Technical Briefs deals with technical concerns of interest to a wide audience, addressing topics as diverse as vanguard technology and ancient arts. It will also answer questions from readers and respond to suggestions, always trying to explain in simple terms the magic and wonder of working with metal. For this article, Tim will discuss fire stain on brass.
Fire Stain on Brass
I’m looking for help in removing a copper deposit that forms on brass and bronze alloys during pickling. I’ve been using either hand finishing by abrasion or bright-dipping, but both have their drawbacks. Is there something else I can try?
– Philip Smith, Norristown, PA
Your problem is basic to copper alloys. At high temperatures copper and its alloys combine with oxygen to make several oxides, the most important being cuprous oxide and zinc oxide. The latter is an easily dissolved film that comes off in just about any subsequent rinse. If the work is pickled in a mild sulphuric solution (like Sparex #2) the copper oxide is reduced to pure copper. This is the pink-orange film you’re finding on your work. As you might have noticed, increased temperature will create a thicker layer of copper Polished surfaces will oxidize more slowly than rough ones.
Most commercial solutions to this problem rely on bright-dipping, or a quick bath in acid to attack and dissolve the copper. Typical solutions use (by volume) 25% nitric acid, 60% sulfuric acid and a trace of hydrochloric acid. These are generally used at room temperature. Parts are dipped in pickle, rinsed, dipped in bright dip, rinsed and thoroughly cleaned. In a factory setting that deals with a single alloy and repeats steps with great regularity it’s possible through experimentation to arrive at a concentration of bright dip and a system of application that will give consistently excellent results. For the studio craftsman who works with several alloys and whose line involves diverse work, the process is less satisfactory. The additional hazard of this super-strong acid is enough drawback to make many people shy away from bright-dipping.
A less common and somewhat safer pickle specifically for copper and brass can be made of:
- Concentrated sulfuric acid, 4-10% by volume
- Sodium bichromate, 4-8 oz. per gallon
Water, remainder Ferric sulfate may be substituted for sodium bichromate in the above recipe, at the rate of 10-25 oz. per gallon After a standard soaking in Sparex, rinse the work and immerse it in this solution at room temperature. The duration of the soak will depend on the depth of the copper stain. Alter removing from this second pickle, wash the metal thoroughly with soapy water.
If abrasion is the route to go, your next thought would be to investigate tumble (also called barrel) finishing. One choice will be vibratory polishing, a process in which abrasive-packed plastic or ceramic pieces are cascaded over the work. This is quite last, easy and versatile. The alternate choice is tumble finishing, in which the work is packed into a sealed drum along with steel media and a liquid cleaner. The drum is rotated by machine, tumbling the work over and against the media. This is slower but permits the use of heavy material like steel. These burnish and work-harden simultaneously.
Send questions for this column (double-spaced, typewritten) to:
23 Mill Road Circle
Boylston, MA 01505.