You might notice that this one has a significant addition of Cobalt, half and half Custer (Potash) and F4 (Soda) Feldspars, calcined talc, and 2% Manganese. Typical that I changed too many things to give a really useful side-by-side comparison. But I suppose when I’m coming up with new variations, that’s always been my style.
Some observations on this one:
Cobalt goes a long way and pretty dramatically alters an oilspot. With a .25%-.5% addition you get a nice shift from brown and russet glaze matrix to a darker solid black glass. Beyond 1% you can get some really nice silvery qualities to the spots. The drawback is that the more you add, the more refractory the glaze tends to get – and the longer it takes for the glazes to heal.
Feldspar is a pretty dynamic one, obviously. Since Kona F4 isn’t really wildly available anymore, I wont expound too much on what I saw as the differences (if there even were any…) in oilspots… but it’s fair to say that diversifying the Feldspars usually gave the fields of spots a little more depth. Nothing scientific or tangible about that feeling though… just kinda my opinion.
Calcined Talc was an addition that aided in the application. Anytime you see calcined ingredients in my recipes, it’s to reduce plasticity in the raw glaze. I.E. the thick layers of glaze keep falling off the pots or taking forrrreeevvveeerrr to dry. Tossing glaze materials into a bique kiln ‘pre-shrinks’ them and mitigates a lot of the expansion and contraction that happens during glaze application. This a super handy move for slip-based Tenmoku recipes.
Manganese is a pretty magic one when it comes to oilspots. There’s no doubt that it’s a flux, and it definitely adds to the fluidity to the glaze during the peak temperature ‘heal over’ stage of the firing. It also adds a wispy, fuzzy characteristic to the quality of the spots – owing, I’m sure, to the separate (and distinctly different from Iron) crystallization phase that Manganese goes through as the glaze moves from the molten to solid phase. To me at least, it adds a bit of glittery, hazy, nebulous background. Again, that’s more speculation, but suffice it to say that Mn behave a whole lot like Iron in oilspots, but it’s uniquely different.