Patina a 'layers' approach
The quantity and kind of alloying elements of the bronze have influence on the color of the metal and on the colors that develop when under influence from chemicals.
The most common alloys used in art casting are silicon, tin and lead bronze.
The following examples make clear that the decisive factor in choosing an alloy can be COLOR.
Color of the metal
Photo with three polished bronze plates
Tin bronze has the strongest reddish tone, silicon bronze the strongest yellow colour.
Transparent patina on xx finished bronzes
Photo with three plates
The plates are patinated with a chemical which produces an thin copper oxide layer on the bronze, kaliumpermangate. The patina consists of a layer red and black copper oxide and oxides of the alloying elements.
The lower part of the plates is finished with a coating of clear beeswax. Tin bronze strongest reddish brown tones, silicon bronze more yellow cold....... .
By using a top coat of colored wax or lacquer the tone of the patina can be influenced further, but this can only compensate the color partially and diminishes the typical brilliance of the metal.
The change in time of the patina depends on properties of the statue and its environment.
Statue properties are alloy, patina, top coat and maintenance. Factors from the environment are
sunlight which destroys organic topcoats, chemicals in the air, gases like water, oxygen and salts like natrium chloride.
From the photo's it becomes clear that the differences are LARGE? and that an some artists and some owners have interest in these aspects of alloy choice.
City center, 60 km from the sea, xx years, regularly cleaned and waxed
Anne Frank , Mari Andriessen, Binder Haarlem 1975, sand cast, ounce metal
Original patina: ?
Photo: date, maker, +
On the sea coast Polich Tallix New York, lost wax ceramic shell, year xx
.... Tom Otterness
Now reddish with soft green areas.
In the US when this colour development is not wanted, it is called 'red bronze disease'.
Mariani Pietra Santa, 1999?,
Igor Mitoraj, ' Light in the moon'
Patina now, black with growing green patches starting here on the lips and lower right cheek
2. ' as cast' skin
3. surface worked on with tools
5. sandblasting - polishing
10. top coat
11. environment and time