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Concept
Powder mill
Summary
A powder mill was a mill where gunpowder is made from sulfur, saltpeter and charcoal.
Crude grinding and mixing operations such as the Frankford Powder-Mill of Philadelphia were a cottage industry until the industrial revolution brought improved product quality through the following procedures:
Charcoal was often manufactured nearby from locally available trees, but the heating retorts were typically separated from the other buildings to minimize fire danger. Trees with low value as sources of lumber were debarked, dried, and cut to uniform length to fit into iron retorts with cast iron doors. The retorts were carefully packed to leave as little air space as possible, and the retort doors were closed and sealed with clay to prevent entry of air as the retorts were heated by external fires. Volatile gas generated by the heating process was vented through a small flue at the top of each retort. After the external fires were extinguished, the retorts were allowed to cool before the doors were opened to remove the charcoal.
High purity sulfur usually required little preparation other than grinding to a powder. Separate grinding mills reduced cool charcoal and sulfur to fine powders.
Crude saltpeter (KNO3) was dissolved in a pot of boiling water. After boiling, a small amount of floating glue might be added to the hot water to aid skimming off floating impurities. The hot solution was then carefully decanted to avoid suspending insoluble solid impurities settled to the bottom of the pot. The hot decanted liquid was distributed into shallow wooden vats where evaporative cooling would remove excess water. Purified potassium nitrate recrystallized while the cooling solution was gently agitated with broad hoes to prevent formation of large crystals.
Powdered sulfur and charcoal were combined in appropriate ratios with moist saltpeter crystals, and the moist paste was mixed and compacted by kneading beneath heavy rotating wheels in a press mill.
Official source
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