@kellerwerks – you asked this question, but we quickly got derailed.
Glazes can be hugely complicated, but here are some basic thoughts.
Plus, I recently audited 2 of our glaze classes, and they were missing one bit of info.
Glazes get their color in one of 2 basic ways. Either they have a frit (a finely ground glass that’s been formulated to not melt at pottery firing temps), or they have a combination of minerals that will combine to produce the desired color once the kiln gets up to temp. I think of these as “crayon colors” and “chemistry glazes”.
Amaco’s Celadon line, the Satin Mattes, and the underglazes are all (pretty much) fritted. 2 exceptions in the Celadons are Cobalt and Iron. Both cobalt oxide (I think that’s an oxide) and iron oxide are so reliable that those 2 Celadons just use the oxide rather than jacking with a special frit. I also believe that Obsidian is using more of a chemistry formula. Otherwise you wouldn’t get the results you get when you layer one of the Potter’s Choice glazes (those are all 100% chemistry) over Obsidian. Obsidian is actually changing the color of the PC glaze, and the only way it can do that is if it’s a chemical reaction that gives the Obsidian its color. (As a side note, there is no true black in heat chemistry. Black glass is actually super-dark purple, most times. Black glazes tend to have blue notes because they use cobalt as one of the minerals to approach black.)
The Potter’s Choice line from Amaco is all chemistry glazes. You will notice that the color of the liquid glaze has nothing to do with the color of the fired glaze (in most cases). One important note for glaze users is that you need to be careful not to contaminate a Potter’s Choice glaze with another PC glaze, at least in the jar. You may change the color, and not in an expected way. Well, if you were familiar with the reactions that different minerals cause, you might be able to predict the color shift. The problem there is that Amaco’s glazes are proprietary, and they’re already a blend of different minerals.
Folks who are interested in how chemistry glazes are compounded might find John Britt’s book on glazing. It’s an industry standard.
The “base glaze” can be different in different glazes. Mostly, glaze is shiny. The matte glazes contain an ingredient that doesn’t melt at the ranges we fire too. That’s why there really isn’t such a thing as a clear matte glaze. It’ll be foggy at a minimum, because the matte ingredient doesn’t melt.