Flame and Ash

Fire is something that we are all familiar with. Everyone has experienced the simple fact that certain things can burn and in doing so are irrevocably changed. For mankind, fire has been an agent of change from the beginning of its use. A simple campfire can be thought of as a crucible where organic matter is destructively distilled and oxidized to carbon dioxide and water and where inorganic matter is consolidated to metal oxides, carbonates, and phosphates.

The flame of a campfire sits in place over the fuel source, appearing to be stationary. But in reality, a flame consists of hot flowing gas. It is the combustion process that is stationary.  A campfire is a kind of air pump pulling air in from the sides and lifting it upwards due to the buoyancy of hot combustion gases. As the gases rise, microscopic particles of glowing carbon are lifted above the wood giving the appearance of an envelope of glowing gas.  Properly mixed propane or natural gas give a flame that has a bluish appearance with much less luminosity. Reading is possible by the light of a campfire. It is not so good by the blue flame of a camp stove.

A wood campfire will consume the wood down to ash. But before the wood becomes ash it can be observed to change from a fibrous solid to a glowing ember of black carbon. The early phase of burning is characterized by the evolution of abundant volatiles that distill into and fuel the flame. Early gas lighting used the flammable gases destructively distilled from coal to provide flame lighting for streetlights and home lighting. The problem with coal gas was that it was free of particulates so the brightness of the flame was poor. The problem of poor gas flame luminosity lead to invention of the limelight and the lantern mantle.

The lantern mantle was developed to overcome the problem of poor gas flame luminosity. A fabric bag soaked in thorium nitrate solution (with 1 % cerium) was dried and then attached to a burner. The gas ignition process burned the fabric and caused the thorium to calcine in place, forming a gossamer webbing of thoria ceramic. The heat capacity (Cp) of thoria is relatively low and the melting point is exceptionally high. Low heat capacity materials require less energy to raise the temperature to a given point relative to high heat capacity materials. The result is that a flame of ordinary energy can raise the temperature of the low Cp thoria to produce high luminosity. The ceria in the mantle dampened the green tinge of glowing thoria to produce a relatively natural light.

Thoroughly burned wood produces an ash that is largely inorganic in nature and at one time was considered quite valuable for soap making and gunpowder. Wood ash was used to provide saltpeter for early gunpowder formulations.  In the early days of gunpowder, saltpeter was extracted from various sources and used with mixed results. The potassium nitrate or nitre form of saltpeter is found in wood ashes. Elsewhere, potassium nitre would appear in damp patches of organic-rich earth as a whitish solid clinging to twigs and plant matter on the ground looking much like hoar frost. Caverns have long been a rich source of sodium nitrate saltpeter. Mammoth Caves in Kentucky and Carlsbad Caverns in New Mexico were mined for their nitrate rich sediments long before tourists began tramping through them.

In 15^th and 16^th century England, saltpeter was systematically cultivated and extracted on saltpeter farms.  These farms had deep beds of manure and plant matter that underwent air oxidation and were covered to shield them from rain.  After an aging period, the beds were transferred to a large basin and leached with water. The leaching solution was then boiled to dryness to give crude saltpeter. This crude nitrate was carefully recrystallized from water to produce a purified white crystalline product.

Saltpeter is a nitrate salt comprised of a nitrate anion and a cation like potassium, sodium or calcium. In the early days of gunpowder, quality and reliability of the powder was highly variable. One of the variables was the extent to which gunpowder attracted moisture. Powder makers eventually learned by trial and error that gun powders made from wood ash saltpeter were much less likely to be passivated by humidity than those made from sodium saltpeter. It became common practice to combine wood ash with saltpeter extracts from another source to produce what we now know to be potassium nitrate.

As an interesting aside, an important development in gunpowder came along when it was prepared in a form that was much less powdery. A technique known as “corning” was applied to the composition that made it more granular in form. This gave much improved burning characteristics.

Saltpeter from the guano beds of Chile were rich in sodium nitrate while material from the great nitre deposits along the Ganges river in India were substantially potassium nitrate. Indian nitre was an important commodity of the East India Company and strategic material for the British Crown.  Until the invention of the Haber-Bosch process of synthetic nitrogen fixation in 1909 and subsequent oxidation of ammonia to nitrate, the world’s guano beds, wood ashes and cave soils were the major sources of nitrates.

The first World War has been called the chemist’s war in part because of the tremendous casualty counts due to the mass implementation of nitroaromatic explosives like trinitrotoluene and picric acid. Haber is notorious for his part in the development of war gases, but the subsequent production of nitrates from his process was of no less consequence.