Definition

• Patina.  Surface coloration; any of a variety of finishes that are applied to a metal surface to give it color and protective finish. The term derives from the green finish
  – incrusted patina – of bronze buried underground. This process, known since ancient times, is a very slow chemical reaction; incrusted patina forms from the copper in the bronze in contact with sulfur in the soil or surroundings to form green copper sulfate. Buried bronze coins and statues give the best evidence of incrusted patina.
  Fortunately the two metals most suitable for applying artistic patinas as a modern process are also excellent in casting and striking – copper and silver. Each metal composition, however, will have its own patina requirements. When artificially applied, silver and silver alloys have a narrow range of color patinas, grays and blacks. Copper and bronze have a very wide range of color patinas, predominantly browns and greens.
  With extreme skill, however, a patina finisher (a patinaeur) can color bronze any color of the rainbow as well as white and black. Also of importance, if a patina color is unsatisfactory, the piece can be repatinated –the old patina can be removed and/or a new patina can be applied over it.
  Patinas add permanent color to a metal surface, give it protection and enhance the beauty of the design. It does not add texture to the surface, nor does it create relief. Patina is not paint, it is not a coating, it is not transparent. Instead a patina will color the top layer of molecules of a metal surface by changing their chemical properties. Thus it is permanent unless the surface is treated with chemicals again.
  Patinas for medallic items are the same as those applied to statues. However, since medallic items are comparatively small, their processing can be done on a production line bases and often more varied. Statues are usually one of a kind with larger surface area, and for repatinating must be done on location where the sculpture cannot be removed to a workshop. Medals are processed for patina work in the finishing department of a medal plant and done by the batch process under far more controlled conditions.
  Techniques of patina applications.  Patinas are not simple immersions of medals in some liquid that magically transforms a raw medal into a colored medallic item, and to dip the medal in another liquid to get a different color (like Easter egg coloring). Instead a patina finish is a combination of the coloring agent – a chemical – and technique of its application plus the skill of the finisher.
  Typical patina applications are wet wheel, torch finish, brush finish, immersion finish, appressed finish and vapor finish. Brush finishes have a wide range of applications with the use of different brushes: stipple brush, wire brush, bristle brush, scratch brush and bristle wheel. The chemical can be applied at room temperature, hot, boiling or applied with a blow torch. Application methods differ widely adding to the many variables of these techniques. Detailed descriptions are given in the entries on these patina applications.
  Variables affecting patinas.  The alloy composition of the metal object to be
  finished is the foremost variable, followed by the chemical(s) used to effect the color desired; temperatures and time are also major variables in patina work. What metals make up the object's alloy? The tiniest amount of lead or phosphorous in the bronze alloy, for example, will greatly alter the outcome of the applied patina. The pH factor (degree of acidity) of the chemicals is always important in acid patinas.
  How fresh are the chemicals? Are they full strength, or have they been diluted?
  Some patinas require the chemical to be applied to a hot surface, or the chemical itself to
  be boiling. Is the temperature above a minimum, or does it require a precise degree? How long should the chemical be applied?  Should it be applied with a cloth, a sponge, or a brush? Should it be brushed on or stippled? With a bristle brush or wire brush. Does it have to be buffed? With a wet wheel or a crimp-wire wheel?  Or does it require the item
  to be immersed?  For a few seconds?  Or for several days? Is a single color satisfactory? Or are two or more colors required? How strong must the undercolor be? Does the overcolor have to be translucent?
  While extensive experience is required for applying a patina, knowledge of the numerous variables is also required and how to correct an unsatisfactory color. Fortunately most objects can be repatinated until the correct control of variables can be mastered and the desired color can be achieved. A particularly attractive patina process, perhaps developed in secret, is known as a proprietary patina if the developer wishes it to remain secret. But because of the large number of patinas published in Hughes & Rowe (Bibliography T11) it could probably be duplicated by others.
  Color samples.  An astute artist will want to exercise control over the patina of any medallic item he or she creates. The artist will often want to determine which patina his creations are to receive. Often the artist requests to be shown several proposed patinas – color samples – applied to the first cast or struck items so he can make a selection of the most appropriate patina finish.
  Such a selected patina will become a patina standard toward which all produced later will be matched for color control. Even in a batch process, there are so many variables in patina work that no two specimens are exactly alike. Thus the patina standard is the color toward which all those produced are to be matched. (The degree of deviating from that color standard among the entire production run is a decision of the finishing department foreman.)
  About 1910 the Gorham Company created a set of sample color panels in an attempt to standardize patina finishes (for small bronze statues). It also served the useful purpose of being able to show these panels to prospective customers to ascertain which patina finish to select that the firm could provide.
  Types of patina.  After extensive literature research and their own experiments, Hughes and Rowe lists 1,126 possible metal patinas. Of this number they recommended 344 formulas. Of this there are about four dozen that are widely employed by experienced patinaeurs in the art field (for statues and medals). An exact number cannot be stated because so much of what these craftsmen do is either proprietary (fine-tuned to their own needs) or their formulas and applications have not been published.
  For numismatic purposes we can examine the patinas in two American art medal series and reveal these patinas and the process for their creation. These series are The Society of Medalists (129 medals) and the Great Religions of the World (18 medals). Here are the names and a typical bronze medal bearing that patina:
                      Bronze Patinas                       
                                                         Typical Medal  
    Patina Color:                              SoM         GRW 
    Blue (turquoise). . . . . . . . . . .      –         Catholic
    Bronze, light.. . . . . . . . . . . .        #1      Salvation 
                                                                        Army
    Bronze, light, glossy . . . . . . . . #101                    
    Brown, dark.. . . . . . . . . . . . .      #8  Presbyterian
    Brown, dark with second color   #41    Methodist
    Brown, chocolate. . . . . . . . . . .  #14       Lutheran
    Brown, chocolate with green highlights #11         
    Brown-gray. . . . . . . . . . . . . .     #84           Baha'i
    Brown, light with green overcolor . –          Islam
    Brown, light with silver highlights .  #87 Judaism
    Brown, light with medium highlights . #83           
    Brown, mahogany.. . . . . . . . . . .    #2                    
    Brown, medium.. . . . . . . . . . . .     #5             
    Brown, russet.. . . . . . . . . . . .       #25    Buddhism
    Brown with green highlights . . . . #3             
    Brown, white. . . . . . . . . . . . .          –      Christian
                                                                       Science
    Copper antique. . . . . . . . . . .     .#122             
    Gray-bronze.. . . . . . . . . . . . .       #90    Episcopal
    Green.. . . . . . . . . . . . . . . .           #70             
    Green-blue. . . . . . . . . . . . . .        #73             
    Green with brown highlights . . .   #6             
    Green, pale.. . . . . . . . . . . . .        #85             
    Metallic bronze.. . . . . . . . . . .     #74             
    Red bronze. . . . . . . . . . . . . .       #65             
    Red-brown, pale.. . . . . . . . . .   .  #40             
                                                           
    Plated patinas.                                        
    Goldplate (even yellow).. . . . . . . #20             
    Goldplate, deep yellow. . . . . . . .   #4              
    Goldplate, reddish (rose gold). . . #17             
    Silverplate antique.. . . . . . . . .      #12             
    Silverplate, steel gray.. . . . . . .      #42            
  From the above chart one can observe the preponderance of browns and greens for bronze patinas. A wide range of browns are in an experienced patintaeur’s repertoire.   This is due to the nature of copper and its reaction with common patina chemicals. But it should be noted that a wide variety of other colors can be achieved as well.
  Further, that plated items (more so silverplate than goldplate) can be given patinas just as can nonplated items and silverplating is infrequently one of the early steps of a distinctive patina. For example, the patina finish for Issue #42 of The Society of Medalists (by Cecil Howard) was first silverplated then oxidized with liver sulfide.
  How To Further Beautify A Patina
  Add an enamel.
  Add a second patina color.
  Do both sides in a different color.
  Add a colorful object to the surface.
  Add a stipple color effect (with spots of color).
  Use masks and make in three or more patina colors.
  Add a highlight with a contrasting color.
  Add color with a tint lacquer.
  Add a clear overcolor.
  Add an undercolor.
  Patina chemicals.  The following chemicals are some of the more frequently used in patina finishes:
  Ammonium carbonate  (NH4)2 CO3.H2O.  A mixture of ammonium bicarbonate and carbonate. Used for a bronze patina of bluish-green color.  Also called “hartshorn.”
  Ammonium chloride  (NH4Cl) A patina solution for coloring bronze a verde antique green.  Also called “sal ammoniac.”
  Ammonium sulfide  (NH4)2SO4  An excellent darkening agent for bronze and silver in highlighting during a finishing operation. The objects to be darkened are immersed in this chemical for less than ten seconds – the sulfide is the source of sulfur as the darkening agent.  Also called “sulfate of ammonia” and “sulphuret of ammonia.”
  Barium sulfide  (BaSO4)  Used as a coloring agent for bronze medals for a light brown color, called “Old English.”
  Copper chloride  (CuCl2)  A patina coloring chemical producing yellowish-green color on bronze.  Also called “cupric chloride.”
  Copper nitrate  (CuNO3)  Used for dark blue and green patina coloring on bronze.  Also called “cupric nitrate”
  Copper sulfate  (CuSO4)  Colors bronze green. It is the green corrosion on copper items in an atmosphere exposed to sulphur and moisture over long time.  The composition of incrusted patina.
  Ferric nitrate  (Fe(NO3)2)  For use only by very experienced finishers; colors a dark chocolate color. Care must be used, however, as some of the nitrate salts can spot the surface.
         Liver sulfide  (K2S)  Used for a bronze patina on statues and medals; it produces
  a color from red brown to dark brown.  Also called “liver of sulfur,”
  “potassium sulfide,”  “sulfurated potash.”
  Liquid sulfur.  (S)  Quickly turns bronze and silver a dull black.
  Oil of lavender.  A bronze patina of pale ashen green color, formed by adding yellow pigments to oil of lavender.
  Potassium nitrate  (KNO3) is a patina solution for turning bronze a dark red color.  It is most used for tempering tool steel (heat treating dies) and for chemical analysis. It is also called “saltpeter.”
  Potassium permanganate  (KMnO4)
  History and cataloging.  Bronze statues have been given protective finishes for centuries; often with patinas that are a distinctive color. But it is only since the 1880s that bronze medals have been given patinas for their distinctive color. The French were the first to develop patinas for art medals. This came about from the introduction of sandblasting a metal surface, then oxidizing and relieving this surface (known as french antique). To carry this one step further by using sculptural patinas to medals was a natural progression.
  In cataloging work, medals with a color patina should be so identified. This has
  been observed in the past but only a rudimentary way, as the basic color of the metal finish. It is hoped that color standards can be established within the medallic field in the future and these utilized in cataloging art medals.
  Patina terminology.  Most patinas are named for the end color the formulas and applications produce. Even thought this technology is more than a century old it is still evolving in its terminology. An extensive word list including patina terms are given in the entry finish and finishing.
  The dictionary plural of patina is patinae, but modern workers say patinas. Both are correct. Also the verb is patinate, and past tense is patinated. The noun form of an existing metal coloration is either patina or patination. To do over a patina is repatinate.
  References:                                                                                                                                 
  T1  {1878} Spon.
  T2  {1892} Hiorns.
  T3  {1904} Austen.
  T4  {1907} Hiscox.
  T6  {1925} Field and Bonney.
  T7  {1962} Fishlock.
  T11 {1983} Hughes and Rowe.
  T12 {1985} Brachert.
  O37 {1977} Julian, pp xxxv-xxxvii.

  excerpted with permission from

  An Encyclopedia of Coin and Medal Technology

  For Artists, Makers, Collectors and Curators

  COMPILED AND WRITTEN BY D. WAYNE JOHNSON

  Roger W. Burdette, Editor

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