GERSTLEY BORATE: Replacing It in Your Glazes

There ts alarm across the ceramic comm
aity in North America about the impend-
ing demise of Gerstley Borate. US Borax
has ceased mining and remaining stocks in
the sales channel may last a year,

Laguna Clay hetp:/lagunaclay.com, to its
credit, has stated that “pricing will con-
tinue to reflect material costs and far
mark-ups, and will not increase as the
supply decreases”.

Gerstley Borate (GB) has been mined by
US Borax & Chemical Corp near
Boron, CA and ground by Hammill &
Gillespie and Laguna Clay Co. In fact it is
named after a former president of The
Borax Company named Mr. Gerstley. The
ore contains the minerals Colemanite
(Ca2B6011 ; SH20) and Ulexite
(NaCaBSy9 : BH2O). It has been used in
ceramic bodies, as a bonding agent in
grinding wheels, and as a fire retardant.
Most importantly, this versatile material is
commonly employed in glazes fired at
low, medium anc high lemperatures. For
many years students have formulated
glazes using a GB base because there is no
other inexpensive and safe natural mineral
that melts as readily and vigorously. For
example, firing a sample of GB powder to
cone 010 (about 1600F) completely trans-
forms it into a molten mass. lt is little
wonder that so many glares depend on it
when one considers that the main altema-
uve, feldspar, does not begin to flux vigor-
ously for another 400 degrees! Many frits
do noteven fuse as well. There is one other
COMMON INEX pensive ceramic mineral that
fuses to produce visually superior glazes:
lead oxide. Since the demise of lead it is
easy to understand why GB has been sa
important to hobby and educational ce-
ramics and small scale manufacturing.

There are other factors that suggest that
GB may not be the dream material many
have thought it was, For a number of
reasons this maternal is largely unused in
industry. The sodium and caleium borates
found in it are very slowly soluble in
water, The Na+ tons produced tend to gel
and flocculate a normal collotdal suspen-
sion. This necessitales additions of water
that induce greater shrinkage that compro-
muses bonding to the body, which in tum

d

Tony Hansen

cin cine glaze crawling, Some have sug-
bested additions of epsom salts (ie. 0.5%)
can help by introducing Mg++ ions that
inhibit the agglomeration of particles while
others have suggested the opposite, using
a deflocculant, This is an indication that
this type of gelling is mol an cusy problem
todeal with. Itisasomewhat temperamen-
tal material, with variation in chemustry
nnd physical properties reported by many.
Shipments have had sizeable amounts of
oversize particulate. H&G have claimed
thal they cannol euarantes. quality from lot
to-lotas the material is strictly" mun-of-the-
mine’ (they have to take what they get from
Lhe US Borax mine }and vanations can and
do occur.

GB can contin gypsum that can cause
glaze pop-owts if leftin the glaze mix. The
fusibility of GB is actually a curse as well
as a blessing, It is often used in excessive
amounts or at higher temperatures than is
necessary and little regard is paid tw its
unusual chemistry, The result has been a
Beneration of flux saturated glazes that
lack hardness and resistance to leaching in
acids.

Ttis common to blend GB with clays in the
proportions necessary to make the mix
melt at the desired temperature. Since it is
plastic like a clay, blend glazes can shrink
excessively during drying on ware and
tend to crack and then craw] during firing.

Finding a Replacement

Laguna Clay has stated that they are work-
ing to find a replacement and having good
results with two matenals. While many
will decide to wait for this replacement, [
feel that a better solution is to stop and
evaluate maticrs. GB is a very unique
material and it is not easy to believe that.
another just like it will surface soon (acte-
ally it may be possible, keep reading).
People are pointing to CadyCal as a possi-
ble successor but as you will see below,
you can’ ¢ just substitute them one-for-one.

The real secret to replacing GB in your
glazes is lo understand thal il continbules a
chemistry and physical propertics to the
glaze slurry. GB glazes have a creamy
consistency that will not be duplicated if

Potters Guild of Bnush Columbia

you simply substitute a dead’ mineral of
frit powder. Replacing GB needs to be
done on a case-by-case basis using an
appropriate strategy for each,

You may have no desire wo dig into the
chemistry of this issue and want a quick
foolproof fix. You won't be getting it
soon! This may not be a Y 2K situation but
itis fairly serous if you depend heavily on
GHB. [ will try to present this information in
away that most people with litle chemis-
try backpround can understand it, Lf you
would like io pay someone else to help you
convert your glazes, please write to Ron
Roy al ronroy @astral.mamec.ce. If you
would like to lemm to understand and con-
vert your own glazes keep reading.

There is no denying that the principal
reason for employing GB is for its chem-
istry. Itisasource of bore oxide or boron.
Youcan learn more about boron (203) ar
WWW. cenimicseanch. com/oxide!, Few in-
soluble natural materials contain signifi-
cant boron; soluble materials don’t work
well in glazes becouse they soak into the
clay body and cause it to melt or warp
during fining. These soluble forms of bo-
ron are typically used to make boron frits.
GB has a formula that we would like to
think approximates this: 2CwO . S3B203 .
SH20. However, it isn’t so simple. GB is
actually a lot more complicated

Some Technical Information About
‘(Gerstley Borate

Quoted chemistries for GB are very ap-
proximate. Tom Buck has researched some
fustory on this maternal to determine the
nature of the variations that can and have
occurred over the past decade, He said in
parm: “The colemanite content (calcium
borate pentahydrate) can be guesstimated
iM 30-34%, ulexite (sodium calqium bo-
rate} content 26-30%, shale low -iron clay!
feldspar/quartz) 20%, limestone 20%.
These proportions would yield a chemical
analysis close to H&G's, ignoring of
course, the unknown quantities of other
tramp minerals in the ore. One could there-
fore consider that today’s OB is only two-
thirds the equivalent of the once common
and familiar colemanite; that is, to get as

March 2000