Statuary finishes are conversion coatings. In conversion coatings, the metal surface is either converted into a protective film, usually an oxide or sulfide of the metal involved, or a compound is precipitated which forms a surface film.
The use of chemical solutions is generally termed "oxidizing," although the oldest method and the one which produces the widest range of brown to black stages on copper alloys actually produces not an oxide but a metal sulfide finish by the use of alkaline sulfide solutions. Originally liver of sulfur was employed, this being a crude mixture of potassium polysulfides and thiosulfate, also called potassium sulfuret.
Modifications of these formulas called for the use of sodium, potassium, barium, and ammonium sulfides, which were claimed to produce different shades, but almost all sulfide colors are now produced from solutions of polysulfides which are sold in concentrated form under a number of trade names, usually called "oxidizing liquid."
All sulfide films require wet or dry scratch brushing for good appearance and will look better longer if protected by oiling, waxing or, more permanently, by a good top coat of clear lacquer. The desirable contrast in color can be produced by scratch brushing with a pumice paste, or by use of a "greaseless" polishing compound on a buffing wheel. In any case, the sulfide solution employed should be fairly dilute, since concentrated solutions can result in a brittle film which may be non-adherent.Back to Top
The metal surface should be degreased with trichlorethylene, or similar solvents. This not only cleans the surface, but enhances the cutting quality of abrasives if subsequent mechanical finishing is to be done before applying the color.
Clean to a bright satin finish using a mixture of 5% oxalic acid and water together with fine India pumice powder. The cleaning should be done using a fairly stiff short-bristled cleaning brush in the direction of the grain. The metal should be recleaned using the above mixture and a wet, virgin clean white cloth and applied in conformance with the original motion. The work should be cleaned with a virgin cloth rinsed in clean clear water and allowed to dry.
Finish the metal with abrasive belts, abrasive pads or wheels, or greaseless abrasive compounds on portable buffing wheels.
As the final operation, give the metal a hand rub with a fine abrasive pad (e.g., Scotch-Brite) and a slurry of pumice and water in order to insure complete removal of all surface films of oil and grease. Then remove all. traces of pumice by wiping with a clean damp cloth or sponge.Back to Top
Statuary Finishes On Bronze
Statuary finishes can be produced in light, medium and dark brown depending on both the concentration and the number of applications of the coloring solutions.
Solutions of 2%-l0% aqueous ammonium sulfide, potassium sulfide ( liver of sulfur) or sodium sulfide ( liquid sulfur) are swabbed or brushed on. Oxide pretreatment may be employed to enhance adherence. Final hand toning or blending may be required to achieve acceptable color match and color uniformity.
The following should produce a medium shade of brown:
- Ammonium Sulfide Process
- With a sponge or pad fairly well wrung out, swab on a thin film of a solution containing 5% to 10% by volume of polysulfide (dark) and water.
- Rinse thoroughly.
- Immediately follow this with a slightly heavier film of either a 5% solution of copper sulfate in water or a 0.5% solution of sulfuric acid in water. Apply by swabbing with a sponge or pad.
- Do not contaminate the solutions by using the same sponge to apply both.
- Rinse thoroughly.
- Tone by rubbing in the direction of the grain with a fine abrasive pad (e.g., Scotch-Brite) while still wet.
- Remove solution residue by wiping with a clean damp cloth or sponge and dry.
- Repeat until the desired depth of color is achieved.
- Alternatively, small objects may be immersed in the ammonium polysulfide copper sulfate solutions.
- Potassium Sulfide Process
- One gallon of 1.5% sharp water (2 oz oxalic acid in 1 gallon tap water).
- Fine beach sand.
- Potassium sulfide ( liver of sulfur) mixed in tap water (1/4 lb. liver of sulfur to 1 gallon water).
Clean the metal. Wipe down the solution of potassium sulfide using a virgin clean white cloth, in the direction of the grain of the metal. Wash down with a clean white towel wrung out in sharp water. Apply fine beach sand with another clean wet towel going against the grain to even out the color.
This procedure is followed until a medium statuary color is attained. It may have to be repeated several times before uniformity appears. After the desired uniform color is attained, neutralize the work with a wash of clear water.
- Other Processes/Procedures
- Clean with fine pumice (0, 1/2) on a clean cloth moistened with a 10%-20% solution of oxalic acid and water.
- Wipe off with a clean soft cloth
- Apply a 5%-l 0% solution of potassium sulfide or ammonium sulfide using another soft cloth dipped in the solution and well wrung out.
- Follow while still wet with a wipe of sharp water (about 2 oz of oxalic, sulfuric, or nitric acid in 1 gallon water) using a clean soft cloth well wrung out.
- Repeat steps 3 and 4 to achieve a depth of color slightly darker than the desired shade.
- Relieve the surface by rubbing with fine beach sand on a clean damp cloth until the desired color is reached.
- Rinse and dry.
- Clean copper with pumice and water or pumice and solvent to remove all dirt, grease, oil or tarnish.
- Brush entire surface with a 2% solution of liquid ammonium sulfide (technical grade) in water.
- Let dry. Even out color by rubbing lightly with pumice and water, using a stub or fine brass wire brush.
- Repeat steps 2 and 3 to obtain the desired color.
Regularity is the key to a successful maintenance program. A schedule should be arranged providing periodic cleaning with regular inspections in the interim. The schedule should differentiate between interior and exterior surfaces and those surfaces subject to handling, scuffing and abrasion. When a regular maintenance program is followed, most installations can be maintained by oiling or waxing, some by lacquering and a few by polishing.
Surfaces prefinished or naturally weathered to the statuary bronze shades should be maintained by periodic oiling with Lemon Oil, U.S.P.; Lemon Grass Oil, Native E.I.; or a high grade paraffin oil.
Mixtures of Carnauba wax and wood turpentine or beeswax and wood turpentine as well as quality commercial paste waxes have been found satisfactory. The comparative costs of waxing versus oiling should, however, be weighed.
Oil and wax coatings look best when applied with a well-impregnated, clean soft cloth followed by rubbing with a second, clean soft cloth to remove excess oil or wax. Frequency of oiling or waxing is as important as the oil or wax used. Newly installed metal should be oiled weekly for the first month in order to build up a protective film. Metals subject to heavy traffic should be oiled or waxed at one to two-week intervals. Where traffic is moderate to light, monthly treatment may suffice. In non-traffic areas, quarterly or semiannual applications are feasible.
Considering a typical building entrance, door handles, push plates or bars, and kick plates as well as the door stiles and rails, would normally be exposed to heavy traffic. The door frame and adjacent window wall framing usually receive less handling and would be considered a moderate to light traffic area. Transoms, canopies and similar metal elements normally out of reach would be classed as non-traffic areas.Back to Top
Bronze and other copper alloys can be restored to their original appearance even after years of neglect. Restoration of neglected surfaces may require the advice of specialists engaged in maintenance work.
To restore statuary finishes, the surfaces may be cleaned with a 5% oxalic acid and water mixture together with finely ground India pumice powder. Wipe dry with soft clean cloths and apply the statuary finish solution as outlined.Back to Top
Long-term protection can be achieved by applying a clear organic coating. Air-drying formulations are the most convenient to use and among them the INCRALAC formulation has proven to be the most protective.
INCRALAC is an air dry, acrylic lacquer for field or shop coating of copper and copper alloys. In research initiated by the International Copper Association, Ltd.), INCRALAC provided the best protection of all air dry coatings tested.
When applied to a properly cleaned metal surface, INCRALAC provides excellent protection indoors or outdoors, even in highly corrosive industrial and marine atmospheres.
The use of abrasive pads (e.g., Scotch-Brite) followed by washing with a cleaning solvent, provides a surface for maximum performance. Steel wool should not be used because it sometimes contains a corrosion inhibitor which may cause discoloration later on.
If abrasive pads are not available, the surface should be thoroughly washed with a solvent or alkali cleaning solution, or by vapor degreasing.
INCRALAC is designed for spray application and should not be brushed. Conventional spray equipment can be used, applying first a mist coat, followed by a wet coat. Two coats are recommended, with at least 30 minutes air dry between coats.