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Green Patina Finish Activity

I. Purpose:
To create a protective coating of a blue-green patina on a small piece of copper jewelry or statuary.

II. Background information:
The decorative natural protective coating of a blue-green patina characterizes many of the older copper roofs, statues and other copper surfaces. The copper roofs of many ancient cathedrals, the Statue of Liberty, and the dome of the South Carolina Capitol are all covered with a green patina finish. Patina is actually the product of copper corrosion on exposure to the weather. The chemicals in the atmosphere produce patinas in a process that require at least five years in an industrial or coastal environment and many more in rural areas. Because of the time required to do this, much research has been done on artifical patination.

The rate of patina development and its chemical composition are a function of the prevailing atmospheric conditions. The main constituent of patina is a mixture of basic copper carbonate and sulfate. This is the mineral brochanite, Cu4SO4(OH)6. In industrial areas the basic copper sulfate predominates. In coastal areas copper chlorides prevail while in rural areas it is predominately basic copper carbonate. The color variations from greenish to blue-gray depend on the difference in chemical composition.

The weathering process begins with the tarnishing of the copper surface into a uniform brown coloration after a few weeks. With continued weathering a deeper hue develops until eventually a green coating develops. In cities with much pollution the time required may be as little as ten years but in less polluted rural environments the process can take more than twenty years.

The decorative coating protects the underlying copper from further corrosion by acting as a barrier between the atmospheric chemicals and the copper. As the rate of patina formation decreases, the corrosion essentially stops.
If the patina is damaged the areas will self-repair as the exposed copper weathers again. In artifical patination, success seems to depend on the application method, weather treatment conditions, and most importantly, on the exposure climate.

Because of the number of variables involved, the chemically-induced patinas have several shortcomings. They are prone to flaking. They excessively stain adjacent materials and lack color uniformity over large surface areas. Since the process may require five years or longer, these factors should be considered when attempting to duplicate the natural patinas.

Artificial patinas for architectural applications such as copper roofs and large statuary require that the chemical solution must be brushed or sprayed upon the large surfaces. Immersion and fuming techniques may be used for smaller objects such as decorative accessories and jewelry.

III. Materials:
copper object
1M H2SO4
cotton swabs (q tips)
rubber gloves
TSP (tri-sodium phosphate) - found at paint stores
Lemon or raw linseed oil
For method A For method B For method C
ammonium sulfate (tech. Grade) ammonium chloride lemon juice
copper sulfate brush salt
concentrated ammonia mist bottle ammonia
plastic bucket vinegar
plastic spray bottle
stirring rod
For method D (fumed)
clear box or aquarium
shallow cups

IV. Procedure:
A. Preparation of solutions
1. Method A (sulfate)
The solution should be prepared in a plastic bucket or other corrosion-resistant container.
Dissolve 111 grams of "technical grade" aluminum sulfate in one liter of tap water. Add 3.5 grams of copper sulfate (blue vitriol). Add the concentrated ammonia solution slowly, while stirring the solution. Do this with adequate ventilation. Be exact with the quantity of ammonia! Place in spray bottles for application. One that delivers a fine mist is best.

2. Method B (chloride)
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 surface.
3. A patina made from common household materials:
¼ cup lemon juice
¼ cup table salt
¼ cup household ammonia
¼ cup vinegar
Apply to clean metal and let it react.
4. Fumed method
Shallow cups with ammonia and vinegar solutions

B. Cleaning of surface
The surfaces must be free of any dirt, oil or grease, which will interfere with the chemical action of the solution.

1. Go over the surface with a commercial chemical metal cleaner, TSP, following directions on container. After a thorough rinsing, the rinsing water should spread uniformly without beading.

2. Remove the oxide coating by swabbing the surface of the copper with a cold 1M sulfuric acid solution. WEAR RUBBER GLOVES AND TAKE CARE TO PREVENT SPILLING ANY ACID. Thoroughly wash any spill with water. Immediately after swabbing, the surface should be thoroughly rinsed with clean water.

C. Application

Method A (sulfate)

1. Spray the clean object rapidly, using fine drops. Use too little, rather than too much!

2. Allow the solution to dry after first spraying (about 10 to 15 min on a warm, dry sunny day; longer on cool or cloudy day). Spraying and drying are repeated five or six times. The color does not show up immediately but should appear as covered with a "glassy" coating. The ideal weather conditions following treatment would be very high humidity. Placing it near a humidifier would result in a blue-green patina if left undisturbed for at least six hours.

Method B (chloride)

1. Brush or spray on clean surface. Several applications may be required.

Method C (common household)

1. Apply to clean metal and let it react until desired color is reached. Rinse excess chemical away.

Method D (fumed)

1. Place copper object on bricks or something, in an enclosed container, plastic box with lid or aquarium turned upside down with the vinegar and ammonia each in shallow cups - at a level below the copper so that the fumes will travel up to the copper. You may watch the copper turn colors through the glass or plastic. Remove when it is a pretty color. If left too long or if chemicals are too strong, the copper will turn black.

D. Maintenance:
To arrest further weathering, apply a suitable oil, raw linseed oil or lemon oil. Applying two thin coats may preserve the finish in excess of ten years.

V. Observations:|
Appearance before cleaning___________________________________
Appearance after cleaning_____________________________________
Changes during application____________________________________
Final appearance_____________________________________________
Any signs of flaking or streaking________________________________
Other observations of note______________________________________

VI. Questions:
1. Find and write the formula for basic copper carbonate.
2. What causes the corrosion and patina formation on copper in coastal areas?
3. What industrial pollutants contribute to patina formation?
4. What industrial sites are located in your area?
5. Locate and name five buildings, statues or artifacts that are examples of natural patinas.
6. Give several examples of artificial patinas that are common today.

VII. Hazards:

1. Wear gloves when handling the TSP as the solution is strongly basic and caustic.
2. Wear rubber gloves and take care when using the sulfuric acid as it is corrosive. Wash thoroughly if there is a spill.
3. Concentrated ammonia is corrosive so prepare solution for method A under a hood or with adequate ventilation.

VIII. References:

Young, R of Sculpt Nouveau

This activity was developed for Charleston County Public Library by Norma L. Ashburn, retired chemistry teacher (Hanahan High School, Berkeley County, South Carolina). It is intended for use in high school chemistry classes and is especially relevant to "Chemistry and Art", the theme of National Chemistry Week, 2001.