Working with Plainsman Clays
(https://plainsmanclays.com) and others and we
created a successor to GB under the trade name BORAQ. It was used for a number of years until Laguna began processing another GB stockpile they found at the quarry.
- Its Advantages
Boraq targetted a compound of materials that was chemically, mineralogically, and physically very similar to what Gerstley Borate was at its very
best. However a 'one-mix-fits-all' solution did not emerge, the chemistry-mineralogy-physics
interplay is too complex
- The Boraq 1 Recipe (for low fire)
Boraq 1 employs Turkish Ulexite,
Cadycal, Soda feldspar and Hectorite (get the recipe by clicking above). Boraq 1 compromised chemistry (it was a little
lower in CaO, MgO and SiO2 and higher in B2O3 than GB) to make it physically melt more like GB at lower temperatures.
- Boraq 2 (for middle and high fire)
Boraq 2 was recommended for cone six and above. Boraq 2 was not sold, people had to make it by using this recipe:
84 Boraq 1
For example, if a recipe contained 50% GB, they used 50x0.84=42 of Boraq 1, 50x0.08=4 of dolomite
and 50x0.08=4 for whiting. The Boraq 2 chemistry matches GB formula more closely.
- Boraq 3 (where high CaO is needed)
Boraq 3 was recommended for cone six and above where high CaO was needed (e.g. for chrome tin reds and pinks and for glazes that
rely a lot on the boron-blue effect, that is, a lot of calcium borate crystalization). Good examples of
this type of glaze are transparents that have a lot of bluish-white cloulding that opacifies them. It was 84 Boraq 1 and 16 whiting.
- Boraq 1 Physical Properties Data Sheet
Click here to
visit the data sheet.
- Boraq is more than Gerstley Borate was
Boraq (like GB) makes it possible to produce very interesting
glaze effects that are impossible with frits. Some glaze surfaces at
cone 5-6, for example, have crystalline, phase, color with thickness,
rivulet, and speckle variations all in the same surface! This effect
is a product of the 'alphabet soup' of mineral particles in raw glazes
using Boraq (fritted glazes are just ground homogeneous glass)
combined with the aggressive melting behavior of boron. Boraq melts so
well that you can make glazes at middle and low fire that excel in
visual character, even more than high fire glazes.
- The Importance of Chemistry
Although many of the substitutes will produce a glaze of similar
character and surface, many will present problems when you add your
coloring oxides. In glazes color is a product of chemistry and if a
substitute does not have the same chemistry then many colors may look
Comparing Boraq and Gerstley Borate
- As noted above, we formulated the Boraq 1 as a 'base recipe'. As is it
melts like GB at cone 06. The Boraq 2 variation (its chemistry is much more
similar to GB) is recommended for cone 2 and above (see recipe above).
- Boraq was designed to have a lower hectorite clay mineral content so that
over-gelled GB glazes would work better (hectorite or bentonite can be added
if better glaze suspension or hardening is needed). Boraq thus creates a
gels less (it still gels enough to prevent dripping). This gives it better flow and thus better application
properties. After an overnight sit
the Boraq containing glaze will settle leaving a layer of water at the top.
However high Gerstley Borate glazes usually gel completely and it is
difficult to get them moving again without the addition of more water.
Because Boraq glazes gel less and dry faster after lay-down on the ware they may
not paint on as well. You can either add some bentonite or hectorite, perhaps
2-4%, or even better, emply a CMC Gum solution. Commercial glazes use this to
improve brushing properties. It is basically a glue, it hardens the glaze and
also slows down drying (which is great for painting). Add CMC powder to boiling
water to form a gel then use this to replace part of the water when
mixing the glaze. Experience makes it evident how much to
- Boraq glazes build up a layer on the ware faster even if they contain more
water, thus you do not have to
hold them in the glaze as long. Thus it is easier to get Boraq glazes on
too thick. The glaze layer should dry
faster (although still slower than typical fritted glazes). Again, the lower
hectorite content in Boraq recipe is responsible for this behavior.
- Boraq glazes still bubble and blister if applied thickly and fired quickly
(especially if they employ a high content of colorant that gases during
- High Boraq cone 6 glazes have the same tendency to run during melting as Gerstley Borate
- Boraq has the same tendency to produce variegated surface effects and
responds to small amounts of lithium to heighten the effect. Infact it
appears to be an even better variegator.
- Boraq 2 is needed to produce the intensity of red in iron red glazes.
- Boraq 2 glazes may run more even if they are applied at the same
thickness, try adding a few percent silica if this happens (since Boraq 2 is
lower in SiO2 than GB).
- Boraq 1 has a higher formula weight than GB so less may be needed in your glazes. However it does not fuse as early so a little more may be needed to
melt. Thus it appears the it is a one-for-one substitute for low fire, but
be prepared to increase or decrease as needed.
- Boraq 1 does not melt into a transparent glass below cone 06 as Gerstley
Borate, however it 'catches up' and fuses most glazes from cone 06 and up
the same as GB.
- In high lithium glazes lithium borate and calcite crystals tend to form in the slurry using
Boraq (lithium carbonate and soluble portions of calcium borate exchange
ions to form the lithium borate and calcium carbonate). This tended to
happen with GB also but not as much as Boraq because the colemanite (calcium
borate) that it contained was less soluble than the Cadycal (calcium borate)
we use. These cannot be removed, of course, without changing the chemistry
of the glaze. However they can be ball milled back into the glaze. However be cautious of the food safety of these, some contain up to
30% lithium carbonate whereas this material is normally used in amounts less
- Boraq based glazes may have a less dense lay-down, especially if applied
by spraying (the wetter, more plastic gerstley borate lays down in a denser
layer). The resultant fired glaze layer will thus be thinner. This can be
solved by adding more hectorite or bentonite to the Boraq glaze version or
applying it in a thicker layer.
A Study of Specific Glazes
Click here for lots of information on dealing with substitution in a variety of common glazes.
Making a blend of materials that has the same chemistry as
GB is a process that requires ceramic calculation and physical testing.
Nirvana Has Not Been Reached
Still the 'Nirvana' of glazing with boron is not here. See the
Learning page for more.
Here are some examples of one of the tests we have been doing. In the first
two one recipe was compared using GB and a substitute. The third shows pure GB
on the right and an early substitute on the left. This flow test method works
extremely well and enables us to see an excellent comparison between two
specimens both in the degree of melt and the behavior and character of the melt.
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