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Historic Preservation - Technical Procedures

Spectitle:

Patinizing Exterior Copper Elements

Procedure code:

0575003R

Source:

Developed For Hspg (Nps - Sero)

Division:

Metals

Section:

Ornamental Copper

Last Modified:

02/24/2012

Details:

Patinizing Exterior Copper Elements



PATINIZING EXTERIOR COPPER ELEMENTS


PART 1---GENERAL

1.01 SUMMARY

    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
         water.  

         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
         FORMULA.

    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

         3.   Submittals

         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).

1.02 DEFINITIONS

    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%.


PART 2---PRODUCTS

2.01 MATERIALS

    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
              expectorant.

         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
              ammonia*.

         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
         chloride above.

    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

2.02 EQUIPMENT

    A.   Soft wiping cloths

    B.   Soft bristle brushes

    C.   Heavy gloves and protective gear

    D.   Paint spray gun


PART 3---EXECUTION

3.01 EXAMINATION

    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
         these first.

3.02 ERECTION/INSTALLATION/APPLICATION

    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
         of water.

         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
         yards.  

         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