Alum Shale

The Alum Shale Member forms the upper part of the Whitby Mudstone Formation and was deposited in a reasonably well-oxygenated marine environment around 186 million years ago.

The shales were extensively quarried between c.1604 and 1871 for the Alum Trade which flourished here on the coast and hills of Cleveland and North Yorkshire. The quarrymen would look for shales that contained the fossil bivalve Nuculana ovum knowing that these rocks were depleted in calcium carbonate (CaCO₃) but still contained pyrite (FeS₂) or fool’s gold. Calcium carbonate would negate the effect of the sulphuric acid (H₂SO₄) formed during breakdown of pyrite during the alum making process and ruin the finished product.

The Alum Trade

The Alum Trade came to the Cleveland hills between c.1600 and 1608. The location of this once strategically important industry is restricted to this part of the country because of the occurrence of the alum shale required to produce it. The Alum Trade endured for over 260 years before the final works in the area closed at Sandsend in 1871.

What is Alum?

Alum is a useful substance, used as a mordant (fixative) when dyeing cloth as well as being employed in tanning leather. It still is a vital chemical in many developing and industrial societies used for such processes as water purification.

Once it was essential and scarce. Long before it could to be manufactured in Britain, ancient Chinese and Arabic cultures employed alum as a key ingredient in alchemy and magic, in addition to other uses, e.g. as a remedy for toothache, in an elixir of life when combined with mercury and cinnabar, and as an ingredient in the many attempts at turning base metals into gold. For these reasons, the processes involved in making alum remained a closely guarded secret.

By 1459, alum production was under the control of the Papal States. By the time of the reformation and the beginnings of Protestantism, Britain’s supply was in jeopardy and a secure source was required prompting a number of searches.

The man responsible for bringing the alum makers secret to Britain was a North Country gentleman called Sir Thomas Chaloner. Whist touring Europe he is reputed to have visited the Papal Alum Works at Civitaveccia in Italy, where he found that the rocks quarried there were very similar to those on his land back home in England. In order for him to produce alum he needed more information but was prevented from seeing inside the alum-houses where the processing took place. Undeterred, Chaloner is said to have persuaded two alum workers to abscond with him. This was a very dangerous undertaking and legend has it that he smuggled them onto his ship in barrels and sailed away before they were missed. When the Pope realised that his monopoly of supplying alum to the whole of Western Europe was in jeopardy he excommunicated Chaloner before cursing the man, his family, and its future generations.

When Chaloner returned to England, he spent a number of years experimenting at Belman Bank, near Guisborough, before producing useable alum. Once this was accomplished however, he became the instigator of Britain’s first commercially important chemical industry. As the value of the local Alum Shale was realised the secret of alum making spread as landowners with suitable shale on their estates began to cash in on their good fortune. This development transformed parts of the once quiet coast-line and hills into industrial areas.

The Secret of Alum Making

The process required to make alum can be broken down into a series of stages:

• Quarry the shale and build it up onto heaps up to 10 metres high and 50 metres long (known as clamps) with brushwood interleaved. Up to 100 tons of shale might be required to produce each ton of alum, and of that, 99% of the material quarried ended up as waste! The distinctive pink or orange calcined waste still lines the beds of streams and washes up on many beaches in the area.
• After quarrying, the enormous heaps of alum shale were set on fire in order to start a complex series of chemical reactions. This part of the process (calcining) produced sulphuric acid that helps break down the minerals within the parent rock. The clamps often burned for months before the next step could be undertaken.
• After calcination, the clamps were broken open and clean water poured through them. This dissolves the sulphates, chemicals required in alum making, leaving some of the impurities behind. The resulting liquor (known as Mother’s) was allowed to settle before being channelled into nearby alum houses for evaporation.
• Next, the alum makers would add either potash, acquired by burning seaweed, or ammonia in the form of urine. Barrels of urine arrived from Sunderland, Hull, London, Newcastle, and other cities in boats specially adapted for landing on the rocky scars beneath the cliffs. The ships would later leave carrying alum in the self-same barrels.
• There was one last problem to overcome. The alum was in solution with unwanted ferrous sulphate, and a worthless product would result unless the alum could be isolated. Separation is a relatively simple exercise, namely boil the liquid to drive off some of the water and at some point alum will crystallise out of the solution. The problem was that very soon after this point the ferrous sulphate would also crystallise out ruining the batch.
• The alum makers needed a reliable way of indicating at what stage of evaporation the liquor had reached in order to identify the critical point at which the alum was ready. This knowledge was the alum makers secret.
• The answer they found is as ingenious as it is simple. As any salt crystallises out of solution, the density of the liquid varies. What the alum makers required was a way of monitoring the change so that at the right moment, the liquid containing the unwanted ferrous sulphate could be drained off leaving pure alum. Who found the answer no one knows, but in the years between 1600 and 1608, the secret of monitoring this process at last came to light and proved to be as ingenious as it was simple. A fresh hen’s egg placed in the evaporating liquor would initially sink, but as evaporation proceeded and the specific gravity of the liquor changed, then the egg would suddenly begin to float. What’s more, this phenomenon would occur at precisely the same time as the alum crystallised out…

EUREKA!

Alum Process Flow Diagram (Click image to open in new tab)

Alum working might still be carried out here if aniline (self-fixing) dyes and a synthetic method of sulphuric acid production had not been discovered in the 1800s. Following the latter discovery, alum could be made using colliery waste thereby doing away with the vast quarries required in the past. These discoveries led to the decline and ultimate demise of alum making on the coast and in the hills of Cleveland and North Yorkshire.