Today we know that the chemical elements are capable of showing a range of behaviors in the category of reduction and oxidation (redox). Unlike our predecessors who attempted to wrap their arms around redox phenomena without the benefit of data or atomic theory, we are able to refer to tables of information which give details on the magnitude of redox phenomena and allow us to predict outcomes of transformations.
Reduction and oxidation has always been with us and for most of human history we were blissfully unaware of it as a distinct and complementary phenomenon. Beyond the conduct of redox in biology, for most of human history the major use of redox as a tool was combustion. I would argue that humans began to do chemistry in earnest when we learned to generate fire and use it at will. The introduction of fire allowed humans to apply significant thermal energy to materials in contrast to mechanical energy. Thermal energy changed the composition of materials in a way that was visible to us. With fire we could boil, dry, pyrolyze, combust, sinter, fracture and melt materials. Food once cooked was forever changed. The combustion of wood produced much heat, charcoal, and ash.
Fire could provide warmth and destruction. It could be used as a weapon of war. The Chinese would become renowned for their command of deflagrations, explosions, rocketry, as would the Greeks for their Greek fire. Chinese adepts learned to produce deflagration and explosions with energetic redox compositions centuries before the Europeans. With the spread of gunpowder formulation around the world, the problem of finding it’s components would plague adopters of this technology.
The basic rules of controlling fire were determined very early in human history. Some things burned and other things didn’t. The effects of air might have been inferred by the simple act of lighting kindling and blowing on it. Blowing on an ember can sustain it for a time and gives rise to increased heat. Fire can be accelerated by blowing air on it but may also be extinguished by too much wind. Clues to the basic nature of fire were there all along, but we lacked vocabulary, theory, and analysis.
The color of a wood fire can range from yellow/orange to bright yellow and it can warm you from a distance. Smoke was something that issued from fire and was perhaps troublesome. Fire and smoke always seem to rise upwards. More clues to to the behavior of matter, but as before, we lacked the tools of science until only in the last few centuries.
Today we can use atomic and quantum theory, thermodynamics, and the physics of radiation and buoyancy to explain and quantify fire and its many attributes. Today we can confidently state that a fire requires an initiation (the energy source), a reductant (the fuel), and an oxidizer (air). I think early man would have had a fairly concrete understanding of heat source and fuel. But the need for an oxidizer may have been less obvious. After all, air is all around us and is invisible. Nobody knew about the fire triangle or Smokey the Bear.
The development of oxidizers as a class of substances whose participation in chemical change was held back owing to the obscurity of the concept and the lack of a good theoretical basis like atomic theory. Humans had been perishing by suffocation forever. Everyone has experienced the effects of oxygen deprivation whether it was by running from a sabretooth tiger or holding ones breath on a dare. But without the knowledge of oxygen and its function in respiration or in combustion, oxidation was the answer waiting for the right question.
Reducing materials as fuels for combustion or for the reduction of metal ores to the metal was common knowledge for a very long time. The introduction of oxidizing materials beyond the ever present air around us was a much harder nut to crack. If we set the oxygen in air aside and focus on strongly oxidizing substances, we can begin to see the development of oxidizers as a class of materials.
One of the earliest oxidizers to find use was nitrate, commonly called saltpeter or nitre. Nitre was found in some damp locations that were rich in decaying organic materials. Nitre beds were often observed as having a white crust that migrated to the surface of the ground. Early references of these nitre beds come from China and India. Nitre was capable of having multiple counter-ions. The early users of nitre were unaware of this of course. Later in history, makers of gunpowder would come to prefer potassium nitrate over the sodium salt owing to it’s lower aptitude for hydration. Hydrated saltpeter will passivate gunowder. The story of gunpowder is well documented and the reader can pursue that trail on their own.
The discovery of oxygen in 1772 by Scheele could be considered a major step in the development of oxidation technology. While chemists were misguided by the theory of phlogiston, the isolation of a substance that supported combustion was a crucial conceptual leap. Scheele and later Priestly would show that this new “air” would support combustion. In 1774 the discovery of chlorine by Scheele was the next major oxidizer to be identified. Chlorine was produced by the action of HCl on MnO2 (pyrolusite). The bleaching effect of Cl2 gas was soon discovered by Scheele. The discovery of Cl2 soon lead to the discovery of bleaching powders. The earliest bleaching powder composition comprised of lime and chlorine was patented in 1798 by Charles Tennant in England. By the close of the 18th century, three important oxidizing compositions were produced: oxygen, chlorine, and calcium hypochlorite. Chlorine and lime bleaching powder went into mass production at the beginning of the 19th century.
In a real sense, the development of oxidizers is very much like the invention of the lever. A level is used to amplify mechanical force. An oxidizing agent is used to amplify the extractive force on valence electrons. A strong oxidizing agent is able to bring energy to bare on select transformations that might not be otherwise available. With the advent of this kind of transformation, new possibilities unfolded in history. By the middle of the 19th century, molecules with pendant oxidizing groups would be capable of self reaction to produce tremendous outbursts of energy. Nitroglycerine is one such molecule containing both reducing groups and oxidizing groups in one molecule. Oxidizers and oxidizing functional groups would change how we dig tunnels, extract minerals, carve canals, wage war, and eventually, compress uranium or plutonium into a critical mass for a nuclear explosion.