[TheForge] Jewelry Finishes

[Bruce Freeman]

John,

The process is called "enameling" and there's a whole (jewelry) art to it. =
 When enameling on copper, the problem with the glass breaking off is =
minimal.  It is certainly possible to enamel on steel, but I don't know =
the details.  Ideally you match the thermal coefficient of expansion of =
the glass to that of the metal, then cool slowly enough that they don't =
break apart anyway.

The simplest would probably be to buy commercial glass frit intended for =
use as enamel on steel.  I can look up catalogs for you if you want, or =
just google-search the subject:

+glass +enamel steel=20

maybe?

There is a test for compatability of glasses that might be applicable to =
metals.  For glass, you heat and soften two rods of glass, fuse them =
side-by-side while hot, then draw them out to a "wire".  Let it cool.  If =
the "wire" stays straight, the glasses are completely compatable.  If it =
bends or curls, they have different thermal coefficients of expansion.  =
The greater the bend the worse the match.

Conceivably you could use thin steel sheet and enamel one surface.  AS IT =
COOLS, observe whether it tends to bend.  If so, the glass and metal are =
not compatable.  However, be aware that thin metal is a bitch to enamel.  =
Thick metal is always better.  So even if the glass and metal are =
compatable, the glass could spall off the thin sheet when it gets cool.  =
The reason is probably mostly stress due to too-rapid cooling. =20

As you say, slow annealing is always better for glass.  Typically this is =
done by placing the piece in a furnace at the appropriate temperature =
(which depends upon the glass, but may be as low as 800 F) and then =
turning off the furnace and allowing it to cool naturally.

Bruce

Bruce