Metal finishing Q&As since 1989
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Black marks appear on pewter a few days after tumbling
Q. Hello good people,
I am currently producing my own jewellery using lead free pewter which is cast in black rubber moulds on a centrifugal casting machine. The moulds are powdered using ordinary talc between castings.
Castings are then, once prepared, tumbled with small rounded stones and a polishing soap (powder) recommended to be used with the stones for metal polishing.
The initial result is fine but then after a few days, small dark marks can start to appear on some of the pieces of jewellery. Sometimes left longer the marks can get bigger.
My castings are not badly porous, the marks appear where there is very slight pitting after tumbling, but also where there is none. I am not sure whether this is linked or not. Can you help me?
Lawrence Gibsonjewellery - Helston, Cornwall, England
2004
A. Hello,
Your problem could be drying. Try using corncob or maize to dry it. Also you can try continuous processing.
Regards,
Niranjan Kulkarni- New York
2004
2004
A. Hi Lawrence.
This is a real stab in the dark (proper you might say) The rest of them won't know what that means!
So ...
What is actually doing the staining? Or is this pitting corrosion or of mechanical origin? - Is it residue left by the polishing medium? - stones or powder.
Or slight corrosion from moisture left on the items? - How well do you wash and dry them?
Lead-free pewter is mainly Tin with Antimony and some Copper, and I don't know what the various corrosion mechanisms with that type of alloy would be - but I would guess the most likely cause, if the effect appears after some delay and over different areas, would be from the tumbling, possibly from sulfur-containing media (excuse the odd spelling guys). Send me one and I'll have a look on the electron microscope - I'm not far from you.
Regards
Martin Rich- Plymouth, UK
A. All of the above are good suggestions. Just thought I'd mention that all metals need protection against oxidation after surface finishing. In fact, oxidation will occur faster after such processing; therefore, either use a hard clear coat over parts or a thin film dry to the touch oil.
AF Kenton
retired business owner - Hatboro, Pennsylvania
2004
A. I worked in the pewter industry for 4 years casting and finishing. To tumble pewter the right way use stainless steel shot in the tumbler filled up with water and plain washing up liquid (about half a cup). When tumbling do for about 1 hour then rinse out water replace with fresh water and washing up liquid tumble again for about half hour. Then rinse all the pieces and dry with a fan heater (using a soft rag while drying) this stops any stains!
If you want to before tumbling place pieces in an old electric frying pan filled with black shoe polish.Melt the polish to liquid state then leave pieces in the hot polish for about 15 minutes strain off excess polish. When tumbled the polish stays in any detail e.g., letters or etchings.
Hope this helps somebody.
- Uk
2005
Q. I tried tumbling a few pewter crosses I made and on the last tumbling step with the fine polish powder I let it tumble for approximately 10 hours. the pewter crosses came out black. the edges were shiny and silver. my goal is to take a cross I made out of the mold and shine it to as close as a mirror finish as possible. please help!
mike florio- danbury, Connecticut usa
May 3, 2014
A. It sounds as if you ran a wet process media with maybe a deburring compound. After an hour in in a close loop system your parts or any parts will start to darken due to a reaction of metal, parts and media. You need to run a flow through system or you should consider a dry organic polishing media. If you do need to remove material, run a short cycle then into dry media.
AF Kenton
retired business owner - Hatboro, Pennsylvania
May 5, 2014
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