Green Patina Finishes

The much admired natural protective coating of a blue-green patina characterizes older copper roofs, including ancient cathedrals as well as bronze statues and other copper metal surfaces exposed to the weather. Because of the time required to achieve this, much research has been done on artificial patination, with varying success.

In natural patination the major coloring agent in the film is basic copper sulfate. Carbonate and chloride salts of copper may also be present in varying concentrations. In seacoast locations, chloride salts may form an essential part of the patina film. The basic chloride salts of copper are not only fairly soluble, but photosensitive as well.

In artificially producing or accelerating the formation of a patina, success seems to depend on the manner in which the solutions are applied, the weather conditions under which the treatment is carried out, and perhaps most important, on the climate to which the treated surfaces are exposed.

Because of the number of variables involved, chemically induced patinas are prone to lack of adhesion, excessive staining of adjacent materials or inability to achieve reasonable color uniformity over large surface areas. These potential shortcomings should be considered when attempting to artificially duplicate a weathering process which may take from five to seven years in coastal or industrial environments, longer in rural areas, and may perhaps never develop in some climates and atmospheres.

Artificial patinas for architectural applications such as copper roofs, grilles, and statues have an inherent requirement which dictates that the solution has to be one which can be brushed or sprayed because of the large surface areas normally involved.

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The copper surfaces to be colored must be clean, as any dirt, oil or grease on the surface will interfere with the chemical action of the solution. This involves removal of the residual film of oil left on copper and brass sheets from mill rolling operations, and fingerprints and dirt deposited on the surface during handling and installation.

A few hard rainstorms may clean the surfaces sufficiently to start operations. However, it is always advisable, particularly if the coloring is to be done immediately after installation, to go over the surface with a commercial chemical metal cleaner. Cleaners of the trisodium phosphate type should be satisfactory. Avoid cleaners which leave a coat of oxide on the copper surface.

Follow cleaning with a thorough rinsing to remove all traces of the cleaning compound. If cleaning has been properly done, the rinsing water will spread uniformly without beading (the formation of globular droplets); in other words, the water should wet the copper surface uniformly. If necessary, cleaning should be repeated until this condition is obtained.

Oxide film on the copper will cause poor adherence of the patina. Copper roofs which have weathered for six months or more should have the oxide film removed before the coloring operations start. This is done by swabbing the surface with a cold, 5%-10% solution of sulfuric acid.

NOTE: The operator must wear rubber gloves and take care to prevent spilling any acid on himself or on adjacent wood or stonework.

Immediately after this swabbing, the surface should again be thoroughly rinsed with clean water. This should leave a roof surface, whether old or new, in good condition for coloring.

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Of the three basic processes for accelerated patina formation, one uses a sulfate solution and two utilize chloride salts. Their formulas are as follows:

Ammonium Sulfate
per liter per gal
Ammonium sulfate 111 g 15.0 oz
Copper sulfate 3.5 g 0.5 oz
Conc. ammonia 1.6 ml 0.2 fl oz
Tap water 1.0 l 1.0 gal

The quantities required per 1,000 sq. ft. of roof surface are:

Ammonium sulfate 6 lb
Copper sulfate 3 oz
Concentrated ammonia 1.3 fl oz
Tap water 6.5 gal
Total solution 7.25 gal

The ammonium sulfate should be "technical grade." Agricultural grade may be used if the solution is filtered to eliminate dirt. Copper sulfate is commonly known as blue vitriol the concentrated ammonia should have a specific gravity of 0.900. One fluid ounce of concentrated ammonia contains 0.936 oz by weight of ammonia.

Preparation of the Solution

The solution should be prepared in a corrosion resistant plastic-lined container. Wooden barrels and tubs are also satisfactory if all exposed metal parts are lead covered.

First dissolve the ammonium sulfate in the water. When completely dissolved, add the copper sulfate. This is best done by removing a few gallons of the ammonium sulfate solution and dissolving in it as much of the copper sulfate as will dissolve. Then pour this back into the original solution and remove another batch, repeating the operation until all the copper sulfate has been dissolved.

Then add the concentrated ammonia slowly, while constantly stirring the solution. It is important that the quantity of ammonia be exact, as the correct ratio of ammonia to water must be maintained.

Method of Application

The solution should be applied by spraying. A satisfactory sprayer is an ordinary, plastic or galvanized steel, garden-type tank sprayer, with the inside coated with bituminous paint. Spraying should be done rapidly, using a fine spray. Avoid large drops, which tend to run together, causing streaks. It is better to use too little rather than too much solution at a time.

Allow the solution to dry after the first spraying (about 10 to 15 minutes on a warm, dry, sunny day; longer on a cool or cloudy day). Spraying and drying are repeated five or six times.

The color does not show up immediately. When the spraying has been completed, the copper surface should appear to be covered with a "glassy" coating somewhat resembling a dark, heavy coat of varnish.

The development of color depends on suitable weather conditions. Rain within six or eight hours may wash away some of the solution before it has had a chance to act on the copper.

Ideal weather conditions following the treatment are a moderate to heavy dew, a light mist or fog, or other condition of high enough atmospheric moisture to give a relative humidity of 80% or more. The atmospheric moisture combines with the deposited solution to react chemically with the copper, and the desired blue-green patina results The colored layer should be of a satisfactory depth, if the action continues undisturbed for at least six hours.

Where this has occurred, the next rain should wash off the remaining deposit and bring out the blue-green of the patina. At first the color will be somewhat bluer than natural patina, but it should weather into a natural color in time; the time depending on climatic conditions.

Ammonium Chloride (salammontac)

Dissolve sufficient ammonium chloride crystals (commercial sal ammoniac) in water to form a saturated solution, that is, until no more will dissolve. Brush or spray on a thoroughly clean copper surface. Several applications may be required.

This formula was favored by Frank Lloyd Wright. Wright specified that the solution be mixed 24 hours prior to its use. Two applications were made with a lapse of 48 hours between the two. Twenty-four hours after the final application, the copper was sprayed with a cold water mist. Wright emphasized the fact that dry weather was required throughout the entire period.

Although this coloring method was used with apparent success on the Price Tower in Bartlesville, Oklahoma, it failed to last on the copper roof of a residence in Dallas, Texas. After five years, the blue-green patina which had developed initially completely disappeared, leaving the copper a light russet brown. The ammonium chloride solution tends to chalk and flake if applied too heavily, and is also apt to dissipate in heavy rain. Both factors may have contributed to this failure.

Cuprous Chloridelhydrochloric Acid

Dissolve the following in a small amount of warm water:

per liter per gal
cuprous chloride 164 g 22 oz
hydrochloric acid 117 ml 15 fl oz
glacial acetic acid 69 ml 9 fl oz
ammonium chloride 80 g 10.5 oz
arsenic trioxide 11 g 1.5 oz

When dissolved, dilute with water to either 1 liter (1,000 ml) or 1 gal.

Apply by spray, brush, or stippling. Store and use in nonmetallic containers; do not use aluminum containers.

NOTE: Wear suitable protective clothing and equipment. The solution is both acid and toxic.

It can be applied to either bright or weathered copper. If possible, the desired color should be attained in a single application. Reapplication - particularly in direct sunlight - may cause a reaction between the solution and the salts initially deposited, producing a smooth, hard, colorless film similar in appearance to varnish.

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No maintenance is required for an existing natural patina or one which is in the process of formation.

If a natural statuary finish is desired on copper, weathering can be arrested at the desired point by applying a suitable oil, e.g., raw linseed oil or lemon oil. Depending on the prevailing climatic conditions and the degree of exposure, the frequency of oiling may be at intervals of from one to three years. Instances have been recorded where the initial oiling applied in two thin coats has preserved the statuary finish in excess of ten years.

Copper and its alloys are made of nature's pure elements and as such harmonize with other natural materials with which people find an instinctive affinity. The fact that they change color as they weather gives them, and the buildings they adorn, an extra measure of life and character as first class building materials.

Copper, brass and bronze are resistant to destructive corrosion. The patina which forms naturally is in fact a protective film. The copper metals are lightweight, easy to work, easy to join, attractive and extremely durable. This accounts for their use for centuries for roofs, fascias, gutters, downspouts, flashing, storefronts, railings, grilles and other architectural applications of many descriptions.