Production at Carlton alum works
by Ian Miller
In chemical terms, alum is a group of double salts that contain aluminium sulphate in combination with a second sulphate (either potassium or ammonium). Alum can form as a natural mineral (alunite) under volcanic conditions, where it solidifies from solution to form large crystals of a characteristic shape. It does not occur in a pure state in this country, although at some stage it was discovered that alum could be manufactured from certain types of shale. Exactly who was responsible for this discovery is uncertain, and the origins of what was to be England’s first chemical industry remain rather unclear.
Early attempts at alum manufacture were made in Britain during the 16th century. These were financed by the Crown and focused predominantly in Ireland and Dorset, but proved to be unsuccessful. A breakthrough occurred in the early 17th century, when attention was centred upon north-east Yorkshire. Here, the Jurassic Upper Lias shales that outcrop in a thick band in the coastal cliffs to the north and south of Whitby, and in certain inland locations, were discovered to contain the vital ingredient of alum - aluminium sulphate. The production of English alum was concentrated in this region for the next 250 years, stimulating the development of other industries and helping to lay the foundations for the Industrial Revolution that was to follow.
The first successful alum works was established in 1604 at Slapewath, near Guisborough, and although several other works were built in the area shortly afterwards, it took nearly 30 years for the industry to become commercially viable. The reasons for this were twofold. In the absence of any real scientific understanding, the process of manufacturing alum from shale had to be developed entirely by trial and error. It also proved hard to come by working capital to finance the new industry in an age when an adequate commercial banking system had yet to develop.
Nevertheless, by the mid-17th century, the alum industry was thriving, and a number of new works appeared in the region. One such works was built on Carlton Bank, which lies near the crest of the north-facing scarp of the North York Moors, at a point where a thick layer of shale outcrops. A large quarry and its associated spoil tip on this remote hillside provide a rare testament to this former industry (see below).
The quarries represent the first stages in the manufacture of alum. After its extraction by pick and shovel from the quarry face, the shale was piled into large heaps, which could be as high as 30m, on top of a thick layer of brushwood or similar combustible material. The heap was then ignited and allowed to smoulder for several months in order to roast, or calcine, the shale. This process created a weak sulphuric acid from the moisture in the heap, which in turn reacted with the aluminium in the shale to produce a soluble form of aluminium sulphate.
The next stage in the process was to extract the aluminium sulphate from the calcined rock by soaking it in water. This was done in huge stone tanks called steeping pits, and required a vast amount of fresh water, the constant supply of which called for careful resource management. The steeping pits were usually laid out in a range, but the number of pits varied between different works; Carlton alum works was equipped with six steeping pits.
After one pit of shale had been thoroughly steeped, the liquid was transferred to an adjacent pit, and so on until sufficient aluminium sulphate had been adsorbed into the water. This water, known as raw alum liquor, was then transferred to a storage tank where it was allowed to settle before passing on to the secondary processes of manufacture. The calcined shale within the steeping pits was then discarded, and in many cases this survives as vivid red spoil tips on the hillsides below former alum works.
The secondary processes were undertaken in a purpose-built factory called the alum house. These were often some distance from the quarry, so the raw liquor had to be transported. A wooden channel known as a liquor trough was built for this purpose. The construction of a liquor trough sometimes demanded considerable feats of engineering over and above ensuring a gentle gradient along its entire length to aid the flow of the liquor by gravity. The liquor trough at Boulby Alum Works, situated on the coastal cliffs north of Whitby, for instance, was over a mile long and passed through a tunnel for part of its length in order to negotiate a collapsed section of cliff.
Once at the alum house, the raw liquor was boiled and concentrated to a point where the aluminium sulphate would crystallise with the addition of an alkali. During the early period of the industry, the source of alkali used was human urine, which contains ammonium sulphate. For a small payment, households kept their urine for collection by the alum manufacturer’s agents, although as the demand outstripped local supply, it was brought by boat from as far afield as London! The crystals were then purified through a sequence of washing and re-crystallising cycles. The details of this process were a closely guarded secret, and no contemporary accounts survive.
As nearly all the alum works were situated in remote locations, the transportation of the finished product presented another challenge in an era long before railways or reasonable roads had been developed. In practice, the sea provided the main transport route.
The process of manufacturing alum from shale was superseded during the second half of the 19th century by new techniques, particularly those patented by Peter Spence in 1845. Spence discovered that he could manufacture alum more cheaply and on a larger scale by treating shale from coal mines with commercially produced sulphuric acid.