Odd polishing problem

[ErichCDesigns1]

Greetings All!

I’ve encountered an odd problem with polishing that I thought I
would pick your brains on.

I have been working on polishing some flat, sterling silver stampings
after laser engraving. The stampings get in engraved and then jump in
the pickle for a few seconds to remove oxidation. From there, they
are polished. However, as I go through the process of polishing the
stampings to a near mirror finish, no matter what I do (sand, bobbing
compound, 3M wheels, etc.) the stampings polish out with a somewhat
"frosted" appearance where the metal grain can actually be seen. To
be clear, the surface is polished to an extremely flat finish (i.e.
no scratches left). Yet, the physical appearance of the grain still
shows up and creates a frosted appearance rather than a
"liquid-silver" or true mirror-like appearance as when anything else
is polished. I’ve tried re-sanding, re-polishing, etc. and this
routinely shows up. Has anyone else ever had this problem or know how
to rectify it? In 14 years of jewelry work, this is a first to me and
I’m honestly quite baffled by it.

Thanks for any help you can offer!
Sincerely,
Erich C. Shoemaker
www.ErichCDesigns.com

[Paul3]

huh - sounds just like it’s been etched to show up the grain
boundaries etc. could be the acid is etching the crystal structure,
or especially the grain boundaries.

I encountered this sort of techniques when doing metallurgy at uni
etc.

There are techniques for electropolishing that might restore a
mirror finish - don’t know how commonly available they are.
Electroplating (or electro-less plating) might be easier…

[Paul3]

huh - sounds just like it's been etched to show up the grain
boundaries etc. could be the acid is etching the crystal structure,
or especially the grain boundaries. I encountered this sort of
techniques when doing metallurgy at uni etc. There are techniques
for electropolishing that might restore a mirror finish - don't
know how commonly available they are. Electroplating (or
electro-less plating) might be easier... 

I’ve just remembered a silver plating technique used by my uncle
(scientific glass blower) for backing mirrors. Uses sugar silver
salts and ammonia to plate silver onto surfaces… a quick polish
would be needed and I’m not sure how well it would stick - should be
OK if clean… never done it yet.

[Charles_Allenden]

Hello Erich,

Without physically seeing the pieces it is hard to be sure but what
you are describing sounds to me like an ‘orange peel’ effect caused
by the annealing of the stampings after they have received
insufficient cold work. What size would you estimate the grain size
you are seeing to be?

Charles