In recent years there has been an increased interest in patination and metal coloring. This may be seen as a response to the use of nonprecious metals in jewelry. A trend towards objectmaking in North America has produced relatively large surface areas in base metals that invite patination of various kinds.
Richard Hughes and Michael Rowe’s monumental work the Colouring Bronzing and Patination of Metals appeared in 1982, providing carefully researched compilation of coloring solutions and application methods superior to anything previously available on the subject. However, many of the solutions and chemicals are unsuitable for the average small studio. Some are toxic and corrosive and others impractical to make because of difficulties in obtaining small amounts of the required chemicals.
This abbreviated paper addresses the need for simple, less toxic solutions for small-scale studio use. Since there are thousands of solutions, pastes and atmospheres that affect metal surfaces, there is often more than one method of achieving the same or similar color on a specific metal. As this paper is intended as an introduction to metal coloring, basic procedures are emphasized.
Patinated surfaces should not be worn in contact with the skin or used in anything connected with food. Many patinas are toxic if ingested and may cause skin irritation in close or constant skin contact. Protective sealers are often used with patinas. They must be used if the object will be in body contact. The irresponsible use of patinations in the fashion industry is likely to lead to a backlash when the first major skin reactions occur in customers. When used in jewelry, patinas should be isolated from all surfaces in skin contact. They must adhere well to the metal and be properly sealed.
Natural patinas formed on copper alloys vary with the environment. Industrial environments tend to produce a copper sulfate base, the ocean a copper chloride base and the mountains a copper carbonate base. Varying conditions produce combinations of these and others. Natural patinas include:
|Bright Blue||Copper Hydroxide||Cu(OH)2|
|Green-Blue||Basic Copper Carbonate||CuCO3•Cu(OH)2|
|Green-Blue||Basic Copper Sulfate||CuSO4•3Cu(OH)2|
|Green||Basic Copper Chloride||CuCl2•3Cu(OH)2|
These patinas take four to 30 years to develop well, depending on the location. All of them can be produced faster with chemical treatments. The best durability, color control and speed of application are produced using solutions and patinas that rarely occur in nature.
The oldest traditional method of obtaining green patinas on copper alloy surfaces was to expose them to urine, often aged to developed a stronger ammonia content. Works have been buried in manure, coated with pastes containing urine or sealed in urine atmospheres. I have heard of a Korean metalsmith who had over 40 dated jars of aged urine in his workshop, the older ones being more prized. Diet and physical condition would add minerals, salts and chemical compounds to the solution and subtly affect metal surfaces. Burial in alkaline or acidic earths has also been used to create patinas and may be considered a kind of paste application.
Patination agents included in this paper are clear household ammonia, vinegar, table salt and cupric nitrate. Ammonia and vinegar produce good results as fumes. Salt functions as a direct corrosive agent. A combination of salt and ammonia fumes makes for fast results but adhesion is not as good as with the cupric nitrate patina.
In order to organize the materials, we will deal with the process in the order in which the patination proceeds:
A clean surface is essential. All mineral and organic oils, greases and oxidation must be removed. The process chosen should activate or roughen the metal surface.
There are four common methods for cleaning the surface: chemical, solvent/detergent, mechanical and electrolytic. One or more of these techniques may be required in any given situation.
The solution is used at 60-70C. and a current density of 1-3A/dm2. The work is treated for 1-3 minutes as a cathode and 5-10 seconds as an anode. Dip in 1:10 sulfuric acid to neutralize the electrocleaning solution and activate the surface (15 sec.). Rinse well in running water.
The clean part may be stored for a period of time in distilled water to inhibit the reformation of oxides.
The final appearance of the object is influenced by both the chemical agents used an the application technique. The following details some of the typical applications methods.
Tape, wax, lacquers and other resists may be used with these agents. Layered metals like mokume, doublée and selectively plated surfaces can be considered as a type of resist. Different reactions occur when the various metals are exposed to the agent. Gold is nonreactive and serves as a good resist. It retains its bright yellow color against the green of the base metal below. “Gold Painting.” a gold fusion technique on copper, provides a useful way of developing patterns using gold as a resist.
Next we will consider some common agents. Clear household ammonia produces greens and blues on copper alloys, olive greens on bronze. It works best in thin films. This may be provided by treating the object in an ammonia atmosphere with a damp surface. (Without prior dampening it will condense on the metal.) If the metal is wetted with a salt solution rather than water, a bright blue is produced in about two hours. If left overnight, it intensifies but is far less adhesive. Vinegar works in a way similar to ammonia. Moistened shavings work well as an application technique.
Salt may be sprinkled as dry crystals and then left in a humid place (laundry room) to react with the metal. A day or more is required. Solutions of salt produce varied results. Salt patina is characterized by variegated bright green and orange on copper. Salt also works well in combination with other solutions and fumes. An example is the Rowe/Hughes solution #3.129 (cupric nitrate 200g, salt 200g, water 1000ml), which may be applied hot or cold and produces azure and orange surfaces. This produces a loose patina good for non-wearable work, unless well sealed or built-up.
The balance of this paper will deal with the application of cupric nitrate patinas. This general-purpose patina may be applied hot or cold and may be built up to cover surfaces evenly with excellent adhesion. The solution is most easily applied to brass but holds better on copper. It is a surface covering and may be used on silver, nickelsilver, bronze, steel, iron and even gold. Caution: a good fume hood is necessary when working with this chemical. Nitric acid fumes are released (corrosive, irritant) and the possibility of metal-fume poisoning exists. Plans for a small-scale fume hood for patination designed by Theo Jansen and James Evans are available from the Ontario Crafts Council in Canada. To test air-flow speed use a smoking rope to see the draft in action. The smoke should rise at an angle of no more than 10° from the metal as it moves toward the vent. Rubber gloves and cleanliness are essential. This patina should not be used on surfaces next to skin. No eating, drinking or smoking when working with this patina.
The steps in the application are:
Repeated applications and removal of all loose material by scrubbing in water between exposures is recommended for the best adhesive qualities.
A wide range of surface colors are available from the more common agents:
Correctly applied, the patina is tough and not liable to damage easily. Sealers are necessary for jewelry use. These would include waxes, thick oils, acrylics, lacquers, etc. Acrylic resins and resins like Envirotex® are good for over-all thick coverings. Samples should be made as a palette to choose from. Oils and waxes tend to darken the color. acrylics to make it resemble green paint. I usually spray the latter from a distance for a subtle effect and matt the surface with a cloth before it dries. Tapes, string, leaves, etc. may be used as masks to obtain patterned or mottled areas.
It is worth noting that a number of metal coloring procedures and patinas build up a layer of material over the surface, effectively hiding it. The only difference between them and the use of suitable paints is recent historical context. There is no reason, save tradition, why paint, sealed colored pencil, plastic resins and other durable surface coverings should not be used. Where esthetic choice demands and a durable medium is available, that medium should be used if appropriate to the intent of the work. Paint, for example, is faster to apply than many patinas and presents far fewer safety problems. Let the demands of the piece decide surface coverings and not tradition. And if you choose to use patinas care and enjoy the process.
Charles Lewton-Brain is a jeweler currently teaching at the Alberta College of Art.