Salt and the salt industry

by Albert Frederick Calvert

INTERIOR PENNY’S LANE MINE, NORTHWICH

The story of the exploitation of the top and bottom beds is one that is soon told. The top bed was worked until the mines began to fall in and the subsequent breaking in of fresh water converted the old workings into brine reservoirs. In 1779, the discovery of the lower bed of rock-salt at Lawton prompted the owners of the Marston Mine at Northwich to sink below the top bed in which they were working, and, in 1781, a trial shaft which was sunk from the top mine by means of a horse gin, demonstrated the existence of the bottom deposit in that district. Other owners transferred their operations from the top to the bottom bed, and for the next fifty years practically all the rock-salt was excavated from that source. In 1830 the roofs in these workings began to crack, and attention was directed to the insufficiency of the pillars by which they were supported. A competent surveyor, who did not hesitate to declare that the workings were in a dangerously insecure condition, was regarded as an alarmist by the old salt proprietors, who commissioned other “experts” to examine the pits, and were satisfied with their assurance that they considered each pit to be entirely free from any danger, and that they should not hesitate to work in any of them. Three years later the roof of the first bottom-bed mine fell in, others collapsed in rapid succession, and by 1840 some twenty mines had collapsed, let in water, and become filled with brine. In 1881, only nine rock-salt mines were at work, and eight of these had a combined area of 123 acres.

Rock-salt mining in England is a dead industry, but it will be of interest to outline very briefly the methods that were employed in Cheshire during the comparatively short period of its existence. The old top-bed mines were operated, in the first place, with one shaft to each mine, and they were ventilated by means of an air-pipe and a fan. A horse gin was used for winding, but the winding-shaft in which the gin rope worked did not go into the rock-salt, but only to within a short distance of it, and it was out of this shaft, at a distance of 2 or 3 yds. from the bottom, that a side drift was driven. From this side drift a windlass pit was sunk into the rock-salt, and it was up this windlass pit that the rock-salt was drawn to the drift and thence taken to and up the gin shaft, the part of the gin shaft below the drift being used as a sump or lodgment for water. These top-bed workings did not usually extend more than 100 yds. from the shaft, but, as the number of the mines increased, the workings from adjoining shafts occasionally become connected. In this way one shaft became a downcast and the other an upcast, and the air-pipe and fan at each were able to be dispensed with. The thickness of rock-salt worked averaged from 30 to 36 ft., and pillars of natural rock-salt, usually about 5 yds. square, were left to support the roof and superincumbent strata.

Although the bottom-bed mines were worked upon the same plan, the inadequacy of the supports employed in the top-mines was rectified by an increase in the size of the supporting pillars and in the thickness of the rock-salt roof that was left between them. Steam engines with direct shafts to the bottoms of the mines were substituted for the horse-gins and windlasses, and improved methods were introduced for preventing water from breaking into the shafts. Two winding-shafts were sunk, placed about 10 to 15 yds. apart, and a pump-shaft was sunk to the depth to which the surface water penetrated. One of the earliest precautions taken in the rock-salt shafts, and afterwards in brine shafts when they came to be sunk through rock-salt, was to protect the sides from the ravages of fresh water. All the shafts were roofed over to keep out rain or snow, and the wood casing, which was originally used, was replaced, in 1845, by cast-iron tubings, similar in construction to those used in colliery shafts.

As soon as the miners had sunk the shaft to the depth of the sole or floor of the mine and had made an opening large enough for their purpose, they proceeded to blast off enough rock to form a chamber about 5 ft. high. This formed, they advanced by blasting off the rock-salt from the face of the seam. The salt was loaded into waggons, which ran along small railways to the mouth of the shaft. The men engaged in blasting the rock and squaring the walls and pillars (for these were left quite square and well hewn) were called miners ; those who loaded the trucks and conveyed them to the shaft were ferriers. They were a fine set of men, and their occupation, compared with coal-mining, was a very healthy one. The mines were of an equable temperature, and were sufficiently warm for the men to dispense with their shirts. Being lofty, the air was pure, except when excessive blasting was undertaken. The greatest number of men employed in one pair of shafts was about eighty, and the quantity of blasting powder used by that number in the course of a day averaged 1 cwt. Safety fuses were seldom used, the charge being fired by a straw filled with fine powder, which was lighted from a candle.

Many of the mines were of considerable size, and some of them increased at the rate of about an acre annually. The quantity of rock-salt mined was small compared with coal. No mine in the district yielded above 40,000 tons per annum.

Rock-salt is more free from danger than most kinds of mining; no explosions occur, for there are no deleterious gases, and accidents are rare. In a general way the rock-salt strata are remarkably free from carbonic acid gas, and in only one instance in Northwich, and twice at Meadow Bank, Winsford, does fire-damp appear to have been met with, and then only at pipe veins and in very small quantity. There are no falls of earth, as in coal mines, for the rock-salt is extremely tenacious, and the miners never undermine it but blast it, which is a much safer operation. The two great dangers to which rock-salt mining is exposed, though they rarely result in loss to human life, are the falling in of the mine bodily, or of the shafts and neighbouring earths, and the breaking in of brine either at the head of the top-rock shaft or from old mines, long disused, and full of brine.

Neither the name of the first mine that fell in, nor the date of its collapse, is recorded. We know that a mine in Witton fell in in 1750, and another to the north of the Northwich Town Bridge followed in 1759, and that many others collapsed before 1770. Lakes, or “flashes” as they are called locally, have formed over the larger of these sinkings, but the sites are more commonly marked by what are known as rock-pit holes, and large tracts of country are scored with these funnel-shaped indentations. There can be no doubt that a number of these old mines were worked with pillars that were too few and slender for the purpose, and these supports gradually weakened to their ultimate collapse under the pressure of the superincumbent earths. As the sinking did not take place evenly all over the mine, but most frequently occurred near the shafts and at the greatest distance from the sides of the cavity, the roof would curve down towards the sinking centre and the falling-in formed the V-shaped apertures on the surface which are described as rock-pit holes. But, while in a percentage of cases the collapse of the mine could be traced to the crashing of the pillars, the destruction of the majority of the mines was caused directly by the influx of water, although this water, having become saturated with salt, would, if undisturbed, cause no further havoc in the interior of the mine.

But the manufacture of white salt from brine, which was temporarily surpassed in importance by the rock-salt industry, was not discontinued, and a copious supply of brine flowing over the rock head of the upper bed, was tapped by shafts and pumped to the surface. When, about 1850, this supply showed signs of failing, attention was directed to the enormous reservoirs of brine in the old inundated mines, into which had drained a great quantity of the rock-head brine. The attempt to pump brine out of the abandoned workings was successful, and for some years an abundant supply was obtained. After 1870 the pumping operations caused further collapses, the land overlying the mines subsided, and lakes were formed which, at intervals, broke into the partially exhausted reservoirs, and, pouring through the top-rock workings into the mines in the bottom bed of salt, replenished the supply of brine. A great collapse which occurred in the Dunkirk district in 1880 let down the waters of Cranage Brook and the Wadebrook, together with a huge quantity from the river Weaver. The subsidence resulted in the formation of a large lake, which, following upon a later subsidence in the same area, suddenly disappeared into the earth and literally flooded all the underlying strata.

Surprise has frequently been expressed that in a salt country in which brine has been manufactured for over twenty centuries, the existence of the rock-salt deposits should only have been discovered in the last two hundred and fifty years, but it must be borne in mind that not only was the brine the best custodian of the secret of its own source, but that, when the problem of the supply had been solved, the danger of tapping and controlling it had still to be overcome. When the supply of brine in many of the springs was cut into, it proved so copious that the sinkers had to flee for their lives and to ascend the shaft among the brine. The fact that the depth at which the brine would be encountered was unknown, explained the inability to provide a safeguard against the sudden inrush of brine, but subsequent observation showed that when the workmen met with the “flag,” or bed of hard marlstone that existed above the top of the rock-salt in many districts, the brine might be expected to be found at high pressure. It was then the practice to case the shaft sides down to the flag to prevent the entrance of surface water, and either to blow through the flag with powder or pierce it with boring rods. At a later period, the shaft was sunk to the approximate point of encounter with the brine, and cased with iron cylinders, the bottom cylinder being furnished with an iron bottom pierced with two pipe holes. A column of pipes was erected in the cylinder, and a set of boring rods was let down each pipe, so that when the flag was bored through, the brine rose until it attained its level in the pipes, while by means of a tap attached to each pipe it was possible to stop the entry of the brine and to empty the shaft.