Potters & Sculptors - Making Rock from Mud
Gerstley borate was mined by US Borax up until around 2000, when production ceased. It was an inexpensive source of B2O3 in a fairly insoluble form. The impurities in the mineral lent some rich variegation to glazes and it has been a very common glaze ingredient for many, many years. That said the mineral has been problematic in that the chemical assay varied considerably as it was mined from natural aggregates that were quite inconsistent. Add to that the very high Loss on Ignition, and the partial solubility in water, and gerstley borate is less than ideal for glazes. Nevertherless many potters continue to use Gerstley, and have variously documented their results. The two-penny consensus is to use a more reliable source of borate where the results prove satisfactory, and use gerstley borate where others fail expectations for a more variegated, trailed, glaze matrix.
See Laguna Clay about their stockpile of previously mined G.B.
Laguna says their stockpile is quite consistent in content http://www.lagunaclay.com/support/pdf/Chemical_Composition_of_Gerst...
A whole lot more at http://digitalfire.com/gerstleyborate/index.html
In a previous life I was a lab tech at US Borax in Boron, California. More soluble forms of borax were mined there, dissolved, and crystallized into 5 and 10 mole "borax". Other processes fused the crystals into anhydrous products. It was awesome to see the fluxing effect of molten borate on hard fire bricks 2 ft thick. The furnaces had to be rebuilt when they got too thin, because the molten borate actively dissolved the interior face of the bricks.
I've been very impressed with white-burning Bentone EW from China, available from Trinity Ceramic Supply in Dallas TX for about $5 a pound, if you can't find it elsewhere.
Much less expensive than VeeGum which from my experience has only about 20% of the suspending power of Bentone EW. Digitalfire describes it as a macaloid, but again in my experience has far more suspending ability than macaloid.
Bentone EW is a really important ingredient for reformulating many Gerstley Borate glazes in the Insight-Live database, as Gerstley Borate is often the only suspending clay added. When Gerstely Borate is replaced with a frit, there is often not enough "room in the recipe for more than a few percent Kaolin or Clay. The only other alternative is using soluble Boric Acid or Borax Pentahydrate - far from a good solution.
Using just 0.5% Bentone EW, or less, I can create the suspending ability of 10% Kaolin, Clay or Gerstley Borate.
We have two "complete glaze" ^06 red and yellow cadmium-sulfide frits, CM-940 and CM-941, which fire black if 2% VeeGum is added. I've fortunately been able to suspend these frits using only 0.1% Bentone EW and calcium chloride without much noticeable change in the bright red or bright yellow glazes.
When ever I see a recipe which uses something like 7% bentonite, I know the author was using extremely low-quality bentonite which sells for $0.47 a pound as opposed to the $5 a pound for Bentone EW. So that recipe becomes 2% VeeGum or 0.5% Bentone EW.
There were roughly ten "Colemanite" glaze recipes without photos I deleted in the Insight-Live database. All were identical to other recipes in the database which used Gerstley Borate, with the mere exception that Gerstley Borate had been replace one for one with Colemanite - which hardly qualified as a unique recipe, especially without a photo.
At our studio we mix two parts Laguna Gerstley Borate with one part Mason Stain and six parts water to create our 20 "underglaze" color bottles. The resulting product has more flux than a typical underglaze and sticks better - creating essentially a non-flowing glaze when used alone at cone 6. More often people use these as tints for clear and white glaze - about one part tint to 4 or 8 parts glaze. For the next 20 to 30 years the remaining Gerstley Borate is being sold for less than what it previously sold for.
I had no idea you once worked at US Borax. As you know my only industrial experience was with a electric and gas utility and the oil company Chevron. But both used ceramics.
Before I post a question, let me say I am impressed by the amount of informational and technical resources on this sight. It is apparent that a great deal of time has been spent locating and correlating all of it into one place. I have been experimenting with Calcium Borate for the last year. Have you had the chance to use this product? The general rule of thumb I use in glaze mixing: boron is water soluble, and mineral borate is not.