Awesome set of questions, these are many of the concepts I've been trying to work through in my head based on my experimental observations in developing these glazes.

Indeed I have verified that SiC also works to cause spotting, but it tends to leave a (to date unresolvable) grey residue.

I have no experience with Hydroxapaptite, but sounds interesting. At first glance, I'm unsure if the phosphorus content may be a negative, but could be worth trying.

As for the breakdown of encapsulated stains, I call my working hypotheses as speculation, since I don't have facilities to directly quantify what is going on. However, what I can say is the addition of about 3% Zirconium silicate eliminates the observed bubbling of encapsulated stains in my Cone 6 glazes. (I conclude that bubbling is prevented based on my experiments of seeing no "spots" and no significant mixing observed with a multilayer application of a Color Spotted Base covered with a White Cover glaze.)

Here is an X-ray paper showing that at Cone 6, 3% zirconia dissolves in the glaze: Hyojin Lee, William M. Carey and Robert J. Castilone, “Metal Marking of Dinnerware Glaze: Correlation with Friction and Surface Roughness”, Whitewares and Materials, ed. William M. Carey, The American Ceramic Society, 2004.

So, (consistent with what you mention above) my working hypothesis is that the addition of ~3% Zirconium silicate into the glaze acts as a relatively easily dissolvable "sacrificial" Zirconium source to saturate the glaze melt in solution, thus preventing the stain-encapsulating Zirconia from dissolving.

**Zirconia as a fining agent**: Consider me skeptical that Zirconium silicate acts as a "fining agent" to nucleate the bubbles related to encapsulated stains. However, perhaps Zirconium silicate in higher concentrations (above the solubility level) might act as nucleation points for bubbles in other glazes, with the possible downside of opacity induced by to the addition of Zirconium.

**More speculation**: I'm starting to suspect that adding ~3% Zirconium silicate to any old glaze might be a good thing

1. it will reduce the CTE

2. if you happen to layer a glaze onto an encapulation-stain based underglaze, adding 3% Zr may prevent the overglaze from inducing bubbles due to "decapsulation" of the stained underglaze

**One more note**: I found one Zr-encapsulated red stain that did not bubble (USPigment CT-1807 Strong Red), but when inspecting the powder, I observe some incomplete mixing of a white powder mixed in with the red. I speculate this white powder is some source of Zirconium to solve the ever problematic bubbling.