Metallurgy and smelting are perhaps some of the most important innovations in human history as they changed transportation, warfare, trade, agriculture, and more. Smelting and the working of metals provided a currency system and made the Industrial Revolution - from steam to electricity - possible.
Metallurgy as it is known today developed over a period of about 6,500 years. The invention and subsequent development of metallurgy and smelting came to be relied upon by civilizations for weapons, tools, agricultural instruments, domestic items, decorations, etc. The first metals used were gold, silver, and copper since these occurred in their native or metallic state. The very earliest forms were likely gold nuggets that were found in the sand of riverbeds. The first known copper artifacts were found in Iraq and date to 8700 BCE. Toward the end of the Stone Age, these metals were used decoratively and practically. It was discovered that gold could be formed into larger pieces through cold hammering, but copper could not. Beginning around 7000 BCE, Neolithic peoples started to hammer copper into crude knives and sickles; these tools lasted longer than stone tools and were just as effective. Egyptians made weapons out of meteoric iron from about 3000 BCE. Between the Stone Age and Bronze Age, a transitional period occurred which has been named after the combination of the materials copper and stone - the Chalcolithic Period.
The discoveries that metals like copper could be extracted from its ore by heating the metal beyond its melting point and that such metals could be shaped by melting and casting in molds was critical in leading to the Metal Age. The first metal to be smelted was in the ancient Middle East and was likely copper. The earliest known artifacts that were shaped through melting and molds are copper axes from the Balkans dating to the 4th millennium BCE. Furnaces with forced-air drafts were invented to reach the high temperatures required for smelting. The furnaces were fueled by coal until the 18th century when coke - a solid residue formed from heating bituminous coals - was introduced in England.
The next critical discovery was that the combination of copper and tin made a superior metal - bronze. Serbia used copper and tin to make bronze objects, marking the beginning of the Bronze Age. Different types of bronze were made for different purposes, and smelting technology spread - likely through trade and migration - from the Middle East to Egypt, to Europe, and China. By 2500 BCE, a technique called brazing - joining metals by melting and flowing a filler metal into the joint - was in use, as evidenced by the gold drinking vessel made for Queen Puabi in the Sumerian city of Ur. There are also several examples of this technique from Troy and Egypt around this time. This technique is still in use today.
While there is no way to pinpoint the beginning of the Iron Age exactly, iron artifacts dating to 2500 BCE were found in Hattic tombs. Scholars believe the Hittites invented the process of extracting iron from its ore and forming a workable metal, though small pieces of iron were made naturally in copper smelting furnaces. By 1800 BCE, India had begun to work iron, and apparently Imperial Rome considered India to be excellent cast iron workers. Anatolia was making iron weapons on a large scale, and thus, this is typically considered the true beginning of the Iron Age. By 1000 BCE, iron working was introduced to Europe, and its use spread westward rather slowly. Ironmaking had reached Britain at the time of the Roman invasion, around 55 BCE. Though some regions had not even implemented the technology, evidence indicates that other regions were using hardening processes to improve the sharpness of swords, etc. By this time too, East Africa had started working with steel.
Tempered martensite was found in Galilee, dating to approximately 1200 BCE. Tempering increases the toughness of alloys like steel and cast iron. Large amounts of steel were being produced in Sparta by 650 BCE, and by 600 BCE, wootz steel was being produced in India. The next several hundred years saw many developments in the metallurgy industry; Song, China created a method for using less charcoal in a blast furnace; the Romans improved mining organization and administration; East Asia invented the process later called the Bessemer process. From 1623 AD, Pascal's Law impacted the heat treatment of metal, and the first iron foundries were set up in the UK in 1700 AD. The electric arc furnace was developed in 1907 AD. Many more developments were made, including the publication of metallurgical knowledge in the book De Re Metallica during the 16th century by Georg Agricola, who is regarded as the "father of mineralogy." Today, some of these techniques are still used, though many developments have been made to modernize the process.