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Historic Preservation - Technical Procedures
Patinizing Exterior Copper Elements
Developed For Hspg (Nps - Sero)
Patinizing Exterior Copper Elements
PATINIZING EXTERIOR COPPER ELEMENTS
A. This procedure includes guidance on patinizing exterior
copper architectural elements when these have been
replaced with new ones and need to match the color of the
existing surface or other elements.
B. The development of the natural green patina on a copper
sheet or architectural element can take from ten to
twenty years, depending on the location and the
atmosphere. To speed up this process, patina can be
produced artificially - by chemical solutions. The
patina solution listed in this procedure includes the
mixture of copper ammonium chloride, ammonium sulfate and
NOTE: THERE ARE NUMEROUS TYPES OF PATINATION FORMULAS
RECOMMENDED FOR DIFFERENT USES. FOR A LIST OF SOME OF
THESE FORMULAS, SEE 05030-02-S. THE RHPO SHOULD BE
CONSULTED FOR GUIDANCE IN SELECTING AN APPROPRIATE
C. See 01100-07-S for general project guidelines to be
reviewed along with this procedure. These guidelines
cover the following sections:
1. Safety Precautions
2. Historic Structures Precautions
4. Quality Assurance
5. Delivery, Storage and Handling
6. Project/Site Conditions
7. Sequencing and Scheduling
8. General Protection (Surface and Surrounding)
These guidelines should be reviewed prior to performing
this procedure and should be followed, when applicable,
along with recommendations from the Regional Historic
Preservation Officer (RHPO).
A. Copper is initially bright reddish-brown in color, but
when exposed to the atmosphere, it acquires a protective
patina that turns from brown to black to green over an
8 to 10 year period.
B. The patina is a copper carbonate or copper sulfate formed
on the surface of the metal when hydrogen sulfide
combines with oxygen or sulfur dioxide. The patina is
actually a thin, tough layer of natural corrosion that
usually prevents deeper and deeper layers of corrosion
(such as rust, which can totally consume iron) to form
because of further exposure to the atmosphere; therefore,
even though copper corrodes, it is corrosion-resistant.
C. The color of antique copper, which is a little more
orange than that of new bronze, was much admired in the
late 19th century. Victorian cast-iron hardware was
sometimes copper plated, although brass-plated hardware
was more common. Cast-iron stair railings and newel posts
were sometimes copper plated.
1.03 PROJECT/SITE CONDITIONS
A. Environmental Requirements:
1. Do not attempt patinizing of the copper in raining
or foggy weather.
2. Ideal relative humidity for patinization is between
85% and 100%.
NOTE: Chemical products are sometimes sold under a common
name. This usually means that the substance is not as pure as
the same chemical sold under its chemical name. The grade of
purity of common name substances, however, is usually adequate
for stain removal work, and these products should be purchased
when available, as they tend to be less expensive. Common
names are indicated below by an asterisk (*).
A. Copper Ammonium Chloride:
1. Ammonium chloride is a white crystalline volatile
salt that is used in dry cells and as an
2. Other chemical or common names include Ammonium
hydrochloride; Chloride of Ammonia*; Hydrochloride
of Ammonia*; Muriate of Ammonia*; Sal Ammoniac*.
3. Potential hazards: TOXIC; CORROSIVE TO FLESH;
CORROSIVE TO CONCRETE, STEEL, WOOD OR GLASS.
4. Available from chemical supply house, dry cleaning
supply distributor, drugstore or pharmaceutical
supply distributor, or hardware store.
B. Ammonium Sulfate:
1. Other chemical or common names include Sulfate of
2. Potential hazards: CORROSIVE TO CONCRETE, STEEL,
WOOD OR GLASS.
3. Available from chemical supply house, drugstore or
pharmaceutical supply distributor.
C. Sal ammoniac (a paste in cake form): See ammonium
D. Trisodium Phosphate:
NOTE: THIS CHEMICAL IS BANNED IN SOME STATES SUCH AS
CALIFORNIA. REGULATORY INFORMATION AS WELL AS
ALTERNATIVE OR EQUIVALENT CHEMICALS MAY BE REQUESTED FROM
THE ENVIRONMENTAL PROTECTION AGENCY (EPA) REGIONAL OFFICE
AND/OR THE STATE OFFICE OF ENVIRONMENTAL QUALITY.
1. Strong base-type powdered cleaning material sold
under brand names.
2. Other chemical or common names include Sodium
Orthophosphate; Tribasic sodium phosphate;
Trisodium orthophosphate; TSP*; Phosphate of soda*;
(also sold under brand names such as).
3. Potential Hazards: CORROSIVE TO FLESH.
4. Available from chemical supply house, grocery store
or supermarket or hardware store.
E. Clean, potable water
A. Soft wiping cloths
B. Soft bristle brushes
C. Heavy gloves and protective gear
D. Paint spray gun
A. Before proceeding with steps to artificially patinize
copper elements, it is important to first inspect and
check for other damages. Clean surface and/or repair
A. Wipe the surface clear of dust and other debris with a
clean soft cloth or brush.
B. Remove all greasy patches by swabbing with a mixture of
trisodium phosphate in water. Rinse the treated area
with clean, clear water and allow to dry.
C. If possible, rather than washing new copper, allow it to
weather through two or three heavy rains, but not longer
than three months.
D. Allow the copper sheet/architectural element to dry
before application of the patina formula.
E. Mix a solution composed of 6 ounces of copper ammonium
chloride, 3-1/2 ounces of ammonium sulfate, and 1 quart
NOTE: The double salt, copper ammonium chloride produces
a pure green patina, and the addition of ammonium sulfate
to the solution gives the true-green shade of the
naturally formed patina.
F. Apply the solution when outside conditions are dry by
using a paint spray gun to produce a fine dispersion of
the liquid at a concentration of 1 pint per 5 square
1. At this concentration the surface should be covered
with a uniform layer of droplets. Avoid undue
coalescence of the droplets, which may occur if too
much liquid is applied.
2. If the humidity is low, dampen the prepared surface
with a fine mist or spray of water. Spray the
surface so that it will dry within 5 minutes.
Repeat this process at intervals of 2 hours or more
until the required patina has formed.
G. After waiting, the surface should be free from any
appreciable wetting until a certain amount of weathering
takes place. Premature wetting makes the patina flake
and peel. The best results are obtained with a relative
humidity of between 85% and 100%.
H. To age small new pieces in order to match the old, apply
a mixture of sal ammoniac (a paste in cake form) and
water to the surface.
NOTE: Sal ammoniac is used to clean soldering irons and
it is not harmful to copper.
END OF SECTION