9.2.

Seven metals were in use before the invention of writing; gold, found only as native metal; silver and copper, found as native metals but, more commonly, as carbonate and sulfide minerals, or ores; mercury sleeps in pools of liquid metal and as oxide and sulfide ores; native iron can be found in meteorites, but far more commonly as oxide and sulfide ores; tin and lead are found only in ores, most commonly tin oxide and lead sulfide.

Table 9-1. Metals and Their Ores

Copper was the earliest metal to come into common usage because its ores are fairly common and because it is smelted at moderate temperatures. It is a soft metal, however, which makes it of marginal value for tools and weapons. Some ores of copper contain arsenic as a contaminant and copper smelted from these ores contains from 2-6% of residual arsenic. This arsenical bronze, a solution or alloy of arsenic in copper, is much harder than copper alone, which makes it more useful for tools and weapons. Tin also forms an alloy with copper, bronze, which is harder than either metal alone. By 3,000 BC bronze had become the dominant metal, so much so that its use defines the Bronze Age. The higher temperatures required for iron production delayed the advent of the Iron Age to about 1200 BC. Bronze continued to be used into the Iron Age because, unlike iron, it could be cast into molds. Cast iron, a lower melting alloy of iron, was developed in China as early as 500 BC but did not become common in Europe until about 1500 AD.

The fundamental problem of smelting is the reduction of metallic compounds to elemental metals. Different compounds require different treatments, which I shall explain in turn, beginning with the oxide ores. Whereas calcination heats the bejeezus out of a material in the presence of air alone, smelting uses a reducing agent to aid in the removal of the bejeezus. Anything that burns may be considered a reducing agent, the most common being charcoal or some material which turns to charcoal in the kiln.

Equation 9-1. Smelting of Oxide and Carbonate Ores

The oxide and carbonate ores are smelted by very similar reactions, as shown in Equation 9-1. Charcoal combines with the oxygen in the ore, escaping as carbon dioxide (the bejeezus), and leaving the molten metal behind. Mercury and tin are smelted at modest temperatures, lead at higher temperatures, copper and silver at still higher temperatures, and iron at the highest temperature. The high temperature required for melting iron is not achieved in a simple kiln. In practice, iron oxide is reduced to solid, rather than molten iron at 1200C. The non-ferrous minerals in the ore, the gangue, melt under the influence of the flux, leaving a bloom of solid iron suspended in the melt. This solid is removed to an open hearth, or forge, where it is sequentially heated to red heat and then hammered on an anvil to produce wrought iron.

Sulfide ores must be roasted to remove sulfur. In roasting, the ore is heated in an oxidizing atmosphere. Sulfides are oxidized by the oxygen in the air and the sulfur combines with oxygen, the bejeezus coming off as sulfur dioxide gas as shown in Equation 9-2. What remains are metal oxides, which may be smelted with charcoal as previously described.

Equation 9-2. Roasting of Sulfide Ores

I shall now tell you of the properties of these metals and the many and varied uses to which they are put. Gold is without doubt familiar to anyone reading this book. It is a soft, dense yellow metal. Three properties distinguish it from the other metals. First, it has density of 19.6 g/mL, almost 20 times that of water and the highest of the seven metals under discussion. Second, it is exceedingly resistant to corrosion. Finally, it is the rarest of the seven metals, and consequently the most valuable. It is mined largely as particles of native metal naturally dispersed in sand or other ores. The chief problem in its production is the physical separation of the minor quantities of gold from the bulk of the material which contains it. The properties of gold have suited it to its familiar uses in jewelry and as a standard for monetary value. Hammered into thin foil or gold-leaf, it can be used to adorn paper, wood, and even thread. More recently, gold has become widely used for electrical contacts because of its resistance to corrosion. In all times and places it has been a symbol of completion, perfection, and immortality.

Silver is valued next to gold among the seven metals. This soft white metal is moderately resistant to corrosion, though far less so than gold. Though it is found as native metal or as a sulfide ore in its own right, it is a common impurity in galena, the sulfide of lead, and because such large quantities of lead are produced, galena is the principle commercial source of silver as well as lead. Frequently it is alloyed with copper, which produces a harder metal. Familiar uses include the metallic coating of mirrors and the photo-sensitive emulsions of photographic films and papers.

Copper is a common red metal, similar to silver in its resistance to corrosion. Though found as native metal, it is more often found in its sulfide, carbonate and oxide ores. It ranks second (behind iron) in worldwide production. Bronze, as previously described, is an alloy of copper and tin; brass is an alloy of copper and zinc. Copper is used to make inexpensive jewelry plated with silver or gold, as an important coinage metal, and for electrical wiring.

Mercury is a most mysterious and wonderful metal, the only metal which is liquid at room temperature. It is a heavy metal, though not as dense as gold. Found as pools of liquid metal in mercury mines, it is most often extracted from its sulfide ore, cinnabar. Mercury dissolves many metals, including gold and silver, forming solutions called amalgams. "Silver" dental fillings are actually amalgams of mercury and silver. Widely used in industry, its most familiar domestic use is as the liquid metal of thermometers. While metallic mercury was handled with impunity throughout human history, its compounds, such as mercury nitrate or mercury acetate, are generally acutely toxic; the organic compound, methyl mercury, is extremely toxic. But because the general public fails to distinguish the properties of elements from those of their compounds, mercury is widely and incorrectly perceived to be a "toxic metal." Many web-sites and news reports claim that mercury is among the most toxic substances known, a claim which is patently and demonstrably false. Millions of people live to ripe old age with mercury fillings; try having your teeth filled with caustic soda or potassium cyanide if you want to know what a toxic filling looks like. There are legitimate health concerns over both metallic mercury and its compounds but it is wrong to oversimplify them. In alchemy mercury is one of a holy trinity of principles, holding a place of honor as the archetype of the metallic essence, the spirit of the metals and, symbolically, of everything in Nature.

Iron is the familiar gray metal of commerce and industry. Very rarely found as native meteoric iron, it is most commonly smelted from its oxide and sulfide ores. Of the metals, iron is produced in the largest tonnage. Far more susceptible to corrosion than the other seven metals, it must be painted, alloyed or coated if it is to resist the effects of air and water for more than a few years. Its alloy with carbon, steel, is among the most useful metals known. Subtle changes in the concentration of carbon (less than 2%) produce a steel which is harder or softer, stiffer or more flexible. Wrought iron is typically found in ornamental iron work, cast iron in cook-ware, and steel in knives and bridges.

Tin is a metal almost as white as silver and almost as soft as lead. Next to mercury it has the lowest melting point of the seven metals. It has excellent resistance to corrosion. For this reason it was widely used as a coating for iron, "tin-plate," a use which has been largely supplanted by the less expensive zinc-plated, or galvanized iron. The "tin can" was actually tinned iron, now aluminum or galvanized, and "tin foil" has been replaced for household use by inexpensive aluminum foil. With copper, tin makes the alloy, bronze, and with lead the alloys, pewter and solder, and it is here that the public is most likely to come into contact with it.

Like mercury, lead has been widely slandered as a metal, owing to confusion between the properties of the metal and those of its compounds. Metallic lead is extremely un-reactive while its compounds, particularly the soluble ones, are acutely toxic. It is smelted from its sulfide ore and ranks fifth in tonnage after iron, copper, aluminum, and zinc. Lead is a dense white metal which quickly loses its metallic luster on contact with air. It found widespread use during the Roman Empire as lead pipe for plumbing. Modern households find it used as a component of pewter and solder, but its use in automobile batteries is its largest single application.

Consulting a modern periodic table, you will find that nearly 80% of the elements are metals, most of them unrecognized until the nineteenth century. Zinc was alloyed with copper to produce brass during the Roman Empire and nickel was recognized during the Renaissance. The next metal to achieve widespread utility, aluminum, would not do so until the twentieth century.

WarningMaterial Safety
 

Locate MSDS's for copper carbonate (CAS 12069-69-1), tin oxide (CAS 18282-10-5), and sodium carbonate (CAS 497-19-8). Summarize the hazardous properties of these materials in your notebook, including the identity of the company which produced each MSDS and the potential health effects for eye contact, skin contact, inhalation, and ingestion. Also include the LD50 (oral, rat) for each of these materials.[1]

Your most likely exposure is dust inhalation. If a persistent cough develops, see a doctor.

You should wear safety glasses and a dust mask while working on this project. Leftover materials can be disposed of in the trash.

NoteResearch and Development
 

Before proceeding with your work, you must master the following material:

  • Know the meanings of those words from this chapter worthy of inclusion in the index or glossary.

  • You should have mastered the Research and Development items of Chapter 5 and Chapter 7.

  • Know the names, properties and uses of the seven metals whose discoveries pre-date recorded history.

  • Know the ores listed in Table 9-1 and be able to recognize samples of them.

  • Know the equations for the smelting of malachite, Equation 9-1(c), and cassiterite, Equation 9-1(g).

  • Know which metals are alloyed to produce bronze, brass, pewter and solder.

  • Be prepared to handle malachite, cassiterite, soda ash, charcoal and fire responsibly.

  • Know that the properties of an element may be completely different from those of its compounds.

Notes

[1]

The LD50 was introduced in Section 7.2.