I spent two years of testing so far and I am not there yet! So forgive me for not publishing the recipes. But if you read through this article you might get some ideas of what materials I am using.
Recently I have concentrated on a recipe I call OIL26. I have its original version from a Taiwanese source. Fortunately, I had all the ingredients except for black clay, for which I substituted Albany slip. (Luckily nothing exotic like "black chestnut peel ash" or "Suzhou soil"...!). My first test tile was not very promising, and initially I tossed it in the box with all the other (non-promising) test tiles. After a few months, I dug it out again and wishfully found some potential in the "reluctant spotting" at the bottom where it had run.
I then set out a course for adding oxides to improve the spotting. OIL26-1 was with 3% titanium, OIL26-2 with 3% rutile, and OIL26-3 with 4% ziconium. (Note that I do not test with toxic oxides though I know they are more likely to give good results. Manganese especially. However, as I will be using the successful glazes for bowl shapes - for food and drink - I just avoid toxics altogether).
The zirconium test alone looked like it might be twisted into something better, I reduced the calcium content and added 2% bone ash, some gaolin and some silica for good measure as the original recipe had none. These tiny additions made a great improvement (the redder tile is the improved recipe).
This improved recipe tested on a bowl shape came out beautifully on the outside where it ran and rolled perfectly at the bottom drip collecting rim. The only thing I would like to improve is the cloudiness at the rolled rim - though that is quite standard for oilspot pieces, I think it would be better if it was not there:
The inside of the bowl is another matter. Oh-la-la! A Mega Blister.
Just can't get over how pretty the surface on the exterior is. Sigh!
I suspect the zirconium has to go. There's only 3% in the first place, but it looks like the glaze is too viscuos and the cloudiness and blisters in the center of the bowls make me think it is where the zirconium contentrated after running.
The next level of testing will be to see if the zirconium has to be replaced by another whitening oxide (magnesium, tin..?) or whether it can be removed altogether (which I suspect it can).
To be continued...