Bruce Girrell on thu 1 sep 05
A couple of months back I wrote a question to Clayart asking what would be
the best flux for rutile and ilmenite. Thanks to those who responded at that
to to help give me direction. Since then I have conducted some experiments
and I finally have something to report.
Reporting the results necessarily involves pictures, so I have posted them
at Flickr. As I refer to an image, I will provide the appropriate URL for
the photo. The photos were taken with a macro lens to allow small details to
be seen. As a result, the images are large. Those of you with dial-up
connections may want to go to http://www.flickr.com/photos/33323728@N00/ and
choose the smaller sizes of images for viewing. There should be enough
information from context for you to find the proper image to view.
The start of the saga
We had been getting some nice results from a teadust and an ilmenite/rutile
glaze. http://static.flickr.com/28/39396629_dcd890771f_o.jpg shows a
drinking glass with these glazes.
http://static.flickr.com/29/39396628_3722a34a34_o.jpg shows the reverse side
of the same glass. There is actually a third glaze on this glass, another
rutile glaze, around the carved middle area. The clay body is Great Lakes
Clay ovenware and it was fired to cone 10 in reduction. Because I know that
someone will ask, here is the recipe for the ilmenite/rutile glaze:
Custer spar 3280
The glaze is one that Lynne got when she took classes at our local college,
Northwestern Michigan College. The original source of the recipe is unknown,
hence, we call the glaze NMC Ilmenite. Before you run off and mix up a
batch, though, you had better read the rest of this.
The ilmenite/rutile glaze provided a very nice variegated blue. Here is a
closeup of it: http://static.flickr.com/23/39396627_730c1bad75_o.jpg Just
for the sake of completeness, here's a closeup of the teadust glaze where
there are a lot of the green crystals:
But the glaze bucket was getting low. Time to mix some more. And, of course,
the new glaze looked nothing like the one that was doing so well for us.
Here's one of the test tile from the new batch of glaze:
http://static.flickr.com/24/39387070_7927502062_o.jpg Note the little black
specks everywhere, especially near the upper part of the tile where the
glaze is thicker. Most of the ilmenite and rutile granule look exactly as
they did in the unfired glaze. Aside from a smattering of tan flecks, they
have provided almost no coloring to the glaze at all, and certainly not any
This was the point at which I wrote to Clayart asking for help. The
responses indicated that there were two directions to go: 1) add another
flux, such as boron, to improve the melt and 2) adjust the silica/alumina
To test the added flux method, I used some of the glaze that I already had
mixed and calculated the amount of gerstley borate necessary to provide 1%,
2%, 5%, 10% and 20% GB tests. That was the easy part. To evaluate a modified
silica/alumina relationship was a different story. For this, I decided to
make an Ian Currie grid test. If you are not familiar with Currie's method
please see http://ian.currie.to/original/index.html.
There are a lot of details in this section that may not be of much interest
to some, but which are essential for anyone who is interested in
understanding the results. If this part does not interest you, skip to the
Results section, where you get to see more pretty pictures.
The unity formula of the original glaze is
CaO 0.594 Al2O3 0.282 SiO2 3.437 Si/Al = 12.206
Using the calculations from Currie's website, I made Corner C with Custer
205.0, Dolomite 60.0 and Whiting 35 (plus Rutile 15 and ilmenite 9), which
gives a unity formula of
CaO 0.522 Al2O3 0.263 SiO2 1.790 Si/Al = 6.818
I'm not sure where the discrepancies in the flux values come from or if
they're important. I accepted the values for Corner C as given by Currie's
calculations page. The rutile and ilmenite add iron oxide and titanium
oxide. In the Currie grid method, the same fluxes are used for all glaze
combinations. As a result, the CaO, MgO, K2O and Na2O do not change across
the entire tile. Likewise, the amount of rutile and ilmenite are constant
for all glaze combinations. I will report only the Al2O3 and SiO2 numbers
from here on out.
Corner A (custer 123.0 dolomite 36, whiting 21, kaolin 120, rutile 15,
ilmenite 9) gives Al2O3 0.805, SiO2 2.905, Si/Al 3.610. And, yes, I did use
kaolin. Some time had elapsed and I forgot that the original recipe used
OM4. When I mixed the corner glazes I used EPK instead of OM4. As we will
see, I got the results that I needed with the EPK, so there is no need to go
back and re-do the test with OM4. Corner B (custer 71.75, dolomite 21,
whiting 12.25, kaolin 75, silica 120, rutile 15, ilmenite 9) gave Al203
0.846, SiO2 7.220, Si/Al 8.532. And Corner D (custer 102.5 dolomite 30.0,
whiting 17.5, silica 150, rutile 15, ilmenite 9) gave Al2O3 0.265, SiO2
5.558, Si/Al 20.943.
I mixed the 35 glazes and applied them to a tile made of our ovenware clay
body and one made from the porcelain body that we use. Because there was
some question as to whether the original formula had used powdered or
granular rutile, I mixed a second set of glazes to test that variable as
well - a total of 140 different combinations
The previous tests that I had done had been a major disappointment - 50
tiles and not one of them had given a hint as to the proper recipe or even a
direction to go. This time it was different. The gerstley borate test
demonstrated that adding boron was not the way to go.
http://static.flickr.com/27/39387069_597148e972_o.jpg shows the 10% gerstley
borate tile. The boron has increased the melt of the glaze to the point that
the thick glaze application at the top of the tile has run down the full
face of the tile. Still, the rutile and ilmenite appear as dark specks.
The Currie grid tiles, though produced the desired results. Each grid
provided two possibilities as replacements for our original recipe, as well
as two more "surprise" glazes that we'll have to try out later on.
http://static.flickr.com/29/39390810_e27436e201_o.jpg shows a typical grid
tile. The eutectic trough extended along an axis from glaze 4 to glaze
21.The other grids produced similar results, though it was clear that
powdered rutile was the way to go.
http://static.flickr.com/29/39387067_4a01eefbec_o.jpg shows the position of
the 35 glazes on an alumina/silica plot.The numbers of certain glazes have
been placed on the plot for easy reference. The shaded rectangle on the
alumina/silica plot represents the limits adopted by the Matrix glaze
The most promising replacement glaze appears to be glaze 17 (Al2O3 0.474,
SiO2 2.821, Si/Al 5.952).
http://static.flickr.com/28/39383944_980e40d4dd_o.jpg shows glaze 17 on
ovenware and http://static.flickr.com/25/39383943_b3fe09e503_o.jpg shows it
on porcelain. Glaze 17 was the only glaze that came close to the desired
appearance when granular rutile was used
(http://static.flickr.com/24/39383942_7f70a6feaf_o.jpg ovenware with
granular rutile and http://static.flickr.com/23/39382285_9f46f51bcc_o.jpg
porcelain with granular rutile).
Glaze 13 (Al2O3 0.577, SiO2 3.863, Si/Al 6.697) also showed a lot promise,
though it is darker than the target glaze color
(http://static.flickr.com/28/35659524_d19922ea01_o.jpg on ovenware,
http://static.flickr.com/22/35659523_2b921fea99_o.jpg on porcelain). It is a
very nice blue, though, and it retains the blue color over a wide range of
Two glazes showed promise as new glaze recipes. Glaze 4 (Al2O3 0.831, SiO2
5.653, Si/Al 6.800 - http://static.flickr.com/33/35659522_ea213bb1dc_o.jpg
on ovenware and http://static.flickr.com/25/35659521_2065d4eaa9_o.jpg on
porcelain) appears as a deep maroon, almost black, with orangey-brown rutile
crystals throughout. The base color of this glaze was surprising, though it
may be an indication of more of the ilmenite entering the melt.
Glaze 26 was the nicest surprise (Al2O3 0.321, SiO2 1.910, Si/Al
5.948 -http://static.flickr.com/25/35659520_61311e7879_o.jpg on ovenware and
http://static.flickr.com/23/35659519_e6f0a6d094_o.jpg on porcelain). It has
a dark blue background and is a waxy yellowish white where thick, with
abundant crystalline inclusions. This is probably not a stable glaze. Note
that because it is on the left column of the grid, this glaze contains no
silica in the recipe. All of the silica comes from the feldspar and
EPK.Still, as a decorative glaze, it appears to have a lot of potential.
I'm sure that only the most ardent glaze afficianados have made it this far.
Thanks to those who took the time to read this.
We were able to find a likely replacement for a favorite glaze using the
Currie grid method. The glazes that produced the best blues fell in a
diagonal on the alumina/silica plot in a region very close to, but slightly
to the northwest of, the region shown in John Britt's book. The two most
promising glazes fell within accepted limits.
I was surprised to see that boron made no difference at all in this glaze,
other to lower the melting point of the glaze. Clearly, the key was lowering
the silica, which was obviously very high in the original formulation. Glaze
33 was the closest match to the original formulation and, indeed, it showed
only light spotting, with most of the rutile and ilmenite remaining as black
The original formulation of the NMC ilmenite glaze was supposed to produce a
white glaze with gold flecks. Several of the very high silica glazes did
show this character, especially when the granular rutile was used. Why this
glaze started making blue is a mystery. I am guessing at this point that I
made a mistake when I mixed the batch of glaze, possibly leaving out one
weighing of silica. Whatever the reason, it was a happy accident that
eventually led to this study. When you get tiles like these out of the kiln
it becomes easy to forget about the hours of weighing, mixing, and
everything else that went into making them. It always seems like there's at
least one very pleasant surprise on a Currie grid.
If anyone has comments on this experiment or would care to venture any
guesses as to how much difference the substitution of EPK for OM4 may have
made, please do.
Bruce "teacher, my lab report is done now" Girrell
 Britt, John, The Complete Guide to High-Fire Glazes, p. 59
 ibid, p. 107