The processing of alum from shale is arguably the earliest chemical industry in the British Isles.  Alum was principally used in the textile industry as a fixing agent for dyes. It was also used by tanners to produce a more supple leather and in the paper industry in the production of parchment. 

The History

In medieval times the production of alum was carried out in Tolfa in Italy, which was land owned by the Pope, and thus the process was a Papal secret.  Alum was discovered in the Cleveland Hills around 1595 by Sir Thomas Challoner of Guisborough (allegedly, others attribute the discovery to a Thomas Chaloner, his cousin, of Slapewath).  He had been astute enough, on a visit to Rome, to notice that the discolouring of leaves on trees near the Pope’s alum works was very similar to those near his home.  He realised the clay of both regions was similar too and this led him to examine the possibility that the Cleveland Hills might be a source of this valuable chemical – and so it proved.
When Thomas Chaloner, Sir Thomas’ cousin, opened a mine at Belman Bank, Sir Thomas effectively set himself up in competition with the Pope who was exporting alum to England at £52 for a ton while Sir Thomas could produce it for a mere £11 per ton.  Not only that, one of the enduring tales of his early endeavours was that Sir Thomas desperately needed skilled workers to both mine the alum and teach others their skills, and so he bribed some of the Pope’s workers to come to England, smuggling them secretly out of Italy in large casks. Needless to say, he was not very popular with the pontiff and was promptly excommunicated!

 This discovery led to a boom as workers flocked to the area and more alum was discovered both in the moors and along the cliffs, with two of the largest mines being established in 1615 on the cliffs both at Hummersea and to the north of Loftus at Boulby.
It is difficult to imagine how men managed to work on such sites and in undoubtedly dreadful conditions.  The area is remote and in winter the weather can be bitterly cold and wet; there would be little shelter from the North Sea gales.  The process of extraction was very labour intensive, like all mining, and the refining process was chemically corrosive, causing clothes to rot in the fumes.  The work was quite sporadic and the Alum Miners and labourers frequently went unpaid – causing quite a lot of alarm among the more philanthropic landowners (probably because of the spectre – to them – of having to dole out Parish Relief to these poor souls!).

Not far to the south at Kettleness, there were two mines, one producing alum and the other producing iron ore.  However, on December 17, 1829 the cliffs around the iron ore mine collapsed, having been weakened by the excavations and the entire village slid slowly but inexorably towards the sea.  Fortunately, there was an alum ship standing offshore and most of the residents managed to get safely aboard, but the sliding earth carried away both the village and the alum works.  They were rebuilt in 1831 but disaster struck again when they caught fire and burned for two whole years.

The Manufacturing Process

The process involved in extracting alum from alum shale was long and complicated and very inefficient.  The shale was first quarried from inland hillsides, such as Belman Bank, Guisborough or coastal cliffs, such as Boulby, leaving massive quarries scarring the landscape.  It was then heaped into large mounds (often up to 30 metres high) which were made up of alternate layers of shale and brushwood, these were fired and left to smoulder for up to 9 months.  The roasted shale was then tipped into leaching tanks where it was left to soak in water.  The solution, containing aluminium sulphate (the active ingredient of alum), was then drained off and ran along stone or wooden conduits to the ‘Alum House’.  Here the water was boiled away from the solution in evaporating pans.

Hummersea - Remains of Conduits/troughs

Hummersea - Remains of Alum House

An alkali, derived from human urine or burnt kelp, was added to cause precipitation of the alum crystals, the correct point at which to add the alkali being determined by an egg – when it floated to the top of the pan it was time to add the alkali. The crystals were then bagged and transported for sale. The burnt shale left in the leaching pits was either disposed of nearby to form enormous shale tips (inland sites) or simply thrown in to the sea (coastal sites). In some places it forms a thick red layer in the cliff strata. 

This is an over-simplification of the actual manufacturing process – sometimes the process of leaching and boiling would have to be repeated 3 or 4 times, using good alum to “seed”  the liquor, to obtain alum of sufficient purity.  In all it was a very inefficient industry – 12 tons of shale produced at best 1 ton of Alum!

A secondary industry was the supply of urine for the alum Industry – it would be collected from the surrounding area (in much the same way as “night soil”) by scavengers (a trade!) and sold to the Alum Houses.

Ships would also bring supplies of urine from the large cities and coal and beach themselves (called “taking the ground”) to unload their cargoes and take on the alum from the alum works.  At Saltburn are the deteriorating remains of a rutway cut into the shale bedrock to allow carts of standard sizes to negotiate a safe passage out to the waiting ships to take off supplies and transport the finished product back.  There are even traces of trackways coming round Huntcliffe from the Brotton direction (Saltburn Alum mines closed before 1800).

The alum industry in North Yorkshire began in the first decade of the 17th century.  Despite the scale of the industry it was rarely profitable and mines had frequent gaps in operation.  Only three mines in Redcar and Cleveland continued in use into the 19th century, these were at Guisborough, Hummersea and Boulby. 
The death knell for this laborious process came in the 1860s when faced with competition from works in Lancashire, Humberside and Scotland which used the Spence Process (the action of sulphuric acid on the much more readily available coal shale) and the last Cleveland alum mine closed in 1871.

The remains of the alum works have left a dramatic impact on our countryside. The coastal quarries at Boulby and Hummersea have an almost unworldly feel; the red shale heaps giving the impression of being on another planet. Boulby is the best preserved site and surviving remains include the steeping pits and part of the Alum House. The site has been designated by the Secretary of State as a Scheduled Ancient Monument.

(Compiled and condensed from information readily available on the Internet)