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A GREEN PATINA
FINISH
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
Brushes
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)
brick
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
1. 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.
2. Method B (chloride)
1. Brush or spray on clean surface. Several applications may
be required.
3. Method C (common household)
1. Apply to clean metal and let it react until desired color
is reached. Rinse excess chemical away.
4. 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:
http://innovations.copper.org/2000/0009/bodleain_story.html
http://protection.copper.org/green.htm
http://www.asgla.com/patina.html
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. |