In our last post, we saw how the British secured their supply of saltpeter from the largest source in the world. In today's post, we will look at how they refined saltpeter over in England. We will study the refining process at the The Royal Gunpower Mills at Waltham Abbey.
We had actually studied a bit of this process earlier, when we studied black powder several months ago. In today's post, we will look at the process in more detail.
Waltham Abbey is a small town north east of London and fairly close to it. The river Lea flows through this area and there is a large church in town. There was once a large monastery here and the monks had diverted some of the waters of the river towards a watermill that they built to process wool for cloth production (a procedure called fulling). In the early part of the 1600s, the mill was converted into an mill for producing vegetable oils. Around 1665, the Second Anglo-Dutch war started and there was a shortage of gunpowder in England, therefore the oil mill was converted to gunpowder production. At around the same time, it was acquired by a man named Ralph Hudson. At the end of the 1600s, the Hudson family sold it to William Walton and the Walton family ran it successfully as a private enterprise for almost 100 years. In 1787, it was decided that the British government should acquire the mill from the Walton family, to ensure the supply of gunpowder and establish standards for quality, at which point it became the Royal Gunpowder Mills.
Our description of the process at Waltham Abbey comes from a book written in 1915 by Arthur Marshall, who served as a Chemical Inspector, Indian Ordnance Department.
The Royal Waltham Gunpowder Mills acquired its saltpeter exclusively from India. The crude saltpeter (also called "grough saltpeter") was produced in India and then packed in jute bags and shipped over to England. At Waltham Abbey, the process was as follows:
The crude or "grough" saltpeter is dissolved in a larger pot A, which has a capacity of 500 gallons and is fitted with a perforated false bottom, which prevents the saltpeter adhering to the vessel. For each batch, about 25 cwts. (2800 lbs. or 1270 kg.) of grough saltpeter are taken, and 5 cwts. (560 lbs. or 254 kg.) of crystals recovered from liquors, and 5 cwts. of crystals left in the crystallizing cisterns. This is all dissolved in about 280 gallons (1060 liters) of the washings of the purified saltpeter, which also contains a considerable amount of the salt. The fire is lit under the pot, and in about two hours the saltpeter is dissolved and the liquid boiling. Just before it boils, a thick scum rises to the surface, consisting mostly of impurities. This is skimmed off and the false bottom is removed, and cold water is added from time to time to induce fresh scum to form, if it will. The fire is then withdrawn and the the liquid is allowed to settle for about two hours. Then a hand pump is lowered into the pot and the liquid is pumped into filters B, where it passes through linen cloth. From here it runs to shallow copper crystallizing troughs C. As it cools down, the liquid is stirred by a workman in order to make the saltpeter separate into small crystals, and the saltpeter "flour" as it forms is drawn up on to an inclined draining platform D, and from there is passed to a washing vat E. After the temperature has fallen to about 32 °C. (90 °F.), the solution is no longer stirred and any crystals that form after that are treated as grough niter.
The washing vat E is about 6 feet long, by 4 feet wide, by 3.5 feet deep, and is fitted with a false bottom made of wood with small holes bored in it. Below the false bottom is a plug which can be removed to allow the washings to flow away. First, the charge is washed with 70 gallons (264 liters) of water sprinkled over it by means of a rose, the plug being left out so that the washings can drain away into liquor tank F. After draining half an hour, the plug is inserted and the saltpeter is covered with fresh water, which after standing for half an hour is also allowed to drain into F. Finally the salt is washed by sprinkling 100 gallons (378.5 liters) of water, the plug remaining out. The saltpeter is now allowed to drain all night and is then removed to the store house where it is allowed to dry spontaneously. In about three days, the moisture has fallen to 3 or 5 percent.
The mother liquors and other impure solutions are boiled down to about a quarter of their original volume. Any scum or deposit that forms during the boiling should be removed and water then be added. The solution is now filtered and allowed to crystallize. The crystals are treated as grough saltpeter and the mother liquor returned to the evaporating pots.
The author mentions that the methods of refining adopted in France, Germany and other countries are substantially the same as that of Waltham Abbey. He also mentions that a small amount of size (a gelatinous solution) is often added to the pot to assist in the formation of scum.
In our next post, we will look at the production of charcoal in some detail.