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We require silver bright coating on glass beads

2006

Dear Sir,
We have an vacuum evaporation coating plant (for coating on glass beads).

We want silver bright coating on glass beads , we tried using chrome powder & aluminum strips to get silver coating,at temperature 250 °C to -5 to 5.5 vacuum we achieved it, but after putting glass beads in nitric acid it turned to black - jet. (i.e., base color) As aluminum coating is very thin it wears out quickly.
Therefore my question is what other metals or powders which is strong enough to get silver bright coating through vacuum evaporation process on glass beads, and should not be affected by nitric acid.
Thanking You,
Yours faithfully,.

PS: the base color of glass beads is jet black color.
Previously we were evaporating chrome powder only but we were getting hematite or steel grey color at present we require silver bright color on it so what powders or combination of powders we should use.
We would like to know what metal powder we should use as a protective layer for aluminum.
We will appreciate & waiting for your earliest reply.

Mustafa. H. Bakkkallah.
- MUMBAI, Maharashtra, INDIA