Metallurgy

Metalúrgica
Metalúrgica
Fundición y moldeado
Cera perdida
Dorado y plateado
Referencia
  • Introduction

    Certain metals—known as native metals—can be found naturally in their pure form, but most metals are normally contained in rock as ore. Native metals are very malleable and can be worked by striking them with a hard object. This technique, called hammering or beating, is the oldest form of metalworking. The extraction of metal from its ore is a more complex procedure; first, the correct minerals must be identified and mined, often from below the ground. The ore is then purified using fire, often using crucibles and furnaces, allowing the metal to be smelted and separated from the rest of the rock. The use of smelted metal allowed items to be made using moulds, hammers, chisels, and files. This set of techniques and knowledge is known as metallurgy.

    In addition to copper, iron, gold, and silver, human groups have made use of other metals such as lead, tin, and platinum to produce artifacts with different characteristics—color, finish, strength, and flexibility. The alloys most used in ancient times were based on copper, which was transformed into bronze through the addition of tin or arsenic. The result was a stronger, lighter-colored metal with a lower melting point and unique resonant properties, allowing it to be used for making such items as highly sonorous bells. Other important alloys include electrum, a mix of gold and silver that was used in Ancient Greece, and tumbaga, an alloy of gold and copper that was widely used in the northern Andean region in pre-Colombian times.

    The earliest objects made from native copper were produced in the Middle East (Iran and Anatolia) around 9000 years ago. Metallurgy was developed approximately 6000 years ago in the area of Mesopotamia and the Balkans, and from there this technology spread rapidly throughout the Old World, with copper being the most widely used metal. The use of alloys increased around the year 3000 BCE, giving rise to the Bronze Age in Europe and Asia. Around the year 1000 BCE, iron replaced copper as the most common material used to make agricultural tools such as ploughs and weapons of war.

    Metallurgy was discovered independently in the Americas, where the earliest finds of hammered gold and copper sheets date back to around 1500 BCE in the Central Andes (Andahuaylas and Cupisnique), while the first evidence of smelting appears around 1000 BCE in the mountains of Peru. To the South, around Lake Titicaca, the earliest evidence of smelting dates back around 3000 years and consists of slag, the waste rock that is separated from the molten metal. The metallurgy of the Central and Southern Andes (Peru, Bolivia, Northern Chile, and Argentina) was based mainly on copper and its alloys, which were used to make implements and tools (such as knives, axes, chisels, tweezers) as well as ceremonial items and personal adornments (including ear ornaments, rings, masks, and small bells). However, some objects were also made from silver and gold.

    In northwestern South America, in the region between present day Ecuador and Costa Rica, a separate metallurgical tradition developed, the earliest evidence of which is found in copper artifacts dating back to around 500 BCE. By 400 CE this metalworking craft had become very sophisticated, with the use of tumbaga (a copper-gold alloy) and the manufacture of complex pieces using a casting technique known as the lost wax method. Around 1200 CE these groups began to work with platinum, a metal that is far stronger and more heat-resistant.

    Metallurgy in Meso-America was a different story. The earliest evidence is found in western Mexico around the year 600 CE, influenced by the technology of northern South America, as there seems to have contact among cultures and even trade between the two regions along the Pacific coast. It is noteworthy that the people of the Mayan civilization, which built the great city of Teotihuacán, had no knowledge about the use of metals until this spread into the rest of the region around the year 1200 CE.

    Metals were considered sacred materials by many cultures of the Americas, being associated with the gods owing to their remarkable luster and color. In both Meso-America and the Andean Region gold, yellow and untarnished by time, was identified with the Sun and other male deities, and was also linked to fertility and agriculture. Conversely, silver–pale in color and subject to corrosion–was identified with the Moon, with water and tides, and with goddesses, and thus also linked to fertility.

  • Smelting and casting

    Metals must be exposed to intense heat to smelt them out from the ores that contain them. Each metal has its own melting point, which can often be lowered by adding other minerals or fluxes. Different metals can be combined during smelting, producing alloys.

    Metallurgy furnaces can be simple constructions, as long as they can reach the high temperatures required. A stone structure over a fire can work as a furnace, as long as adequate fuel is provided and the air can circulate to feed the flames. The most commonly used furnaces in the Andes were the huayras or huayrachinas, both used in pre-Columbian times and during the Colonial Period. These were cylindrical stone or ceramic structures with holes around the outside that were placed over fires. Air could be blown through the structure to increase the temperature of the fire, often by locating the furnace on a hillside or in other locations where strong winds would circulate the air naturally. Alternately, air could be blown into the furnace using wooden or ceramic tubes known as toberas. The most commonly used fuels were wood and guano. Image 1

    Once the molten metal was obtained, it was removed from the furnace and cast into a shape suitable for its intended use. Pure metal could be saved in the form of drops or ingots, which could be melted again later, or it could be poured into molds to achieve basic forms that could later be reworked into more complex forms for ornaments or tools. Molds were made from carved rock or ceramic, covered with a layer of lime to prevent the piece from sticking. Some were simple, single-sided molds, while others were made with multiple pieces that fit together to create more elaborate shapes. Molds were usually preheated before the molten metal was poured to reduce the temperature difference and prevent them from breaking. Image 2

    Items such as hooks, knives, and axes were usually worked from a basic shape–either a simple rectangle or a shape similar to that of the finished article–that was then crafted with intense hammering to flatten and stretch out the metal and produce fine cutting edges. Cold hammering could be used to achieve the desired shape, but this technique weakened the metal, making it necessary to reheat the piece after working to enable it to regain its strength.

    Compound molds, made from a number of parts that fit together, have the inverse of the desired shape. These molds have an opening or funnel through which the molten metal is poured into the internal cavity and that also acts as a reservoir that provides extra metal for solidification. As metals contract slightly as they cool, the solidified metal takes up a somewhat smaller space than the liquid metal. The process of pouring the metal into the mold must be undertaken with great care, as the liquid metal is still fairly thick, and a number of air bubbles are normally formed when it enters the mold. To prevent this and ensure that the cavity is completely filled, small holes or auxiliary drainage tubes are installed in different parts of the mold to ensure that the metal fills the entire space and expels all trapped air.

    Once the metal has cooled and completely solidified, the mold is disassembled and the cast metal piece is removed. Freshly cast pieces often seem porous and have excess material along the lines where the parts of the mold were joined. These must be cut or filed off to achieve the finished piece, either manually using soft stones or with the aid of acids to produce a smooth, shiny finish

  • Lost wax method

    The lost wax method is a metallurgy technique used in many places around the world that allows metals to be cast into complex, ornate shapes. It was used in both Mesopotamia and northern South America to enable the production of elaborate and fine pieces from molten metal. In the Northern Andean region (from Ecuador to Costa Rica) evidence of the lost wax method exists as early as 400 BCE, including unused initial shapes made from the wax of bees that inhabited the region’s tropical rainforest. In contrast, in the Central Andes (Peru and Bolivia) the same technique used resins from plants such as the retamo (Bulnesia retama). In Meso-America the lost wax technique appears along with the earliest use of metals, having been transferred from northern South America, and both beeswax and copal resin were used to create the base figure from which the mold was made.

    According to the Franciscan scholar Bernardino de Sahagún, who observed the indigenous way of life and technologies in Mexico during the sixteenth century, the lost wax casting method involved a complex sequence of steps. The central core of the piece was first made from a mix of clay and charcoal, and this was then covered with an outer layer of beeswax and copal, which was modeled into the shape desired for the finished piece; this in turn was covered with a thin layer of charcoal and then wrapped in clay to form the mold, leaving a hole through which the metal would be poured. On exposure to the heat the wax would melt and drain off, while the clay would be baked hard. The resulting mold consisted of an inner cavity surrounded by clay, with a clay and charcoal core in the center. This mold was filled with molten metal and then cooled; the outer mold could then be broken away, and the porous inner core dug out, leaving a hollow finished piece.

    One of the key features of the lost wax process is that the outer mold is a single piece that cannot be disassembled, so the only way to remove the metal piece from inside it is to smash the mold. As the molds could not be replicated and were used only once, each metal piece made using the lost wax method is unique.

  • Gold and silver plating

    During the pre-Columbian period in the Americas, jewelry and ceremonial objects made from metal were relatively scarce, and were used as symbols of prestige by officials and priests. Objects made from gold, silver, and copper were not necessarily seen as symbols of wealth but did signify power, as their color, luster, and sound were considered divine.

    A variety of techniques were used to change the color of metals: alloys of different metals were produced (adding tin makes copper more yellow, for instance), and surface treatments applied to make the exterior color different from the interior.

    Deposition gilding and silvering were undertaken in the Northern and Central Andean regions using an elaborate technique that left a thin layer of gold or silver on the surface of a copper or bronze piece. The first step was to dissolve particles of the noble metal in an aqueous solution also containing acidic mineral compounds; the liquid was then neutralized by adding lime and the piece was submerged in the resulting solution, where some of the precious metal was deposited on its surface. This technique is also known as electrolytic replacement gilding. The result is a thin, fragile layer of gold or silver that is heated to bond it to the surface of the metal and then polished to enhance the luster of the finished piece.

    An alternative technique, used in the Southern Andes, was depletion gilding or silvering. In this method, the piece is made from an alloy that contains a proportion of gold or silver. By hammering the surface in the presence of air, partial oxidation occurs: the less noble metal is weakened by the air and removed by hammering, sometimes also with the use of corrosive substances, leaving only the noble metal exposed on the surface. Pre-Columbian master craftsmen were experts in this technique, achieving truly golden or silver surfaces from pieces containing only small percentages of the noble metal in the bulk alloy.

    While the color and appearance of the metal were important for ceremonial and ritual purposes, the internal composition of the piece was also relevant, though not always plainly visible. Some artifacts from the Andean region contain up to 30% gold or silver alloyed in their interior, even though the piece appears to be pure copper; the luster and color of the gold or silver cannot be seen without a special surface treatment to make them show through. This could mean that the ancient Andean peoples considered both the internal composition and external appearance of these metals to be sacred qualities.

  • References

    • CENTENO, S. & D. SCHORSCH, 1996. Caracterización de depósitos de oro y plata sobre artefactos de cobre del valle de Piura (Perú) en el período Intermedio Temprano. Boletín Museo del Oro 41: 165-185, Bogotá.
    • DONNAN, C., 1998. Un ceramio Moche y la fundición prehispánica de metales. Boletín del Museo Chileno de Arte Precolombino 7: 9-18.
    • EASBY, D., 1965. Pre-Hispanic Metallurgy and Metalworking in the New World. Proceedings of the American Philosophical Society 109 (2): 89-98.
    • FALCHETTI, A. M., 2003. The Seed of Life: The Symbolic Power of Gold-Copper Alloys and Metallurgical Transformations. En Gold and Power in Ancient Costa Rica, Panama and Colombia, J. Quilter & J. W. Hoopes, Eds., pp. 345-381. Washington D. C.: Dumbarton Oaks Research Library and Collection.
    • LECHTMAN, H., 1976. A Metallurgical Site Survey in the Peruvian Andes. Journal of Field Archaeology 3: 1-42.
    • PATIÑO, D., 1997. Arqueología y metalurgia en la costa Pacífica de Colombia y Ecuador. Boletín Museo del Oro 43: 48-67, Bogotá.
    • PENDERGAST, D. M., 1997. Rocas y minerales del México antiguo. Revista de Arqueología Mexicana 5 (27). México D. F.: Editorial Raíces.
    • SAUNDERS, N., 2003 Catching the Light: Technologies of Power and Enchantment in Pre-Columbian Goldworking. En Gold and Power in Ancient Costa Rica, Panama and Colombia, J. Quilter & J. W. Hoopes, Eds., pp. 15-47. Washington D. C.: Dumbarton Oaks Research Library and Collection.