.1-0.5 wt%) P2O5 is also useful in
developing the color and the phase separation (milky) effects common to this
type of glaze. To retain the color one must be careful to avoid reoxidizing on
cooling.
I'm not sure about the nature of the complaint mentioned in Mr. Wirt's note or
on Clayart, as I haven't followed the thread, but I think it had to do
non-reproducability of the Ti3+ (purple-blue) color. Assuming that the
source of
titania was stable, and it probably was not, and that firing conditions were
substantially similar, it is likely that one of the raw materials changed in a
normal lot-to-lot variation to trip up the best result. The lesson here is to
develop a scheme by which _all_ raw materials coming into the shop are
characterised before coming into general usage. I have, as usual, avoided
giving
any sort of "recipe" approach to a remedy as I haven't become aquainted
with the
specific composition involved here. However, I did see mention to that ghastly
Gerstley Borate, and would encourage getting rid of that soluble crap in favor
of a frit consistent with the nominal composition desired. This will give more
uniform results and prevent the glaze from gelling.
Hoping this is helpful, I am,
Karl P. Platt
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