Everything we have talked about so far is stone age technology. Stone, ceramics, woven materials, and wood all have thier relative advantages and disadvantages. However it is hard to overestimate the impact that metals have had on the history of our society. Metals can be shaped like clay, are heat-resistant like pottery, but are not fragile. To someone who had never seen a metal before, this was indeed a magical material.
The first metals were found in their elemental or "native" state. Only three metals are found this way, copper, silver, and gold. Evidence for their use goes back at least 8000 years ( approx Dec 26 on our timescale). At first, these fantastic materials would have been used exclusively for ornatmental or ritual purposes--they are just too rare for utilitarian purposes. In addition they are too soft for most uses.
About 7000 years ago it was discovered that certain minerals would yield metals when placed in the fire. Many of these colored minerals were used as glazes to impart color or texture to pottert. So the first production of metals from their ores was probably an accidental byproduct of pottery firing. As time went on, different minerals were found to produce metals with different properties. While we now recognize each of these metals as a different chemical element, for most of recorded history the view was that all metals were composed of the same material at different levels of purity or development. Aristotle wrote that metals grew in the earth much as a seed grows into a plant. The base metals like lead, tin, and iron were relatively young while silver and gold were the most mature.
Gold is almost always found in the native state, i.e. as elemental gold metal. Native copper and silver can be found in some areas. All other metals must be reduced from their ores. The process of reducing metals from their ores by fire is called smelting. All smelting is based on redox chemistry, with carbon in the form of charcoal being used as the reducing agent. For example, the smelting of oxide ores takes the form:
The discovery of each of these metals predated recorded history. They are listed in order of increasing smelting temperature. Mercury, tin, and lead are quite easy to smelt. Campfire temperatures are all that are needed. But these metals are all very soft (in fact mercury is a liquid at room temperature) and so these metals are useless for making tools. Copper requires higher smelting temperatures and people designed special furnaces to reach these temperatures. Copper is harder than mercury, tin, and silver and was the first metal to be used for tools.
Copper is still pretty soft, though, and a copper knife edge dulls easily. The next real advance came with the discovery that copper could be alloyed with tin to produce a metal, bronze, which was harder than either copper or tin alone. Silver is similar to copper both in hardness and in smelting temperature but it is much rarer and has consequently been used primarily for ornamental purposes through the ages.
Iron requires much higher temperatures to smelt than any of the other metals known to the ancients. Consequently its use came later in history after the development of more and more elaborate furnaces. However it much exceeds any of the other metals in hardness and after its discovery it replaced bronze for use in weaponry.
While lead had been known from ancient times its remained for centuries a by-product of the silver smelting industry. During the Roman Empire lead came to be used for plumbing purposes. In fact, the word "plumbing" comes from the Latin word for lead, "plumbum" from which the symbol Pb is derived. After the advent of gunpowder around 1240 AD, lead took on increasing strategic importance for use in bullets. Today, the major use of lead is in storage batteries such as the one in your car.
As chemistry developed it became apparent that there were many more metals than the seven known to the ancients. In fact, we now know that the vast majority of all the chemical elements are metals.
With the exception of gold, metals are almost exclusively mined from minerals made up of compounds rather than elements. The metals in these compounds are positive ions and are said to be in a positive oxidation state. The primary task of smelting is to reduce the oxidation state to the zero or elemental state. While it is easiest to visualize the metals as oxidized when they are combined with oxygen, many ores are composed of non-oxide compounds. The most important of these are the sulfides and carbonates. Lets look at some of these minerals:
| oxide | sulfide | carbonate | |
|---|---|---|---|
| mercury | cinnabar | ||
| tin | cassiterite | ||
| lead | minim | galena | cerrusite |
| copper | cuprite | chalcopyrite | malachite |
| silver | argentite | ||
| iron | hematite | iron pyrites | siderite |
Metals which have been actively mined in Virginia include:
Part of your quiz for this project will be to identify metallic ores. Here is a list of the raw materials. In this project, you are responible for the ones containing metals.
The following metal ores are included in the mineral collection. You will be called upon to identify them as part of the metals quiz. In most cases the materials is far from pure. The chemicals listed are those which are present in largest concentration. These materials are classified as minerals (M, homogeneous) or rocks (R, heterogeneous). Rocks are further classified as igneous (IR), sedimentary (SR), or metamorphic (MR).
| Number | Mineral or Rock | Chemical | Formula | Picture |
|---|---|---|---|---|
| 2 | Chalcopyrite(M) | Copper Iron Sulfide | CuFeS2 |
|
| 3 | Malachite(M) | Hydrous Copper Carbonate | Cu2CO3(OH)2 |
|
| 4 | Azurite(M) | Hydrous Copper Carbonate | Cu3(CO3)2(OH)2 |
|
| 5 | Hematite(M) | Iron Oxide | Fe2O3 |
|
| 6 | Magnetite(M) | Iron Oxide | Fe3O4 |
|
| 7 | Pyrite(M) | Iron Sulfide | FeS2 |
|
| 8 | Pyrrhotite(M) | Iron Sulfide | FeS |
|
| 10 | Galena(M) | Lead Sulfide | PbS |
|
| 13 | Cassiterite(M) | Tin Oxide | SnO2 |
|
| 14 | Sphalerite(M) | Zinc Sulfide | ZnS |
|
Metal ores usually contain significant impurites which must be removed prior to smelting. The process is similar to that used in processing many minerals. The stone is crushed and mixed with water to form a slurry. The slurry is agitated and specialized surfactants (similar to soaps) may be used to float either the ore concentrate or the impurities to the surface. This procedure is called "froth flotation" and is used, for example, in separating kyanite from the surrounding rock. If you go on the field trip to the kyanite mine, you will see this process in action.
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 and comes off as sulfur dioxide gas:
Once the sulfur has been removed, the metal oxides can be reduced by heating with charcoal, which is a reducing agent:
Carbonate ores are also smelted with charcoal:
Often, other materials will be added as flux to aid in the melting of the oxide ores. These days, coke (derived from coal) is used in place of charcoal, but they are both forms of elemental carbon.
The metals quiz consists of three questions on any of these topics:
We will produce a small crucible with metal ore and charcoal sealed inside.
This will produce a reducing atmosphere inside the crucible which will
reduce the ore to metal. Click on the picture to get a slideshow of the
instructions.
When we crack open your crucible, there should be a bead of metal at the bottom. If there isn't, you fail.