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Chemical Finishes for Brass and Bronze Extrusions

Chemical Finishes for Brass and Bronze Extrusions

Many of our customers have had great success in duplicating mature patinas on brass and bronze extrusions.  CDA (Copper Development Association) provided the primer on “Chemical Weathering” below.  In addition, there are many paint, finish and chemical companies that supply brand name commercial products that can make a chemically accelerated patina or statuary finishes easier and faster.  Mark Kulas of EPI (Electrochemical Products Inc,) New Berlin,WI comments that their “B/OX” line of antique finishes includes 10 to 15 different chemical solutions that provide predictable results in an assortment of colored finishes. 

I have visited many jobsites and fabricator shops and continue to be impressed by the number of alternatives for coloring brass, bronze and nickel silver.  Your comments and product suggestions would be appreciated.  I’ll try to keep the Bronze Blog current with your feedback.

Chemical Finishes and Coloring

Because of the time required for copper to weather to the ultimate blue-green or gray-green patina, men have sought for centuries to hasten the process by chemical means.

Coloring however is an art, mainly a matter of craftsmanship and experience. Chemical coloring techniques depend upon time, temperature, surface preparation, humidity and other variables which influence the ultimate result. A wide range of colored finishes may be produced on architectural copper-base alloys by conversion coatings that are chemical in nature. The purpose is to hasten the natural weathered effect that generally results from exposure to the elements.

Several conversion treatments are in general use which produce the patinas (verde antiques) and statuary (oxidized) finishes.

Patinas are primarily developed using acid chloride treatments or acid sulfate treatments. Because of the number of variables involved, chemically induced patinas are prone to such problems as lack of adhesion, excessive staining of adjacent materials and inability to achieve reasonable color uniformity over large surface areas. These potential shortcomings should be considered when specifying such treatments. Due to the large number of requests for historical chemical applications CDA has compiled the most prevalently used chemical processes.

Because production of an artificial patina on copper is dependent upon a number of variables, including temperature, humidity, wind velocity, surface condition of the copper and method of application, wide variations in the result achieved have been experienced. Reliability of all present methods can, at best, be considered only fair to poor

The following treatments have exhibited some degree of success:

Statuary Finish

a) Ammonium Sulfide base.
b) Potassium Sulfide base.

Green Patina Finish

a) Ammonium Sulfate base.
b) Ammonium Chloride base.
c) Cuprous Chloride/Hydrochloric Acid base.

 You can learn more at the Copper Development Association website at www.copper.org.

 Contact Electrochemical Products at www.epi.com.

Please share this post with others who are interested in chemical finishes for brass and bronze extrusions. You can also link to our post on finishes for brass and bronze extrusions.

About Mac Metals

Mac Metals is a “specialty brass mill” in Kearny, New Jersey that provides custom architectural metals to general contractors, original equipment manufacturers, architects, machine shops and architectural and ornamental metal fabricators that require custom extrusions made of brass, bronze and nickel silver. Contact Mark Sisson for more information about extrusion and chemical finishes for brass.

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