On the Antiquity of the Chemical Art

by James Mactear

Of these there are two chief forms, the first being a tank or pan formed of large pieces of slate, with the joints made with clay, and surrounded with a mud wall. The whole is covered with an arch or vault and is filled with the brine, which is then evaporated by surface heat, the fire being placed at one end and the flue at the other.

The other form is very curious and interesting, and is almost identical in its principle of construction with the pan I have referred to as figured in Agricola, only in this case the materials are very different, being, instead of wood and iron, nothing more than clay or mud.

It was described officially by the Japanese, in their publications at the Philadelphia Exhibition in 1876. The Japanese description of this apparatus is highly interesting. It is as follows:—

A low wall is built, enclosing a space of about 13 feet by 9 feet, the bottom forming a kind of prismatical depression, 3 feet deep in the centre line. An ashpit, 3 feet deep, is then excavated, starting from the front wall, and extending about 4 feet into this depression at its deepest place; it communicates with the outside by a channel sloping gradually upwards, and passing underneath the front wall. The ashpit is covered by a clay vault, with holes in its sides, so as to establish a communication between the ashpit and the hollow space under the pan. This vault is used as a fire grate, the fuel (brown coal and small wood) being inserted by the fire-door in the front wall. The air-draught necessary for burning the fuel enters partly by the fire-door, partly through the ashpit and the openings left in the vaulted grate. Through these same openings the ashes and cinders are from time to time pushed down into the ashpit, for which purpose small openings are left in the side-wall of the furnace, through which the rakes may be introduced. A passage in the back wall supporting the pan leads off the products of combustion and the hot air into a short flue, sloping upwards, and ending in a short vertical chimney. At the lower part some iron kettles are placed in the flue for the purpose of heating the lye before it is ladled into the evaporating pan.

With reference to the pan, it is made in a way that requires a great deal of skill and practice. In the first place, beams reaching from the one side to the other are laid on the top of the furnace walls, and are covered with wooden boards, forming a temporary floor. Two or three feet above this floor a strong horizontal network of poles of wood sustains a number of straw ropes, with iron hooks hanging down, and of such a length that the hooks nearly touch the wooden floor. The floor is thereupon covered with a mixture of clay and small stones, 4 to 5 inches thick, the workman being careful to incrustate the iron hooks into this material. It is allowed to dry gradually, and when considered sufficiently hardened, the wooden beams and flooring are removed with the necessary precautions. The bottom of the pan remains suspended by means of the ropes. The open spaces left all round between the bottom and the top of the furnace walls are then filled up, and the border of the pan, 9 inches to 10 inches high, is made of a similar mixture. It is said that this extraordinary construction lasts from 40 to 50 days when well made, and that it can be filled 16 times in 24 hours, with an average of 500 litres of concentrated lye at each filling; but the quantity depends upon the weather, and is less in winter than in summer. During the cold season one pan yields 140 litres (of salt) each time it is filled, and in the hot season from 190 to 210 litres. The average consumpt of fuel is said to be 1500 kilos. in 24 hours.

In Persia, near Ballakhan, salt is still made, and has been made from time immemorial, in a very primitive way, which is described by Bellen, in his description of his journey in 1872 from the Indus to the Tigris, as follows:—

“For several miles our road led over a succession of salt pits and ovens, and lying about we found several samples of the alimentary salt prepared here from the soil. It was in fine white granules massed together in the form of the earthen vessel in which the salt had been evaporated. The process of collecting the salt is very rough and simple. A conical pit or basin, 7 or 8 feet deep and about 12 feet in diameter is dug, and around it are excavated a succession of smaller pits, each about 2 feet diameter by 1½ feet deep. On one side of the large pit is a deep excavation, to which the descent from the pit is by a sloping bank. In this excavation is a domed oven with a couple of fireplaces. At a little distance off are the piles of earth scraped from the surface and ready for treatment. And, lastly, circling round each pit is a small water-cut led off from a larger stream running along the line of pits.

“Such is the machinery. The process is simply this:—A shovelful of earth is taken from the heap and washed in the basins (a shovelful to each) circling the pit.

“The liquor from these is, whilst yet turbid, run into the great central pit, by breaking away a channel for it with the fingers. The channel is then closed with a dab of clay, and a fresh lot of earth washed, and the liquor run off as before; and so on till the pit is nearly full of brine. This is allowed to stand till the liquor clears. It is then ladled out into earthen jars, set on the fire and boiled to evaporation successively, till the jar is filled with a cake of granular salt. The jars are then broken, and the mass of salt (which retains its shape) is ready for conveyance to market.

“Large quantities of this salt are used by the nomad population, and a good deal is taken to Kandahar. The quantity turned out here must annually be very great. The salt pits extend over at least ten miles of the country we traversed, and we certainly saw some thousands of pits.”