Floating Blue (also called Blue Hare's Fur) is probably the most well known
and popular cone 6 pottery glaze. It was popularized by the book The Potter's
Complete Book of Clay and Glazes by James Chappell (in the current edition
it is on page 210 under the heading "Floating Glazes"). The author
calls it "spectacular and extraordinary". In fact, the vase that
graces the cover of the book uses this glaze. Like most other popular glossy
GB base glazes, this one also uses nepheline syenite, silica and kaolin. Its
borate content is about middle-of-the-road compared to the range of recipes that
we have studied. Like the Butterscotch glaze, the fired effects produced by this
recipe are a testament to the variegating effects that 4% rutile imparts to
colored non-opacified boron glazes.
The term 'floating' could refer to two possibilities in
our observation: The blue cobalt colored surface appears to float on a translucent
brown glass layer. This layer is visible where the color breaks to brownish hues
on thinner sections at edges and irregularities in the surface. More likely,
however, it refers to a white opalescent 'boron-blue' layer that often appears to float over
the cobalt blue background (boron-blue calcium borate crystals form in borate
glazes when calcium is present and the glaze is very fluid). This effect can
best be amplified by melting a ball of glaze on a tile to get a very thick pool
of glass. Cappell says
"the colors seem to float on a surface of a darker background of great
depth, reminiscent of a deep pool of water".
An amazing thing about this recipe is that it actually has the potential to
produce six separate
mechanisms of variegation:
- Its color and opacity vary greatly with thickness so it highlights irregularities in the
surface.
- 'Phase separation' in the translucent matrix makes the color 'swirl' as
small rivulets of more fluid glass flow around more viscous phases.
- Titanium crystals in the matrix make it sparkle.
- Calcium-borate boron-blue crystals grow on the surface.
- Bubbles of escaping gases create pools of lighter colored glass surrounded
by darker rings.
- Small black speckles are produced by unground particles of iron.
No wonder this is such a popular glaze among potters!
Case Study Using INSIGHT Software
Lesson 6 in the INSIGHT software manual shows you how to substitute GB by calculating a
mix of materials with the same chemistry. You can get the manual here.
You can also watch a video of the lesson 6 here.here.
The principles shown in this lesson apply to substituting Gerstley Borate in any recipe, the challenge is mainly one of chemistry.
Using Boraq
2826R
Floating Blue with GB |
| | Amount | % | 
This book is available
at www.axner.com
|
Anyone who has used this glaze
will testify to the fact that it is "fickle" as Chappell notes. He
makes a number of recommendations on using this recipe: Use distilled or low
mineral water, force all material through an 80 mesh screen, stir thoroughly
before and during use to prevent settling out of the iron content, apply the
thickness of a dime, fire to cone 6 oxidation exactly, and cool normally. One
interesting comment Chappell makes is: "Don't substitute any other
chemicals for those given". Since that is exactly what we plan to do I
might also comment that we have had success at lower temperatures and slower
cooling should enhance the effect.
People who use this glaze employ a variety of methods to increase the
variation of surface color (i.e. stippling a second layer, brushing a wash of
another coloring oxide, double dipping, applying a wash of rutile, etc.).
|
| Nepheline Syenite |
47.90 |
44.5 |
| Gerstley Borate |
27.00 |
25.1 |
| Silica |
20.30 |
18.8 |
| EPK |
5.50 |
5.1 |
| Iron Oxide Red |
2.00 |
1.9 |
| *Cobalt Oxide |
1.00 |
0.9 |
| Rutile |
4.00 |
3.7 |
| | 107.70 | |
|
2826R
Floating Blue with GB |
2826R1
Floating Blue with Boraq 2 |
|
|
|
|
|
As you can see by the glazed tiles, Boraq 2 produces a visual effect that is very close to what GB does
in this recipe. We have done a flow
test to determine an exact
melting comparison (shown here, click to enlarge), again the two are identical in color and fluidity (the melt
flow test demonstrated clearly that this is a very fluid glaze). However the
runoff pool flow test shows that the Boraq version has slightly less boron-blue
clouding (slower cooling should help if this
proves to be a problem for you). We have not found an improvement with Boraq 3
in a flow test, although this was expected since it has more CaO. If you know why, please let us
know? However we have had good results using the Boraq 2 version of the glaze on
pottery, the visual effect is very similar (click on goblet for larger picture). It takes a little practice to learn
how to apply the glaze to get the right thickness. Like the GB version of the
glaze, it does tend to form dimples in the surface if the ware is cooled too
quickly (due to the 20%+ gases of decomposition that come off during firing).
Remember also that you can deal with slight color variations by adjusting the amount
of iron and cobalt.
|
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Why Not Use a Frit?
There has been a lot of discussion about how to remove the Gerstley Borate
from this recipe and replace it with a frit. However there are difficulties with
such an approach.
- Floating Blue depends on the
GB to suspend it so alternate recipes must provide another material like kaolin
in sufficient quantities to do the job or it will
settle in the bucket. Using INSIGHT Ron Roy reworked the floating flue recipe to use a frit
and he was able to address the suspension question. It is:
Nepheline Syenite 30, Frit 3134 36, Silica 14, EPK 20, Red
Iron Oxide 2.0, Cobalt Oxide 1.2, Rutile 4.0. While this glaze has a similar
look, it is missing some of the variegation mechanisms of the real thing.
However with a little more creative work in INSIGHT it could probably be
matched closer.
- Gerstley Borate the main reason for the unique visual appeal of this glaze.
As already noted, people who have tried to substitute frits have found their results to lack the
variegation and the floating white crystals. They also find them to be more color sensitive to firing temperature and
variegate less with thickness differences. This
is a testament to the fact that while glaze materials do have a chemistry that
can be matched by blends of other materials, they also have a mineralogy and
particle size distribution that plays a big part in how they melt and behave in
a melt of other things. A frit has a homogeneous chemistry across all the
particles whereas GB is a mixture of particles of radically different
chemistries.
Does your floating blue run off the ware?
The Boraq version of Floating Blue does go on thicker so you have to dip
pieces quicker. If it is
thicker it will run more. Much of the visual effect of Floating Blue is due to the fluid nature of the melt,
it is 'on the edge' and it does not take much to make it start running. There is another possibility also. Boraq has a silica content that is a
little lower than GB. We did this for good reasons (as explained elsewhere on
this web site).
Try adding 5% silica if the glaze runs.
Floating Red (Oxblood Red)
Gertley Borate 54.88
Talc 14.63
Silica 30.49
RIO 21.95
This is a dark chocolate brown with streaks of red. It is very fluid at cone 6
and tends to bubble if fired too fast between 700 and 900C (1292 to 1692F) or
reduced. More information coming soon.
