1. At what temperature can I accomplish this using my kiln, and
You would have to experiment. But you might find this difficult to
do with a kiln. Purple is not an "endpoint" in the coloring sequence,
but rather, is an intermediate color in the sequence of colors Ti
goes through. Getting a uniform exact color would require not only
precise temperature control, but precise timing too.
It would be much easier, give a more uniform color, and give you
much more precise control over the exact color you get, to do the
coloring via anodizing instead of heat. That means to connect the
ring to one terminal of a DC variable voltage power supply, the other
terminal is immersed in a suitable liquid electrolyte, and you
immerse the ring in that solution. The color you get is determined by
the voltage, and normally, one starts at a lower voltage, and dials
it up until you get the color you wish. If you miss, keep going, as
the color sequence repeats as you go up, though not indefinately (it
gets "muddier" as the voltage gets too high, then finally just
settles in on a durable darkish grey) The whole process is much more
controlable than heat coloring. You can even hook this up as a
paintbrush brush plating sort of thing, and paint with various colors
in the same piece, varying the voltage as you paint differing areas,
or you can get multiple colors via immersion anodizing by masking off
areas, anodizing at the highest voltage color you wish, removing the
mask, and anodizing at the lower voltage colors, which won't change
the first higher voltage color.
While doing this does require some additional equipment, I'd say
this is the most versatile way to go. But first, try experimenting
with your kiln and see what you get on some scrap Ti. Who knows,
perhaps you'll be able to control it well enough for what you wish.
2. How well would the patina hold up on a wedding band (worn every
day)? The patina would be in recesses only.
The anodized surface of the Ti is reasonably durable, but not
totally so. Higher, exposed areas would wear away within a probably
unacceptably short time for a ring. Jewelry that takes less wear,
like pendants or earrings, would be quite durable enough. But you did
say the color is in the recesses. If they are deep enough to really
avoid the bulk of the wear, then it should hold up well.
Chemically, it's inert, so you don't have to worry about it changing
And just to keep the language consistent, you're not applying a
patina. It's an anodized surface, meaning an oxide layer induced by
applied voltage while the piece is made the anode in the electrical
connection. The voltage changes the thickness of the oxide layer, as
higher voltage forces oxygen deeper into the layer, which by itself
is an insulator and fairly impervious. If you color by heating,
perhaps you could call it an oxide or patina layer, but in both
cases, it's the thickness of the oxide layer (which by itself is
transparent and colorless) which determines what color you see.
If you have little luck getting this to be controlled via heat, and
want to anodize but don't have the tools, I'm certain you can find,
here on the list, any number of folks who'll be happy to do it for
you (including me).