Last week I said that Gerstlet Borate is crap, and I stand by that. No glaze materials has caused as much grief and stress over the last 30 years as Gerstley. Why? It is very inconsistent and relied upon much too heavily.
Without getting too technical, Gerstley Borate is the cheapest, easiest way to get boron into a glaze. Boron is a popular glaze flux in mid-range and raku glazes. Most other sources of boron that are available are water soluble, which we don't want in our glazes. The only other good non-soluble options available are frits, a powdered glass that is made to a specific formula. These have always been much more expensive, which causes a lot of potters to avoid them. We potters are notoriously cheap, you know. Gerstley also has the benefit of being thixotropic, keeping glazes suspended in the bucket very well without the addition of clay. Many popular Gerstley-based glazes have little or no clay in them, so reformulating a glaze with a boron source other than Gerstley results in a glaze that settles badly in the bucket. 80/20 White Crackle Raku is a good example.
The actual mineral content in it can vary widely, causing radically different results in glazes. While it can be somewhat exciting to be surprised by the results coming out of the kiln, you don't want to have to reformulate your glazes every time you open a new bag of Gerstley. We rely on the consistency of our raw materials to make it easy to reproduce glazes over and over. In some cases our livelihood depends on it.
I spoke with a representative of one of the big industry clay suppliers a few years back, and he was appalled that potters ever used Gerstley Borate in glazes, for exactly the reasons I just said. He told me that it had primarily been used to make roofing tiles, which didn't need much consistency at all. Remember, almost all of the materials that we use as potters are mined for industry, not for us. There are not enough potters in the world to keep a clay or glaze material mine open.
Every few years (or less) for the last 30 years, Gerstley's mineral content would change as they worked through the mine, and people would freak out because their glazes didn't look the same, or were very runny, or not runny enough, etc. So they would do what they could to reformulate the glazes until it changed again.
I was working as a tech for A.R.T. Clay in Sturtevant, WI when they closed the Gerstley mine about 10 years ago. Mass panic ensued and I got calls from worried customers every day about what to do. Everyone wanted a direct substitute, but at the time there wasn't one. I did what I could to reformulate my customer's recipes using a boron frit like Ferro 3134, but that didn't always work. Many recipes had 50-70% Gerstley, and 3134 just doesn't fit into recipes like that.
Laguna clay bought up what was left of the warehoused Gerstley when the mine closed, and has been selling it for the last several years. As far as I know they still have some, as it is still in their catalog. Fortunately, some of the materials suppliers realized what was going on and developed great direct substitutes for Gerstley. These substitutes are frits, so they are more expensive, but they are 100% consistent. I use Gillespie Borate from Hamill and Gillespie whenever I run into a recipe that calls for Gerstley. It seems to have a little more melting power than Gerstley, so I always reduce it by 3-5%. It works great, and since it as a man-made product, it should never run out.
So if you're still using Gerstley Borate, realize that it is crap and that it will thankfully run out eventually. Start testing your glazes with substitutes NOW so you are ready to go when it does run out. It can take several firings to thoroughly test a glaze, which can mean at least a couple of weeks without your favorite glaze. Or sell off the Gerstley you have and get into a 100% consistent substitute now and save yourself some headaches.