In Defence of Alum - 2. England

Up to about 1500 supplies of alum for this country came from various foreign sources. But during the 16th C. it was imported mainly from Papal and Spanish mines, neither, as can be imagined, acceptable to Henry VIII. In 1545 the search for domestic sources began. Singer lists no fewer than six attempts, made between 1545 and the end of the century, to produce alum, twice in Ireland; in Cornwall, the Isle of Wight, Dorset and Bournemouth. All failed. The first signs of success were achieved by the Chaloner family at Guisborough in North Yorkshire; one of them, having visited Puteoli and Tolfa earlier, may have gained some idea of alum production, albeit from a different ore.

In 1607 a Company was formed to exploit this find; new sources were discovered near Whitby; a Patent obtained; and imports of alum prohibited etc. But for the next twenty-five years, the Company failed to produce adequate quantities for the home market and thus justify a ban on imports. It suffered heavy financial losses; and it soon passed into the hands of notorious swindlers. It was not until 1635 that output reached the stipulated 1800 tons per annum and not until ca.1648 that the alum monopoly was abolished. As a result new alum works sprang up all along the Yorkshire coast at suitable outcrops of the Upper Lias shale (for further details see R.P.Pickles Bib.58). Under the Commonwealth the Royal Monopoly was abolished, but it reverted to the Crown on the accession of Charles II. In 1667 imports of alum were again prohibited and in 1679 the Crown finally relinquished its Rights over the industry.

The Process

It will be recalled, when describing the origins and growth in Europe of the manufacture of alum from shale, that a descripton of the process was deferred until we had reached the establishment of the industry in North Yorkshire. Despite the fact that the method practised here was a very inefficient one, North Yorkshire maintained a monopolistic position in Britain until the 19th C.[5]

Wherever aluminous shale was mined, the main elements of this ingenious process were much the same, with minor differences imposed by specific conditions found in a given locality. One of the chief advantages of the Yorkshire industry resulted from the exposure of Jurassic outcrops, known to be rich in mineral resources, at near-vertical cliff faces, see Plates 25 and 26. The seams could be mined without having to expend considerable labour in order to get rid of large quantities of overburden. This could be tipped down or over the cliff face. Another advantage was the availability of cheap transport, shipping, for bringing in supplies needed for the process and taking away the finished product.

The method evolved empirically long before its chemistry was understood. For simplicity the process has been divided here into six stages; for more detailed descriptions, see Almond (Bib.59, chemistry) and Marshall (Bib.60, industrial archaeology):-

1. Excavation

Pickmen, working down the cliff stepwise, removed the overburden of shale, sandstone and limestone, before cutting out the grey alum-bearing shale underneath. The spoil, if not wanted below as a level foundation for plant, would be tipped over the cliff.

The alum-bearing shale was then excavated and barrowed along elevated walkways to be tipped over onto a bed of brushwood about 1 m. thick. At a certain point the brushwood was set alight. Once burning, more shale was tipped on top until a clamp of anything from 12-30 m. in height had been achieved. This was allowed to burn slowly for up to a year.

2. Calcination

Almond identifies two objectives for this process:-

  1. to produce active water-soluble sulphate from pyrites (iron sulphide) in the shale.
  2. to apply the sulphate to convert the rock's aluminiun content into a water-soluble form.

The reaction, the slower the better, was affected by the weather. If the clamp overheated during a dry spell, it was damped down with water. Conversion, however, was never complete.

To function effectively the shale had to contain a significant proportion of carbonaceous material (coal) to act as a fuel. The combustion promoted the formation of sulphurous gases from the pyrites and, at the same time, rendered the shale more permeable, which assisted the conversion of the aluminium silicates into a water-soluble aluminium sulphate. To avoid losing the sulphurous gases the outside of the clamp was sealed with a shale paste.

As the reaction proceeded the clamp would shrink to half its original size. On opening it up, the colour of the calcined shale would be seen to have changed from grey to pink; and the desired texture would be soft and porous. The properties were improved by leaving it to be weathered until it was considered to be ready for the third stage.

3. Lixiviation

Quantities of clean water were required for this and for later stages of the process. If a source was not available on site, a reservoir had to be built (Marshall).

The calcined shale was barrowed to a series of stone leaching pits nearby. Almond gives typical dimensions of 9 x 4.5 x 1.5 m. designed to hold ca.30 tons plus of solid, tipped in to a depth of 0.3 m.

The object of this stage was to extract the soluble aluminium sulphate from the calcined mass. To assist this, acidic mother liquor, saved from a later stage, was run into the leaching tanks and allowed to percolate the solids for several days. The supernatant liquor was passed from tank to tank and eventually led thence in wooden channels (launders) to the raw liquor cistern situated near the Alum House, where the later stages of the process were conducted.

Fresh liquid was added to the leaching tanks and the process repeated for several weeks. The waste solids were eventually dug out and discarded.

The Specific gravity of the liquor from leaching rose to 1.12, indicating 12 tons of dissolved salts to 100 of liquor.[6] At this stage the liquor contained iron and magnesium salts as well as the aluminium sulphate.

4. Concentration of Liquor by evaporation

The next step was to clarify and concentrate the liquor, which was passed by gravity through stone channels to the Clearing House, where it was brought to the boil for twenty-four hours in open lead pans (Almond gives dimensions of 3 x 1.45 m.) resting on closely spaced iron bars (to protect the lead) over a furnace. The concentrated liquor was then passed to a lead-lined settling tank. The object of settling was to remove much of the yellow iron silicate known as "slam" and other impurities. Once it had been clarified, it flowed by gravity into the Cleared Liquor Cistern. (By the mid-19th C. this cistern had not only a waste pipe leading from the bottom, but another outlet, situated above a stone partition, which allowed the supernatant liquid to proceed to the next stage, leaving any residual precipitated matter to sink below). The cleared liquor was then subjected to further evaporation in lead pans with a sloping bottom, the contents being reduced to about one third of the original volume by boiling for 12-24 hours.[7]

5. The Formation of alum

The concentrated liquor at this point contained sulphates of aluminium, iron and magnesium. A quantity of alkali (potash from kelp or ammonia from urine, or both)[8] was added to the liquor before passing it to the Cooling House. The object of adding the alkali was to convert the aluminium sulphate into the double salt with its unique solubility characteristics. The precise quantities needed were judged by the foreman, partly from experience and partly based on specific gravity tests (see Note 6). The liquor was allowed to cool in shallow tanks (Marshall, ca.13 x 7.5 m.) in a well ventilated building. The solubility of alum decreases sharply as the temperature falls. As a result small alum crystals are formed. For a few days the remaining soluble elements were allowed to drain away. The crude alum crystals were then dug out and washed with cold water or mother liquor. Alum has a low solubility in cold water in contrast to other sulphates.[9] So this process can be regarded as another stage in the purification of the alum.

6. Final Stage, Recrystallization in Roaching Casks

As mentioned, Alum has the useful property of having a low solubility in cold water in contrast to the other impurities still in solution. This principle is exploited in the final stage of separating the alum from the other constituents. The crude alum crystals (from stage 5) were redissolved in very hot water and transferred to large "roaching" casks and allowed to cool. The alum crystallizes out leaving any remaining impurities still in solution. (See Note 9). The "roaching" casks (Almond, 2 m. high), were made up of lead-lined wooden staves, which could be assembled or dismantled as required. The casks and their contents were allowed to stand for eight days, then the staves were removed, exposing a crystalline mass. After a further eight days, a hole was drilled through this crystalline mass to allow any residual liquid to drain away, again for use as mother liquor. After another eight days the crystals, now in a highly purified state (about 2-3 tons per cask) were bagged up ready for shipment and sale; some of the alum may have been crushed to a powder before it was packed, but it seems there is some uncertainty about its final state.

Next 3. Alum in paper


[5] It has been calculated that in North Yorkshire 100 tons of shale (with a potential of 130 tons) produced only 1-3 tons of alum (Almond). Early in the 19th C., the Hurlet works near Glasgow managed 30 tons alum per hundred shale, with Spence, at Manchester and Goole, 130 tons of ammonium alum per hundred shale. In each of these instances a shale progressively richer in aluminium was used (ibid).

[6] Once extraction of the aluminium sulphate had taken place, the complicated stages that followed, concentration and purification of the liquor, recrystallization etc., required regulating and techniques for monitoring the results. It is a remarkable fact that such a sophisticated method as determining the specific gravity of the liquors was used at such an early date; indeed, that specific gravity was used at all for controlling an industrial process on this scale.

Singer (Bib.57 pp.193,195-7) refers to one method in use as early as 1678; and others, described as a matter of course in contemporary accounts, such as calibrated sticks, weighing bottles and even floating an egg to indicate the saturation point before allowing liquor to cool for crystallization.

[7] Excessive magnesium could reduce the effectiveness of the heating (Almond).

[8] The potash (mainly potassium carbonate) for the North Yorkshire industry was usually obtained from burnt seaweed. Sodium alum was unsatisfactory because it was too soluble in cold water. The urine (human) was shipped in casks from London (See Note 9 below).

[9] The solubility of potash alum increases fivefold between 0 and 40C.; sixteenfold at 70C.; and one hundred and fifteenfold at 88C. (i.e. from 5.7 g. at 0C. to 664 g. at 88C. per 100 g. water). Solubility of sodium alum at 0C. is 106.4 g. and 121.7 g. at 45C. per 100 g. water.

The solubility curve of ammonium alum is not quite so dramatic as that of potash alum. (Thorpe's "Dictionary of Applied Chemistry" 1937 Vol.I pp.295-296).

We couldn't reproduce here the photographs in the book of the cliffs near Whitby, but you can find interesting resources on the web if you search in Google for "Alum Yorkshire".

This essay has four sections, spread over three pages:

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  2. This page - England
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