Lecture:  BMC2004 Genoa 2 December 2004:
Phoebe Dent Weil

PATINA on bronze sculpture
from the historical-artistic point of view

I want to thank the organizing committee for the great honor of inviting me to be here in Genoa at this historic conference, and for the opportunity to revisit a topic that I have found continually fascinating since the beginning of my conservation career:  namely patina and patination in reference to sculpture in general and to outdoor sculpture in particular.

PAIR OF EYES,  ancient Greek,
from a bronze sculpture

This image of a pair of eyes in the collection of the Metropolitan Museum of Art, originally belonged to an ancient Greek bronze statue.  I show them to  you here for several reasons (in addition to getting your attention) to emphasize the importance for conservator and curator alike of two kinds of “seeing”, namely:

  1. -that of maintaining an attitude of fresh observation, or what might be called “Zen seeing”, or looking with an eye uncluttered by prior assumptions at each object under consideration.  It is a fact that incalculable damage to outdoor bronzes occurred unnoticed over the period of a century simply because it was not expected. 
  2. The other kind of “seeing” that I would like to point out is equally important for the conservator: namely that of informed observation, or seeing with an understanding of historical, technical, aesthetic and scientific points of view—what Denis Mahon has described as “the genuine pooling, without distrust and arrière-pensée of the most diverse forms of knowledge”.

Both kinds of “seeing” are fundamental to aesthetic judgment, and are fundamental to the practice of conservation.  No one point of view is sufficient in and of itself, though some scientists might disagree.

The eyes serve also as an example of one of the peculiarities of sculpture as opposed to painting—and here we enter aspects of the old argument or paragone between painting and sculpture.  The eye presents a basic sculptural problem to the sculptor.  The 17th c. sculptor, Gianlorenzo Bernini was perhaps the first to articulate the problem stating: “…in order to represent the bluish color which people have around their eyes, the place where it is to be seen has to be hollowed out, so as to achieve the effect of this color and to compensate in this way for the weakness of sculpture which can only give one color to matter.” In Roman sculpture an important dichotomy appeared in the development of a more sophisticated, sculpture of pure form that did not rely on polychromy but rather on differentiated contours and surface textures to give the illusion of color—what Wittkower called “high-class” sculpture as opposed to the old tradition of Greek sculpture and the more popular and conservative tradition of polychromy, or what Hermann Born has described as “chromatic realism” that continued to exist as a parallel genre.

Two marble heads by Jean Antoine Houdon:
a) carved eyes, b) blank eyes

The tradition of both the “carved” eye and the blank un-carved eye continued to exist from late Roman times up to the present day and both are to be found in, for example, the sculpture of Michelangelo, Bernini, and here in these two heads by Jean Antoine Houdon.  Such subtle differentiations in form are dependent upon a more or less uniform surface coloration in order to be “read” visually.

Fontana delle Tartarughe and detail of a sculpture from the Washington Monument in Richmond, VA

The distortions you see here caused by the deposition of calcium salts on the surfaces of the late 16th c. Fontana delle Tartarughe by Taddeo Landini in Rome and on the right by typical corrosion patterns formed by industrial pollution on an outdoor bronze monument in Richmond, Virginia, interfere with a correct visual reading of the forms and thus inflict visual damage.

Cellini Perseo and Bindo Altoviti

Here you see on the left the Cellini Perseo before its conservation treatment by Giovanni Morigi in the mid-1990’s, and on the right, Cellini’s bust of Bindo Altoviti in the Gardner Museum in Boston.  The un-maintained, corroded surfaces of the outdoor bronze beside the sculpture that has always been indoors are a stark reminder of what has been lost when surface coloration has been allowed to deteriorate.

Foggini at work, pen and ink sketch, Uffizi

This small ink sketch showing the sculptor Foggini at work at his modeling stand on a small preparatory model in clay or wax.  It serves to illustrate another unique aspect of bronze sculpture important in any discussion of sculpture aesthetics, namely, that sculpture is a medium of translation.  The sculptor, Henry Moore has described the problem from the sculptor’s point of view as follows:

"In doing your sculpture you have imagined a certain quality in the bronze.  No sculptor works directly in bronze; you can’t take a solid   piece of bronze and cut it to the shape you want, so all sculptors who   intend having their work in bronze are working with a mental idea of what it is going to look like while they make it in some other material…"

The progress from clay or wax bozzetto to enlarged model, to mold making, casting, chasing and patination, is the metamorphosis that all bronze sculpture undergoes, and that the sculptor must rely on an army of specialist-collaborators in the production of the final work.

Rowlandson cartoon: “A Cognocenti
contemplating ye Beauties of ye Antique”

Daumier cartoon: “The Sad Countenance of
Sculpture Placed in the Middle of Painting”

This cartoon by Rowlandson entitled “A Cognocenti contemplating ye Beauties of ye Antique” shows the aspiring conoissieur using a magnifying glass to observe a variety of sculpture and objets.  The appreciation of sculpture requires a range of experience that differs markedly from that of “flat art”.

This lithograph by Daumier is entitled, “The Sad Countenance of Sculpture Placed in the Middle of Painting”.  This situation unfortunately too often still applies.  Today we are bombarded with flat images and an appreciation for three-dimensional art continues to lag behind painting.

Meadmore, Terpsichore, private collection, Massachusetts

This sculpture by Kenneth Meadmore, entitled Terpsichore, shown here from two different points of view demonstrates some of the unique aspects of a well-maintained sculpture in a beautiful outdoor setting.  Keeping sculpture indoors in order to preserve it in pristine condition is an unfortunate choice because most sculpture lives best in an outdoor setting where changing color, quality and angle of sunlight, seasonal changes in natural surroundings impart a constantly changing visual experience.  It also provides space to view the work from near or far and allows for the necessary movement on the part of the viewer to observe and enjoy the varied forms.  A static indoor artificial light is perhaps one of the least satisfying conditions for viewing sculpture.  This raises the problem for sculpture in an outdoor setting:  that while an outdoor setting may be ideal, it is also perilous, and the sculpture must be regularly and adequately maintained.

Horses of S. Marco, Venice, partial distant view and close-up of surfaces, before treatment, 1972

We have come a long way since 1972 when the Horses of S. Marco in Venice sounded the alarm and began the awakening to the severe damage occurring on the surfaces of outdoor bronzes, particularly those in urban settings.

Marcus Aurelius, Rome, Capitoline Hill

They and the Marcus Aurelius in Rome dramatically demonstrated the fact that bronzes that had existed in an outdoor setting for approximately 2,000 years were undergoing dramatically accelerated corrosive attack.

The cause was identified as industrial pollution with increased sulfur dioxide content in the atmosphere, which should have come as no surprise.  Studies on the sudden, disfiguring changes in the appearance of outdoor bronze sculpture beginning in 1864 by Magnus in Berlin, correctly identified the source of the black, opaque sulfide corrosion that had begun to appear on bronzes formerly having a lustrous and translucent brown oxide patina.

Marcus Aurelius cartoon

Die Patina, book cover with illustration of chemistry equipment

The earliest extensive investigations of patina in outdoor environments and in the scientific study of producing artificial patinas in a great range of colors came along with advances in chemistry itself and primarily in Germany.  There is a certain irony that the development of chemistry as a science and the development of industrialization went hand-in-hand thereby producing both the cause of increased corrosive attack on outdoor metals as well as providing the means of understanding and identifying the mechanisms of change.  This is the cover page of one of numerous recipe books for producing a range of colors on bronze surfaces.

Green copper roofs

The Statue of Liberty

Studies undertaken beginning in the late 1920’s in England by Vernon and Whitby, apparently unaware of the earlier discoveries of the German chemists, again identified atmospheric sulfur compounds as the prime cause of green corrosion crusts formed in the open air.  Vernon’s studies, which were carried out on samples from copper roofs, concluded that the green compound that was produced consisted primarily of brochantite or basic copper sulfate, as opposed to the copper carbonates, malachite and azurite found on buried bronzes which they had expected.  Their studies concluded that this green corrosion crust was stable, protective, natural and had aesthetic value.

Close view of the right hand of the Statue of Liberty
and close view of the left hand of an early 20th
century sculpture in Central Park in New York City

The Statue of Liberty whose copper skin is but a few millimeters thick and has been heated and hammered in its fabrication provides a parallel to what Vernon found on copper roofs—a fairly stable, uniformly colored patina formed over years of exposure outdoors and has a pleasing, homogeneous color—in most people’s minds the ideal of what an outdoor sculpture should look like.  However a surprising and entirely different situation was encountered when outdoor cast bronze sculpture was first examined beginning in the early 1970’s.  The severe and ongoing pitting attack and the mottled and disfiguring surface colorations pointed to unforeseen conditions resulting from the fact that bronze sculptures were made from a copper alloy as opposed to pure copper; that the metallic structure was “cast” rather than “wrought” providing a situation where local action corrosion cells were forming on the surfaces.  The result was a corrosion situation that could be described as neither natural (it was a result of man-made pollution), nor stable, nor protective, and was visually disfiguring.

William Hogarth, Time Smoking a Picture

My work in St. Louis in examining and documenting condition of outdoor bronze sculpture and in working towards treatment solutions meant that the problem of understanding and dealing with the problem of corrosion products, or patina became an immediate concern.  I was asked to present a paper on the topic of patination for a seminar conducted at the United States Bureau of Standards in 1976, and in the course of exploring the topic I made some surprising discoveries:  that the origin of the word patina comes from the Italian term, patena, used to denote a shiny dark varnish applied to shoes, and the first known printed definition of the word in Baldinucci’s Vocabolario of 1681 applied the term to paintings as, “a term used by painters, called by others a skin (pelle), namely that general dark tone which time causes to appear on paintings, that can occasionally be flattering to them.”  William Hogarth’s representation of “Time Smoking a Picture” provides a good illustration of Baldinucci’s definition.

Ancient Etruscan Mirror, c. 3rd c. BCE

The use of the word patina as a term predominately associated with the green corrosion crust found on buried bronzes seems to have developed in the 18th century along with the developing field of archaeology and that still holds today.

Bomarzo: Hell Mouth

However, in the conservation literature at least, the term can refer in the widest sense to the effects of time, weathering, accumulation of particulate matter, corrosion and use, and normally implies a positive or desirable aspect of these changes.   Even biological growth on outdoor stone sculpture, such as this famous Hell Mouth in the garden at Bomarzo, has been described in recent conservation literature as patina.

PATINA:  noble / vile or virulent     natural / artificial

In an effort to bring some clarity and precision to the use of the term, the late John Gettens wrote an important study of the corrosion crusts found on buried bronze artifacts entitled “Patina: noble and vile” after Pliny’s 1st century CE distinctions between “aerugo nobilis”  and “virus aerugo”, describing corrosion that is desirable, stable and attractive and corrosion that is unattractive and virulent.  An additional important distinction that can be found throughout the patina literature since ancient times is that made between patina that is natural and patina that is artificial. 

Ancient Greek Vessel:  noble patina

This ancient Greek bronze vessel has an unusually attractive and stable patina containing green basic copper carbonates produced from burial conditions and could serve here to illustrate a “noble patina”.

Cellini Perseo arm of Medusa, and NY Central Park, Alice

A noble patina on outdoor bronze sculpture of any age is more difficult to find but my proposal here is the patina produced by human touch, provided it is not excessively aggressive and wears away metal.  The frequent touching produces a kind of regular maintenance and a highly attractive and translucent brown, primarily copper oxide, that is very stable if maintained.

Verocchio, Doubting Thomas on Or San Michele in Florence

Another example of what I would term “noble patina” on outdoor bronze sculpture is the very attractive oxide layer that typically forms at the lowest layer of the corrosion crust.  When the patina forms slowly over many decades under slightly protected conditions this layer can be revealed.

Houdon, bronze cast after the marble
George Washington,
St. Louis, cast dating from 1850’s

For the most part, outdoor bronze sculptures in urban environments develop patinas that can only be described as “vile” or “virulent”.

Henry Moore, Family Group, MOMA, NYC

In a particularly aggressive environment like New York City this development can be fairly rapid.  On the left, Henry Moore Family Group, in a photograph taken when it was newly purchased; and on the right, a photograph taken within ten years of exposure outdoors.

Giambologna Mars, late 16th c.

Outstanding examples of artificial patination can be found in Italian bronzes of the 16th and 17th centuries that have never been subjected to outdoor exposure such as you see here in the figure of Mars by Giambologna.

Limet brothers, Paris, late 19th c.

Artificial chemical patination came into general practice only in the late 19th century in France where the Limet brothers whom you see here were among the most famous patineurs.

Artificial patination in ancient Greek sculpture

Because of the high reactivity of polished bronze and its vulnerability to spotting and staining, it has been the norm since ancient times to treat the sculpture in some way to 1)produce a desired coloration, and 2) to protect and stabilize the metal surfaces.

Riace, bronze warrior

Some of the most interesting recent literature has provided new insights and observations on surface treatments and coloration in ancient bronzes.  Formigli’s important observations made during the conservation treatment of the Riace bronzes, as well as investigations by Carol Mattusch, Hermann Born, Alessandra Giumlia-Mair and Paul Craddock have opened up a new and more detailed picture of the ways in which the ancients approached surface coloration.  By and large the aesthetic tendency was predominately one of “chromatic realism” achieved in a great variety of ways, including the use of inlays of colored alloys and other materials, such as you see here in the use of colored stone for the eyes, silver teeth, copper lips, and painted coatings.  Just as the wax coating applied to ancient stone sculpture along with painted polychromy was intended to be renewed periodically, so also was the pine tar pitch applied to bronzes intended to protect, intensify the coloration, and be renewed as required.

Riace warrior, detail of mouth and teeth

Riace warrior, front and back 3/4 view

…such as you can see here in a detail view.

Speculation about original appearance of the treatment of the skin areas now lean in favor of a more naturalistic coloration, as opposed to the dark coloration that they now have. While Formigli maintains that there are remains of an original black sulfide patina,  Hermann Born, John Scott, Paul Craddock and others point to the phenomenon of sulfate-reducing bacteria found in oxygen-deficient conditions of undersea submersion that are capable of producing a black sulfide patina on bronze surfaces.  They reinforce their arguments with newly discovered comparative material that supports the probability that the areas meant to represent skin were polished and then coated with a layer of bitumen or pine tar pitch as described by Pliny in the 1st century CE that both protected and stabilized the metal surfaces as well as imitating the bright glow of tanned skin.  Born also suggests the use of black pigment in the coating to provide coloration for the hair and beard.  Such differentiation was found on a sculpture brought up from the Tiber river and now in the Thermae museum in Rome.

painting bitumen on polished bronze

This shows my reconstruction of the effect of painting bitumen on to polished bronze.  Ancient documents indicate that this coating was regularly removed and renewed as a form of maintenance for bronzes in the same way that stone sculpture was maintained with a wax mixture.

That the situation was far more complex than I can begin to intimate here, I show you an example of what Craddock and Giumlia-Mair have studied in some depth, namely the phenomenon of black “Corinthian bronze” described by Pausanias and others.  This example which has silver eyes and teeth,  is owned by the St. Louis Art Museum.

Corinthian black bronze, Baby Hercules, St. Louis Art Museum

Donatello, David (1430) ;  Magdalene (polychromed wood)

Painted coatings with dark pigmentation, occasionally with accents of gold were used in the early Renaissance with the rebirth of bronze casting.  The example here is the first free-standing bronze sculpture cast since ancient times by Donatello around 1430 in Florence.  The opaque coating unifies the surface and serves as well to disguise casting flaws.  Elsewhere Donatello uses realistic painted coloration on his polychromed wood Magdalene.

Horse—with dark, slightly worn lacquer

Algardi, Innocent X, Museo Capitolino

The use of a dark lacquer on Renaissance bronzes, as you see here in this small bronze horse…

…and in this large 17th century bronze by Algardi in the Museo Capitolino

Pietro Tacca, Peter the Great on Horseback and horse without rider showing unlacquered polished metal underneath the rider

…continued alongside the highly refined lacquer techniques developed by Giambologna and Susini, and here I show you a superb example of Susini’s Peter the Great on Horseback, after a Giambologna model.  I should mention here the recent studies of Renaissance bronze surface coatings presented by Dick Stone, Norman Indictor and Raymond White at the ICOM-CC conference in 1990.

Henry Moore, Standing Figure St. Louis Art Museum

With the advent of the practice of chemical patination using heat and chemicals after the method practiced by the Limet brothers in the late 19th century, this has become the preferred method utilized by most bronze foundries ever since.  This piece by Henry Moore was patinated in such a way to imitate partially corroded bronze.  However if left unprotected it is highly vulnerable to deterioration. When changes in the patina were first noticed in 1971, it was given a protective lacquer coating which has maintained the original appearance.   It is the first example that I know of the use of Incralac on an outdoor bronze sculpture.  The coating has been renewed and maintained several times.

Henry Moore, Reclining Figure, two versions, different patinas

These two identical casts of a Reclining Figure by Henry Moore are discussed in a paper presented in 1995 and published in the collection of essays from a symposium entitled From Marble to Chocolate: the Conservation of Modern Sculpture held at the Tate Gallery.  She discusses the interesting problem of the allowable “range of variation” in Moore patinas.  The upper example has preserved its original light patination by means of a protective lacquer, while the lower example on the Moore estate in England has been maintained with washing and occasional waxing which has allowed the piece to darken in a more or less even way.

Meadmore, Terpsichore, tinted lacquer coating

We come here full circle back to the Meadmore where the bright, lightly tanned coloration has been produced by means of a tinted Incralac—a modern version of the ancient Greek bitumen.

I should like to close by emphasizing two points in particular:

First, it should be clear that sculpture “lives” by the quality of both its forms and its surfaces.  Patina is a useful term for describing surface character and color but it must be modified if it is to be used with any precision.  Surface character and color are highly vulnerable in an outdoor sculpture, and their preservation demands the most urgent item of business for conservators.  Where these have been irretrievably lost or damaged, the complex decisions for the conservator are made more difficult.  Any treatment decisions demand the most of historic, aesthetic, artistic and scientific information that can be brought to bear on the problem.

Secondly,  the fact that it is very difficult to see outdoor bronzes with eyes prior to 20th century conditioning.  The notion that green is the natural and appropriate color or patina for outdoor sculpture is very recent.  Prior to the 1880’s the predominant color of outdoor bronze sculpture was various shades of brown or black with some anomalies such as the golden bronze prized by the Munich founder Ferdinand von Miller.  With the development of the industry of bronze casting in the late 19th century we have come to expect chemical patination of bronze sculpture, perhaps waxed and then allowed to weather “naturally”.  It is interesting to remember that for the most part of the history of Western bronze sculpture from ancient times up until the 19th century, coloration or patination was achieved primarily (with some exceptions) not by chemical means but by painted coatings.

Paper on Patina
top of page