The Process
The actual process of salt making is somewhat of a mystery. As near as we can tell, the furnaces were long trenches dug back along the top of a bank. They were walled with slate about 15 inches thick which was laid with a mortar of clay. The kettles themselves held about 22 gallons each. Sometimes they were bigger.
They sat on top of a trench in a row, with as many as fifty in a string. The furnace was fired from in front, the flames and smoke being sucked along under the kettles and out through a stone chimney at the far end of the pit. Generally they were protected from bad weather by a shed roof supported on poles. It was quite common for two of these long narrow furnaces to be under a single roof.
The water was boiled for about 24 hours. Then it was transferred to a cooler -- a trough which acted as sort of a settling tank. Then the clear, saturated brine was drawn off into the kettles again and boiled rapidly until it began to grain. Sometimes blood or the white of an egg was added to purify the water.
When it began to grain, or form salt crystals, the fires were slackened but not so much as to stop its boiling. The salt was dipped out as it formed and put into baskets to drain.
The drippings were caught in pans and returned to the water in the kettles. These kettles were never allowed to boil dry. When they got too low, water was added which had been previously boiled 24 hours and the boiling down was repeated.
However, after a certain number of boilings the water in the kettles became so charged with impurities that it was necessary to throw it out. The whole process was then started over again.