Sculpture

Metals & Alloys

Objects made of metal are subject to corrosion, which is the oxidation of the metals often in the form metal oxides/hydroxides that may then flake off. This is a particular problem for objects containing iron, but other metals and alloys are subject to corrosion as well. Corrosion can be controlled by attaching a block of a more easily oxidized metal by a metal cable to a metal statue. This is known as galvanic inhibition of corrosion. Blocks of magnesium are often used for this purpose. The chemistry derives from the electrochemical potentials for the two metals:

Mg²⁺ + 2 e^- → Mg(s)    E° = -2.38V
Fe²⁺ + 2 e^- → Fe(s)      E°= -0.41V

Because magnesium is easier to oxidize than iron, electrons will flow from the magnesium block to keep the iron in its elemental form. The magnesium is consequently oxidized to Mg²⁺. The overall process can be written as:

Fe²⁺ + Mg(s) → Mg²⁺ + Fe(s)      E° = +1.97V

Note that the more positive the value of E⁰ is for an overall reaction, the more favorable that reaction is.

A similar way to protect metal objects is galvanizing. In this process, the object is coated with a layer of zinc. The zinc serves as both a coating and as a galvanic inhibitor. Common metal objects like buckets are galvanized to extend their useful life. Of course, once the layer of protective metal wears off or is consumed, the the object is not longer protected.

Aluminum is a notable exception. Even though its standard reduction potential (E° = -1.68V) indicates that it would be more difficult to reduce/more easily oxidized than most other metals, aluminum objects are inert to corrosion. Unlike other metals, aluminum oxidizes easily on its surface but the Al₂O₃ that is formed is intimately bound to the metal underneath, forming a very durable protective layer that prevents further oxidation. On the other hand, this coating is amphoteric, a word meaning that it acts as both an acid and a base. Consequently, the aluminum oxide coating will dissolve in strong acids or bases, which will allow the metal underneath to react. Aluminum powder mixed with iron oxide is used to weld objects, such as the rails used in railroads, that are in remote locations. The mixture is therodyamically unstable but requires an initiator. Once it proceeds, however, it is extremely exothermic, producing Al₂O₃ and molten iron.

Metals are very durable and make long-lasting sculptures, but metals are also dense which poses engineering problems. Practically, only small objects can be made of solid metal. As objects became larger, the practice was to make the objects hollow to save both on expensive metal as well as to keep the weight of the object reasonable. Even hollow objects needed to be carefully engineered in order to achieve structural integrity of the object, especially if the object was not a compact shape. A small statue of the Budha would be relatively stable, but a large Greek statue of a hero with outstretched arms would pose challenges to keep the arms from cracking off under their own weight. Larger objects needed an artist that understood engineering as well as design.

The coinage metals are very malleable and can be beaten or rolled into sheets. This allows for production of objects by shaping and cutting the sheets into the desired shapes such as the copper sheets used to form the rooves of buildings or the external coating of the Statue of Liberty.

Another method of shaping metal was to pour the molten metal or alloy into a mold. This process could create either a solid or a hollow object, depending on how the mold was created. For complex objects, the statue may have been cast in smaller pieces which would then be assembled and welded together, followed by a smoothing and polishing process. Note that the Roman Portrait Figure of a Ruler shown below is hollow. An important technology was lost-wax casting. In this process an initial mould was prepared from a wax model of the object to be produced. This model was then coated with clay which was hardened, producing a negative image of the object. The wax was then removed by melting. Molten metal could then be poured into the clay mold to create the object. Originally, these clay moulds would have been destroyed to release the object, but later the moulds were created in sections so that the the mould could be opened and then later reused.

Examination of the surfaces of Greek statues has also revealed surprises. While most Greek statues, and their modern imitations, are depicted as being in natural stone, recent investigations have revealed traces of pigments indicating that the statues were once painted in bright colors. The British Museum has found evidence for this in statues obtained from the Parthenon. Matthew Gurewitsch in Smithsonian Magazine, July 2008, points out that Euripedes in his play Helen of Troy has Helen confirm the Greek practice of painting their statues with these words,

Aphrodite - Venus de Milo, Alexandros of Antioch(?), Greek, ca. 130-100 BC, the Louvre Museum, Paris, France, Photo by Livioandronico2013, CC BY-SA 4.0, via Wikimedia Commons

Stone objects are prepared by first cutting blocks of stone from rock quarries. Some rocks, such as sandstone or limestone, are fairly easy to cut with copper or bronze tools but more compacted materials such as granite were more challenging. The ancient Egyptians using only copper or bronze saws were able to cut granite by adding sand as during the cutting process. Essentially, the sand became the cutting surface and the copper tool was the instrument by which the sand was rubbed across the surface of the rock. This process has been described by a Nova special on the preparation of a Pharoah's obelisk. Once isolated, a stone block could be shaped using subtractive sculpture in which the stone is removed with chisels to produce the desired shape. Michelangelo used this technique to produce the Pietà and David. Notice that the design of David pictured above has both arms attached to the trunk of the statue as a way of stabilizing them. The tree stump behind David's right leg helps to stabilize the overall composition. Many Greek and Roman statues, such as the Venus de Milo pictured here, have missing arms and other appendages owing to their weight and consequent instability.

Stabization of large statues is also concern for museums in how these works are displayed and stored. It can be particularly challenging in areas prone to earthquakes. A YouTube video about the The Getty Kouros describes the engineering solutions displaying the kouros and presents a good analysis of the potential problems that need to be considered and some of the solutions to those problems. Regardless of whether the statue is real or a forgery, the video gives insight into how museums must treat such objects.

The cutting tools used by the sculptors also left marks on the surface of the statues. These marks can be examined and compared to those of known artists to establish authenticity similar to the way brush strokes are used to confirm authenticity of paintings.

Wood

Boepjusa Temple, South Korea and one of its temple guardians. The original temple was destroyed in the Japanese invasions (1592-1598) but the pagoda was rebuilt in 1624. Photo by K.H. Whitmire.

Like stone and metal, wood has been used for millenia as a material for buildings and sculpture. It is easy to fashion with simple tools. It can be sawn, drilled, sanded and carved to produce objects of all kinds. Unlike metal and stone, wood is susceptible to damage by insects and rot, especially if it stays wet. Changes in humidity can lead to cracking and warping. Nevertheless, many wooden statues have survived for hundreds or thousands of years. Egypt has a particularly rich history of wooden statues that have survived thanks to the arid climate. Included in this are the highly decorated wooden sarchophagi. But Egypt is not the only ancient culture that used wood. In Asia, magnificent temples adorned with carvings and wooden statues abound as seen in the figure of the Boepjusa Temple in South Korea. These buildings and the carvings that adorned them were made of elaborately painted wood. The paint helped to protect the wood from the elements and insects. See Wood for a detailed description of the tools used for woodworking.

Sunken ships such as the Vasa Warship that was recovered from the Stockholm harbor where it sank in 1628, have been carefully preserved. The ship could not be allowed to dry out, as this would cause major damage to the long-deteriorated cells within the wood. This problem was solved by displacing the water in the vessel with ethylene glycol, HOCH₂CH₂OH, whose boiling point is 197°C. Ethylene glycol has some properties similar to water but it does not leave the wood so readily because of its higher molar mass and lower volatility.

Glass

Glass is a less common material for the production of sculpture, but it has become more popular in modern times. Glass, of course, is easily cracked and broken, making such objects much less enduring. There are also challenges in making glass objects large. Glass is shaped by melting the glass and handblowing it, or blowing or pouring it into molds. After it has been worked, glass must be carefully annealed to remove strain. In this process, the glass is heated close to its melting point and allowed to cool very slowly. Small objects can be annealed by hand using a glass torch but this is not feasible for large objects where the heating and cooling would be uneven. The size of objects is limited then to the size of the annealing oven. For this reason, many large glass sculptures are made by assembling them from a number of smaller parts.

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