If I put niobium or titanium in the kiln to 1500 degrees AFTER it
has already be anodized to a specific color, will it lose it's color
or just get better color?
At those temps, if fired in the presence of oxygen, the color will be
utterly destroyed, and you’ll have a deep black color, nothing else.
Reanodizing such metal requires grinding off the black, which is very
thick. Now, if you combined it with glass in an oven capable of high
vacuum, you might preserve the color. Not sure. But maybe. It would
depend on how the glass interacted with that titanium or niobium oxide
layer. If the glass actually wets the surface, you would probably get
at least a color shift. Remember that the color is just an
interference effect with light reflecting from both the top surface
of the oxide (which is actually colorless and transparent) and the
bottom. The thickness of the oxide layer lets these two reflected
beams interact, causing the color. If the oxide is wetted by glass,
not air. the light may be affected as well. Not sure. Also, some
glasses might act as fluxes, dissolving some of the oxide. I’m not
sure, but it seems possible.