duotone magma glaze?

[s6x]

I'm trying to figure out how to get a mottled magma glaze that's alternately black and white.    I played around with Mayco's dark magma and light magma, and while the dark magma is perfect, the light magma is too beige, not white enough.    I tried spraying watered down white clay slip, white glazes, white underglazes to no avail.   Is there some kind of chemical or powder that you can add to a glaze that will come out predictably white and matte?  I  I could potentially do this with paint after the fact, but I would really prefer if it was permanent in the glaze. 

[PeterH]

Starting from the idea that making your own "white" magma glaze might be the way to go ...

From: https://community.ceramicartsdaily.org/topic/14567-lava-glaze-silicon-carbide-vs-barium-sulfate/?do=findComment&comment=10963

I've recently started playing with SC, my first test only used 3.4% SC and fired to ^6 it was white. (There's also a  bit of green glaze showing at the top). Using more SC has just given me a nasty black result.   I would try reducing the amount of SC.

 

Wondering what might cause the darkening I first remembered
https://community.ceramicartsdaily.org/topic/30023-adding-granular-feldspar-or-stone-to-clay-body/?do=findComment&comment=226744
... where decreasing the SiC  grit size tended to be associated with  both a better magma effect and darkening

After some sercing (most info seems to be behind paywall) I found:
https://www.sciencedirect.com/science/article/abs/pii/S0272884221028777
The experimental results showed that in SiC powders prepared using the Acheson process, the contents of O, free-Si, free-C, and Fe impurities are high, and those of other trace impurity elements follow the order: Ti > Al > Ni > V.

No idea if "our" SiC is produced by the Acheson process, but iron seems plausible as a darkening agent. (And as I understand it most of the fine SiC used in pottery comes from lapidary sources, which are probably not too concerned with impurity levels.)   

Confirmation:https://www.arrow.com/en/research-and-events/articles/silicon-carbide-the-future-of-power
Darker, more common versions of silicon carbide often include iron and carbon impurities, but pure SiC crystals are colorless and form when silicon carbide sublimes at 2700 degrees Celsius.

So I'm gradually coming round to the idea that covering a gray magma with a white compound has it's merits!

 

Edited August 15 by PeterH