Summary for chemical polishing bronze Discussion
Chemically polishing bronze is presented as a cautionary tale rather than a success story. In the thread, Harry Parkes of Birmingham, UK, recalls developing a chemical polishing process for copper alloys that worked commercially, but says it failed badly when used on the HMS Birmingham ship’s bell. The bell, which had once been kept bright in service, had developed a dark patina while awaiting scrapping. Colleagues and members of the Birmingham Royal Naval Association brought it to him expecting a restoration, but after setting up a large bath and trying the process, the bell turned a dirty yellow and became rough like sandpaper. A second dip only made it worse. Harry says he had to admit the failure, and his reputation “sank like a stone.” The exchange underscores that a process successful on copper alloys may not be suitable for bronze, and that testing on valuable objects is risky.
Q. Need a chemical or electrochemical method for polishing bronze (10% tin in copper). Since 2006, we have been producing a metal clay that when fired in activated carbon sinters into solid bronze. Normally the bronze is finished by tumbling with stainless steel shot or by hand with polishing wheels. Our new product, extruded by our 3-D Metal Creator has successfully met our Kickstarter goal. Unfortunately the former polishing methods are difficult or impossible, so we are searching for another polishing method that can be used by our customers (jewelers and craftsmen). I’ve got a PhD in chemistry.
Bill Struve
Metal Adventures Inc. – Wilmington, North Carolina
November 6, 2015
A. Hi Bill,
I don’t hold much hope in chemical/electrolytic polishing being an answer. Many years ago I worked on developing both. While copper and brasses were quite easy, bronze was a problem. The only bronze alloys that showed any promise were low tin (<3) sheet materials.
It is a fundamental requirement with these chemical processes that you have a single phase alloy. Even with low tin it was hit and miss as to whether you would get a polish or etching and staining. We had to take the precaution of stating that our processes were unsuitable for bronze. When it came to 10% tin, it was hopeless. I’ve no idea what metallurgical structure you get with your process but castings always present additional problems. Your procedure sounds even worse from the point of view of chemical/electrolytic polishing.
Harry
Harry Parkes
– Birmingham, UK
Thanks Harry. That is probably why I could not find anything online. Metal Coating Process Corp. here in North Carolina has asked for samples to give it a try. I’ll report their results.
William Struve [returning]
Metal Adventures Inc. – Wilmington, North Carolina, USA
? You mentioned mechanical processing of parts; however, you did not specify which process. Did you try or consider a centrifugal system?
AF Kenton
retired business owner – Hatboro, Pennsylvania
Hi Bill,
Yes, please let me know what happens.
When I think about the chemical polishing of bronze I suffer acute embarrassment. I developed a process for copper alloys. It enjoyed a good commercial success.
When the British warship HMS Birmingham was sent to the breaker’s yard, the ship’s bell was given to the local Birmingham Royal Naval Association. During its time in commission the bell would have been kept bright and shining. But during its period awaiting its fate it had developed a dark patina. So some colleagues, and members of the association, knowing of the existence of my magical product brought the bell to me to restore it to its former glory! Well, I set up a large bath specially for the job and set about the restoration. Restoration it was not! The colour was a dirty yellow and the surface was as rough as sandpaper. I gave it a further dip in the desperate hope of recovery – a bit like a gambler in a last throw of the dice. Of course, it made matters worse.
I then had the pleasure of telling my colleagues. My reputation sank like a stone. They, in turn, had the task of explaining to their fellow seamen.
Harry
Harry Parkes
– Birmingham, UK
Q. Kenton, I’ve used tumbling with stainless steel shot and don’t know what centrifugal polishing is.
William Struve [returning]
Metal Adventures Inc. – Wilmington, North Carolina, USA
November 18, 2015
A. Sorry for the delay. There are 3 basic mechanical systems for deburring parts. A standard barrel system produces 1 g of energy force to the parts. A vibratory system operates at 8 g’s max. Centrifugal systems produce 20-24g’s of force.
AF Kenton
retired business owner – Hatboro, Pennsylvania