Alum Industry

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Published on 1st October 2018, this is now on sale at The Book Corner, Saltburn and Guisborough Bookshop. Also available online through Amazon and other online bookstores. Price £15

Written by our Secretary, Peter Appleton, this book draws upon over five years of research in North Yorkshire County Record Office, Northallerton; Palace Green Library, Durham; the private archive of the Marquess of Normanby at Mulgrave Castle, Lythe; and other locations from Falkirk to Poole and from Swansea to Beverley.

This, the first great chemical industry in England, came into being c.1600 and continued in existence until the early 1870s. There were works located in various places, along the northern escarpment of the North York Moors, on the coastal cliffs from Saltburn down to Ravenscar, and also on the sides of the valley of the Esk and its tributaries.

Alum’s chief use was as a mordant in the dyeing industry. A mordant is a chemical which helps the dye pigment “adhere” better to the fibres of the wool, linen or whatever other material was being processed. Its use produces brighter colours that are more fade-resistant. Alum also had many other uses, some of which have stood the test of time and others which haven’t!

There are some two to three dozen disused alum works spread along an arc stretching from Thimbleby, near Osmotherley, to Peak, near Ravenscar. The most obvious visible remains of these are the huge quarries where the alum workers extracted the alum shale in order to process it. A simplified explanation of the process follows…

First the shale was piled up on a base of brushwood to form a clamp. Shortly after starting to build the clamp, the wood was set alight and the clamp was left to burn for anywhere from 6 to 12 months, with more burnt shale being added until the desired size had been achieved. Each clamp could be up to 90 feet high and perhaps as much as 200 feet across. This process is called calcining.

At the end of the calcining, the burnt shale was removed and placed into steeping pits, large, stone-lined tanks dug into the floor of the quarry. Spring water or collected rain water was run into the pits and the shale allowed to soak whilst the soluble chemicals were leached out. The liquor was weighed to establish whether it was at the right level to be run off to the Alum House where all the remaining processes took place. Weighing the liquor was the way the alum makers tested what we would call the specific gravity. What we have here is a chemical industry operating before the science of chemistry had been established. Everything was done on a “trial and error” basis.

When the liquor arrived at the Alum House it was boiled up to increase its weight (i.e. raise its specific gravity). Then some alkali was added to adjust the acid level and to trigger the depositing, as a precipitate, many of the impurities in the liquor. This alkali, in the first two hundred years or so, was derived from burnt seaweed or from stale human urine.

The liquor was then allowed to cool and, in so doing, the first alum crystallization occurred. These were impure crystals, so they were washed, re-dissolved in hot water and then this liquor was pumped into large wooden casks to “roach” – i.e. to form the final, saleable alum crystals as it cooled again.

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