Metals and their alloys have been important since the dawn of civilization. The Stone Age first gave way briefly to the Copper Age around 4000 BC, before being replaced by the Bronze Age around 3600 BC, when the alloying of copper with tin by the Sumerians was found to produce harder tools and weapons. Of course, in Art it was the iconic metal used in Greek and Roman statues. By 1200 BC, bronze had given way to the Iron age, which lasted roughly until about 500 AD. Iron had the advantage of being stronger than bronze.
Copper was one of the earliest metals to be discovered. It occurs in nature as the metal but could also be smelted from its mineral form malachite (see Inorganic Pigments), which is Cu2(CO3)(OH)2. Carbonate and sulfide minerals must first be converted to the metal oxide in a process known as roasting. Heating malachite will release carbon dioxide and water leaving copper(I) oxide (cuprous oxide).
Cu2(CO3)(OH)2 → 2 CuO(s) + CO2(g) + H2O(l)
Smelting continues by heating the metal oxide with carbon, which can reduce the metal directly, but it also converts to carbon monoxide in the first stage of its own burning and is more effective at reducing the metal oxide since it is a gas and can permeate the solid more effectively.
2 CuO(s) + C(s) → 2 Cu(s) + CO2(g)
CuO(s) + CO(g) → Cu(s) + CO2(g)
During the Copper Age, silver metal was also known. It was found in nature as the metallic element by 4000 BC. The use of lead, which dates back to 3500 BC, started with the smelting of the sulfide ore known as galena, PbS. The process for smelting lead was similar to that for malachite but the roasting process produced SO2. Lead is a very low-melting (328°C), soft metal that could be easily formed into a variety of shapes. The Romans used lead pipes for plumbing water, which was problematic because the lead would leach into the water, and it is now known to be a severe neurotoxin that accumulates in the body. Melting lead does not require a very hot furnace and can be done on a conventional kitchen stove.
PbS(s) + O2(g) → PbO(s) + SO2(g)
PbO(s) + CO(g) → Pb(s) + CO2(g)
Metals are a class of the chemical elements that exhibit the properties of ductility, malleability, luster and electrical conductivity. In the periodic table, the metals lie to the left, and most elements are metals. Metals have a range of physical and chemical properties. Some are much better electrical conductors than others, some melt at low temperatures while others have very high melting points (Selected Metal Melting Points), and some are much more easily oxidized than others. The oxidation of copper by nitric acid has become a classical chemical demonstration with interesting color changes that fun to watch. A video version is available on Youtube thanks to the Royal Society of Chemistry.
The ability to resist oxidation means that some metals can be found in nature as the elements, while others had to await discovery until the chemistry was developed sufficiently to allow their isolation. Aluminum is one of the most easily oxidized of the metals and it was not isolated as the pure element until 1825 and cost-effective commercial production did not happen for several decades after that. It is hard to image now, but aluminum was more expensive than gold until late in the second half of the 19th century. On the other hand, gold, silver, copper, mercury and the platinum metals (platinum, ruthenium, rhodium, palladium, osmium and iridium) can be found in nature as the metals. The platinum metals were first used by the pre-Columbian Americans and were first introduced to the Spanish in 1555. Because of their inertness to corrosion, ruthenium, rhodium, palladium, osmium, iridium, platinum, silver and gold are also known as the noble metals. Gold, silver and copper are also collectively termed the coinage metals for obvious reasons. As these metals have become more expensive, their use as coins has decreased and coins of gold and silver are almost exclusively considered collectors' items. Even the use of copper in coinage has come under scrutiny. Since 1982, copper pennies have had a zinc core to make them more cost effective, but even with the inclusion of copper the worth of a U.S. penny is less than the cost of making it.
The existence of the metallic elements in nature relies on how easily oxidized the metal is. Most metals are so readily oxidized that they are always found in oxidized form as a mineral. This can be readily understood by looking at the electrochemical potentials. Normally these are given as reduction numbers and the more positive the number, the more easily the substance is to be reduced. Conversely, the more positive the reduction potential, the harder it is to oxidize the element.
|Reduction Potentials for Selected Elements|
|Reduction Reaction||Reduction Potential, V|
|Au+(aq) + e- → Au(s)||+1.69|
|Pt2+(aq) + 2 e- → Pt(s)||+1.18|
|Ag+(aq) + e- → Ag(s)||+0.80|
|Cu2+(aq) + 2 e- → Cu(s)||+0.34|
|Pb2+(aq) + 2e- → Pb(s)||-0.126|
|Fe2+(aq) + 2 e- → Fe(s)||-0.44|
|Ni2+(aq) + 2 e- → Ni(s)||-0.236|
|Zn2+(aq) + 2 e- → Zn(s)||-0.76|
|Al3+(aq) + 3 e- → Al(s)||-1.68|
|Mg2+(aq) + 2 e- → Mg(s)||-2.360|
|Na+(aq) + e- → Na(s)||-2.71|
|K+(aq) + e- → K(s)||-2.92|
|Li+(aq) + e- → Li(s)||-3.04|
The Structure of Metals
Most metals in the solid state behave as if the atoms were small spheres. When placed together in a solid, they adopt an arrangement in which the amount of empty space between the spheres is minimized. This is known as close-packing. If you place a number of marbles of the same size in a box and shake them around, they will ultimately adopt this arrangement, except perhaps around the edges. A second close-packed layer will fit nicely on the first, with the spheres fitting into the depressions between the spheres - every where ther is a triangle of three atoms, there will be a slight depression in the layer as seen in the figure. When a third layer is added, however, there is now a choice in how the next layer can be added. One possibility is that the third layer lines up exactly with the first layer. This type of packing, which has the layers A and B alternating, is known as hexagonal close-packing because the unit cell of that arrangement adopts a hexagonal unit cell. On the other hand, the third layer could be offset from the first layer in which case the arrangment is an A-B-C stacking. This gives rise to cubic close-packing because the unit cell for this lattice is face-centered cubic. These stacking arrangements are the most efficient with 74% of the space in the unit cell taken up by the spherical atoms. A good exercise is to calculate this yourself using standard geometry.
While hexagonal and cubic close packing are the most efficient ways of stacking small spheres, they are not the only tupes of lattices found for metals. Some metals will adopt a body-centered lattice, while others will assume a simple cubic lattice. The packing efficiency of a body centered lattice is 68%, while that of a simple cubic unit cell is only about 52%.
Alloys are a combination of two or more elements in a random fashion. Alloys differ from compounds in that they do not have a fixed ratio of elements, rather they exist over a range of compositions. Like metals, the hardness, ductility, maleability and melting points of alloys have a wide range and vary with the amounts of the component elements present. Many of the early alloys were discovered by accident in the process of smelting metals that employed ores contaminated with other elements, but even in the early stages pf the development of metallurgy, the observations of different properties of the alloys based on these impurities were exploited. There are two fundamental types of alloys - substitutional and interstitial.
Substitutional alloys contain different metals randomly distributed throughout the structure, and substitutional alloys are found in nature. Electrum, a naturally-occurring alloy of gold and silver has been known since antiquity. The crystal structure of the alloy if only a small amount of a metal is substituted is usually based on the crystal lattice of the major metal present, although the dimensions of the lattice may vary slightly because of the different sizes of the substituted elements.
Jewelers use the karat (K) scale to describe the purity of gold, with 24K being the value of the pure element. White gold is the name given today for alloys of gold with a "white" metal such as silver, nickel or palladium. Yellow gold often contains copper as well as silver, while rose, pink or red gold is a gold-copper alloy with the darker pinks and red gold having a higher percentage of copper. Some samples of electrum are yellow-green in color, and a green color can also be created by addition of cadmium, but this element is toxic and creates health concerns for wearing Cd-containing gold as jewelry. Purple gold and blue gold are encountered far less often and are gold-aluminum (purple) or gold-gallium or -indium (blue) alloys. The ternary phase diagram for Cu-Ag-Au illustrates the variations in color with relative amounts of each element. For an explanation of how to read a ternary phase diagram, check out Ternary Phase Diagram Basics on Youtube. Similarly, the platinum group elements are largely found deposited together in their mineral forms as alloys.
Meteorites, which are one of the few sources of elemental iron found in nature, are most often composed primarily of iron but contain alloy phases with other metals such as nickel or p-block elements such as phosphorus.
Bronze: One of the oldest known substitutional alloys is bronze, which is an alloy of copper and tin. It is likely that the first bronze made was obtained from the smelting of copper that was contaminated with tin-containing minerals. Bronze is signicantly harder than copper, and was therefore useful for making tools and weapons. Tin in the metallic state is rare, but it was first smelted around 1750 BC from the the ore cassiterite, SnO2. It is not clear where the first tin was mined, but deposits from what is now Germany and the Czech Republic are among the oldest sources known. Later deposits were found in Brittany (France) and Devon and Cornwall in UK. Bronze contains around ~12% tin in addition to small amounts of other metals such as Al, Mn, Ni or Zn and sometimes non-metals such as As, P, Si. The Bronze Age lasted roughly from the mid-4th millenium BC through ~1300 BC, when the iron become the dominant metal for tools and weapons.
Brass: Brass is a substitutional alloy based on copper, but instead of tin, zinc is incorporated into the lattice. Brass has also been known since ancient times, but its use was increased in Roman times with the intential production of brass using copper and cadmia, an ore of zinc oxide. As with bronze, other elements may also be present including As, Pb, P, Al, Mn and/or Si. Brass is more malleable than bronze and has a lower melting point.
The other major classification of alloys are interstitial alloys. These are alloys that are made by placing small amounts of smaller atoms in the empty spaces between the atoms of the host metal. The most well-known interstitial alloy is steel, which is iron with small amounts of carbon located between the iron atoms. Iron at ambient temperatures adopts the body-centered cubic structure known as ferrite. The lattice rearranges to a face-centered cubic lattice (austenite), which can accommodate more carbon. In these structures, the carbon sits in an octahedral site between the iron atoms. Ultimately cementite, Fe3C, is obtained. Modern steels are more complex, and the structure may contain a mixture of ferrite, austenite and cemntite at the microscopic level. Steel is often both an interstitial and substitution alloy. Other metals may replace iron, including, manganese, chromium, etc. These complex compositions are tuned to produce alloys with the desired hardness, corrosion-resistance, etc. In the structures of these materials, the substitutional and interstitial atoms often adopt random locations. This means that only a few of their lattice sites in the crystal lattice may be occupied. For example, in the structure of Fe4C0.63 shown below, the average occupancy of C atoms at any of those locations is either 0.06 for some sites and 0.19 for the others.
An interesting column made of iron is the Iron Pillar of Dehli, which was produced during the reign of King Chandragupta II at the end of the fourth or beginning of the fifth century CE. The pillar is notable because of its high corrosion resistance in spite of its age. The iron was found to have a high content of phosphorus which forms a protective iron phosphate coating to the statue that prevents further deterioration.
Alloys, both substitutional and interstitial, can have substantially different properties from the metals of which they are made. This is illustrated in the melting points tabulated below. Identifying pure metals and alloys has been of prime commercial importance for thousands of years. The iconic example is the legend of Archimedes determining that the crown of King Hiero of Syracuse was not pure gold by a water displacement test. Silver has a lower density than gold, so a crown of pure gold would displace less water than a silver-gold alloy. Density measurements are an easy way to get a rough idea of identity of a metal. One simply needs to measure the mass and the volume of the metal object, usually by water displacement as in the case of Archimedes, and calculate the density. A table of selected densities is found below.
Whether this tale is true in all of its details is not so important as the fact that it illustrates why the study of the properties of alloys compared to pure metals has been studied for thousands of years. In fact, metallurgy starting with the smelting and refining of metals and the production of alloys has been a strong driver of the study of chemistry since ancient times. Not only are the properties of these materials crucial to their use but the costs associated with making them are also important.
|Densities, Selected Metals|
|Melting Points, Selected Metals|
|Metal||Melting Point, °C|
|Densities, Selected Alloys|
|Melting Points, Selected Alloys|
|Alloy||Melting Point, °C|
Gilding & Electroplating
Gilding is the application of a metal coating, often gold, to the surface of an underlying object. Gilding is advantageous over making objects of pure gold, which would be prohibitively expensive. There are a variety of methods by which objects of all types can be gilded. These include:
- mechanical gilding
- mercury gilding
- depletion gilding
- replacement gilding
Mechanical gilding is a simple method that takes advantage of the malleability of gold. Gold is first beaten into thin sheets known as gold leaf. The object to be gilded is coated with an adhesive and the gold leaf is pressed onto the adhesive to afix it to the underlying object. The advantage of mechanical gilding is that it is applicable to any kind of substrate material. Picture frames for paintings and the even the paintings themselves could be gilded. For gilding applied to oil paintings, which often used such as accents - especially for halos and embellishments of garments in religious paintings of the Middle Ages - a bole clay was used. This is a finely ground clay, usually red in color, that is mixed with an adhesive such as rabbit skin glue. This bole clay is laid down over a layer of gesso and the gold leaf was burnished onto the surface. It was also possible to produce texturing patterns in the gold by carefully pressing on the layers in the desired pattern using burnishing tools. Mechanical gilding can be recognized by close inspection of the object and the intersections and overlapping areas of the gold leaf can be seen readily on the surface of the object.
Mercury gilding employs an almalgam, which is a solution of a metal in liquid mercury. Many metals such as gold will dissolve in mercury, and amalgams were also routinely used to make dental fillings in the 20th century. The amalgam remains liquid and can be handled as a paint to coat the surface of the object to be gilded. This produces a nice uniform coating. Once painted, the object is then placed in a furnace where the heating process drives off the volatile mercury vapor leaving behind the dissolved metal as a coating. The disadvantages of this method lies in exposure of the artist to highly toxic mercury vapors. Additionally the method is restricted to coating objects that can withstand the heat of the furnace such as less expensive metals. Mercury gilding can be detected by X-ray fluorescence analysis of the surface as there are always traces of mercury left in the gold coating.
Depletion gilding involves making the object of an alloy that contains a small percentage of gold. Copper alloys, sometimes also containing silver, was used in pre-Columbian objects from South and Central America. Once the object has been prepared, it is subjected to an acid bath that preferentially etches away the more reactive metals such as copper, which is considerably easier to oxidize than gold. This 'depletes' the copper in the surface layer making it rich in gold and producing the desired color. Only a small percentage of gold is required to make this method work. The technique can also be identified by X-ray fluorescence as gold as well as the underlying alloy will be detected. Non-coated areas of the object, especially if they are centuries old, will usually show evidence of corrosion that would be absent in an object of pure gold.
Electrochemical replacement gilding makes use of the highly oxidizing nature of the metal ions of gold and also silver. In this process it was common to have a base object that contained copper. In the presence of gold ions in solution, the copper would be oxidized to Cu2+ ions which would be soluble in solution. The gold ions would be reduced to Au0 that would plate out on the surface of the object. In pre-Columbian works, gold was dissolved in a mixture of salt (NaCl), saltpeter (KNO3) and alum (KAl[SO4]2·12H2O) resulting in a solution of AuCl3. Silver ions will also displace copper. The equations for these two processes are given below along with their electrochemical potential (positive potentials indicate that the reaction should occur spontaneously).
Electroplating is a more modern method of estabishing a metal coating on a substrate that is electrically conductive. In this method, the object made of a relatively inexpensive metal such as copper, iron or steel, is placed in an aqueous solution containing metals ions of the element that one wants to used as the coating. The anode material may also be the metal that one intends to plate onto the selected object. Gold, silver and bronze can all be electroplated onto a given object. The object itself becomes the cathode in this electrochemical bath, and the metal anode dissolved creating metal ions in slution that migrate to the object to be plated where they are reduced as an electrochemical potential is applied sufficient to reduce those ions (see the Table of selected reduction potentials given above).
Resources and References
- The Elements, John Emsley, Oxford University Press, 3rd Edition, 1998, ISBN-13: 978-0198558187, ISBN-10: 019855818X.
- Colored Gold, Wikipedia.
- Precolumbian Surface Metallurgy, Heather Lechtmann, Scientific American, 1984, 250, 56-63.
Crystal Structure Data
- Cementite, Fe3C, Fruchart D, Chaudouet P, Fruchart R, Rouault A, Senateur J. P. Journal of Solid State Chemistry, 1984, 51, 246 - 252.
- Fe4C0.63, Nagakura S, Toyoshima M, Transactions of the Japan Institute of Metals, 1979, 20, 100 - 110.
Credits for the Header Image
- Portrait Figure of a Ruler, Roman, 200–225 AD, Bronze, From the Collection of the Museum of Fine Arts Houston, Public Domain.
- Shaman Pendant, Unknown Central and South American, 1–800, Gold, From the Collection of the Museum of Fine Arts Houston, Public Domain.
- Tureen with Cover on Stand, Baldwin Gardiner, c. 1830, Silver, From the Collection of the Museum of Fine Arts Houston, Public Domain.
- Candlestick, Unknown English, c. 1750–1775, Brass, From the Collection of the Museum of Fine Arts Houston, Public Domain.
- Walking Man, Auguste Rodin, French, 1840–1917, 1905, cast 1962, Bronze. From the Collection of the Museum of Fine Arts Houston, Public Domain.