I’m working on an inlay malachite piece in sterling bezel. Due to the
complexity of the design I pre cut the malachite to fit exactly into
the bezel so I just needed to give a quick buff after epoxy. I used a
small felt wheel on the Fordham with liquid spray (not paste) 50k
mesh diamond spray to top off the stones and a black spot appeared. I
went to wheel #6 (finest) on the polisher to gently grind off the
spot using light pressure, fairly short intervals of contact on the
wheel with plenty of water spraying. I seemed to be making progress
when another black spot appeared then a pit formed and the disaster
was complete. I nicked the neighboring stone in the process, which
was why I didn’t want to have to top off the stones in the first
place and gently (really - I was not pushing it after what just
happened) ran it on #6 and a black spot developed on the second stone
which then cracked and pitted. I’ve polished malachite before and
never had this problem. I’ve ground out the stones and am re-cutting.
I know malachite can be brittle but what just happened? Of course it
was the final clean up before photography. I’m too devastated and
dazed to even cuss.
Your problem was friction. It has basically burnt the carbonate to an
oxide and the harder you tried the worse it got. Polish with a slow
speed and use a leather wheel with a drop of vinegar in with the
water with your polish. Tin oxide is my preferred polish followed by
1/3rd micron alumina and then diamond as a third choice.
Malachite can also have a very fibrous structure and may contain
azurite as well. This leads to porosity, hardness differences and
solubility. Felt has too much of a nap to polish malachite without
leaving polishing relief.
In my experience malachite is very, very sensitive to heat.
Overheating produces black copper oxide.
The only thing I can suggest is doing the absolute minimum of
polishing, etc. after the stone is set. It seems to be even more
sensitive to heat after being mounted in a bezel.
Whenever I’m polishing or buffing malachite, I keep a container of
water handy and dunk the stone in it frequently.
Malachite is beautiful stuff, but it’s so soft and heat sensitive
it’s a pain to work with.
Bonnie, sounds like a real disaster. The only polish I ever use on
malachite is ZAM on a cotton buff at high speed. No idea who the
spots occurred unless there was some kind of reaction between the
spray and the silver bezel. Or it may have been something integral to
the stone, which was probably why the pit appeared as well. Malachite
is usually pretty solid but there are pits and spots that hide deep
inside. Try ZAM the next time for you final polish.
I know malachite can be brittle but what just happened? Of course
it was the final clean up before photography. I'm too devastated
and dazed to even cuss.
Sounds like you made some copper. I think you were just unlucky, and
variations in stones can be a pain.
Isn’t it always the way? When you’re at the last stage, and “boom”
something goes araie. I made a solid sterling locket last year, the
second last class before the end of the year, the hinge caught in
the polishing mop, ripped it out of my hands, and broke the hinge!
All went well, as I was allowed to join in with another class to fix
the damage.
Asked my lapidarist spouse what happened here. You got the malachite
too hot and oxidized it when you used the 50k diamond. Then when you
tried to buff it out, the material that was oxidized became too
brittle and chipped out. Unfortunately there’s not much you can do
except to re-cut the stones.