We have two people at our studio who do macro-crystalline glazes below Cone 6 at home. Each type of macro-crystal type will form most actively at one specific temperature. They often cycle the temperature up and down to get exactly the zinc-silicate crystals they're looking for.
All ^6 macro-crystalline glazes I've seen are silica with flux and virtually no alumina. Judy's Silica to Alumina ratio is 16.75, while the ratio is 38.9 for the Frit 3110 recipe below - and an amazing 106.6 for the Frit 3134 under-glaze recipe! Glass with very little structure allows the glass to de-vitrify and form these crystals.
Her information is pretty specific.
27.9% Custer Feldspar
18.9% Zinc Oxide
8.7% Lithium Carbonate
5.0% Strontium Carbonate
#1 Drop temp to1850F hold from 1 to 4 hrs.
#2 Drop temp to 1650F hold 10 min, Raise to 1900F hold 3 hrs, Drop to 1650F hold 10 min, Raise to 1900F let cool. - creates large center ring with border.
Schedules are continually being changed and reversing the hold positions on #2 creates a crystal with a small center ring inside a wide one.
I think I'll try Judi Buchman's macro-crystalline glaze this weekend, using our regular slow-cool to see what I get. I notice she doesn't prefer the elemental symbol Sn for Tin - Stannum never became a familiar to me.
The macro-crystalline people at our studio use frits, very similar to a Teal Blue breaking Honey Bab's has been trying to get more blue coverage on. The person using this makes this glaze twice, once with 3134 and the other with 3110. He applies the 3134 first, then applies the layer with 3110.
103.0% Macro Crystalline Base -- Cone 6
52.0% Ferro Frit - 3110 or 3134 50%
24.0% Zinc Oxide 20%
24.0% Silica 325 Mesh 30%
2.0% Lithium Carbonate 0%
1.0% Titanium Dioxide 1% Red Iron Oxide - Maybe these are a nucleation agents or it night be unnecessary.
0.5% to 4.0% Coloring Oxide, Cobalt, Copper, Nickel.
Just compare how close this recipe is to Bab's Teal Blue breaking Honey - except they use 24% Zinc Oxide instead of 20%.
With our six hour slow-cool between 1800F and 1500F these are the small crystals I got last weekend with Bab's Blue Teal breaking Honey nickel.
I just found what looks to be a very nice ^5 crystalline glaze recipe, with some variations. I know it's generally considered very difficult to get crystals under about ^8, but apparently this lady has worked it out. Does anyone see any problem with it you see any problems with it?
The writing on her test tiles is very difficult to read. So I fiddled with the picture to make it quite a bit more readable. It is attached. I think the MN means manganese but it could also mean magnesium. Which makes most sense with the appearance of the test tile?
Also do you think the Cu is copper carbonate or copper oxide?
I've done a search for Judi Buchanan and can't find anything that fits. Does anyone know her or how to contact her?