The Tankerton copperas works and the copperas industry in the south of England

Background

Tankerton, nr. Whitstable, Kent: Location of survey area.

An increased rate of marine erosion in Tankerton Bay near Whitstable on the north coast of Kent has led to the recent exposure of a great number and variety of timber structures on the foreshore. Amongst these structures was an extensive complex of wooden posts laid out in a triangular pattern and set in a roughly level surface of poured mortar. These structures were believed to be linked to a number of buildings, set at the bottom of Tankerton Slopes, associated with the copperas or green vitriol industry.

The exposed state of the mortar surface and of the timber posts meant that they were in danger of destruction by wave action. An additional threat was posed by a programme of coastal protection works planned by the Ministry of Agriculture, Fisheries and Food. In the light of this the Canterbury Archaeological Trust was commissioned to undertake a programme of excavation and recording in advance of the destruction of the remains.

In the course of the project, historical research demonstrated that the significance of the exposed remains could only be fully appreciated with reference to the documentary records and the results of previous research. The report submitted to English Heritage, of which this is a summary, consists of an account of the character of the copperas industry, its history and the archaeological traces found below Tankerton Slopes.

Copperas

Copperas, or green vitriol, is a form of ferrous sulphate, extracted from iron pyrite-rich nodules, which was used extensively in the textile and metallurgical industries and for a number of other purposes.

In metallurgy it was a key ingredient in the production of nitric acid (aqua fortis) and sulphuric acid (oil of vitriol) from which chlorine was produced. Chlorine was used as a bleaching agent in the 17th and 18th century textile industry while copperas itself was used as a dye fixative for woollens. Additional uses of copperas included printers ink, a tanning agent for leather and in the manufacture of gunpowder.

This extensive range of uses made copperas a valuable substance and a significant industry developed at places where nodules of iron pyrites occurred in Eocene clay deposits. Such deposits included bands of London clay in the Thames basin, in Dorset and in Hampshire. The co-occurence of the important Kentish woollen industry meant that production in the area was of even greater value.

The industrial process

The process of producing copperas is extensively documented in 17th century sources and these accounts, together with the surviving inventories of tools and equipment, make the task of reconstructing the manufacturing process a relatively straightforward one.

The copperas stones were collected from the seashore and placed in ‘beds’ which could measure up to one hundred and sixteen feet long by fifteen feet broad and twelve feet deep. The beds were lined with clay or chalk and filled with stones to a depth of around two feet. Several such beds were attached to the Tankerton works. The stones were left to weather for up to six years, towards the end of which time they would begin to produce a large quantity of liquid, described as liquor in contemporary accounts. This liquid, a dilute solution of hydrated ferrous sulphate and sulphuric acid, flowed down plank-lined channels into a cistern housed in a boiler house. At Tankerton one large cistern measured eighty feet by nine feet and the complex included at least two other, smaller, cisterns. From the cistern the liquid was pumped into a lead boiler positioned over a coal-fuelled furnace set in a firing pit. Scrap iron was placed in the boiler and the liquid added to it. During the subsequent twenty days over which the boiling took place further scrap iron was added. At Tankerton, one of the boilers described in surviving documents measured over twelve feet square and took an initial one hundred pounds of iron, followed by a further fifteen hundred pounds during the boiling process. As the liquor was reduced by evaporation, more was added.

When it was deemed to be sufficiently concentrated the liquor was tapped off into a cooling tank where it remained for up to fifteen days. The Tankerton coolers are described as being twenty-nine feet long by six feet and six inches wide. As the solution cooled the copperas (hydrated ferrous sulphate) crystallised on the bottom and sides of the tank, reaching a depth of around five inches. In some copperas works bundles of twigs were placed in the tank to promote crystallisation. The remaining solution was drained into a second cooler and reboiled.

The ferrous sulphate crystals were collected, heated to melting point and poured into moulds. The resulting cakes were packed in barrels for transport.

Some changes to the process were made during the 17th century, mainly to improve the efficiency of the boiling, but otherwise it appears to have remained relatively unchanged throughout the 18th century.

The history of the copperas industry

The production of sulphate / metal compounds, known generically as ‘vitriols’, was known to the Babylonians and had become well established by Classical times, being described in some detail by Pliny. A number of early medieval accounts describe the manufacture and use of vitriols, and Jabir-Ibn-Hayyan (721-815) distinguished between green vitriol and blue vitriol, ferrous sulphate and copper sulphate respectively. By the 14th century production was centred in Asia Minor and was extensively controlled by a Genoese syndicate. The fall of Constantinople in 1453 drove many of the Italian producers back to Italy where a Papal monopoly was established, later being extended, under licence, to Spain, Denmark, Germany and the Netherlands. During the 16th century the Papacy tightened its control on the movement of vitriol and this stimulated a search in England for a local source as the substance was essential to the massive English wool industry.

Several abortive attempts were made to manufacture vitriol in southern England under Henry VIII and Elizabeth I and considerable state subsidies were granted to entrepreneurs, an indication of the importance attached to finding a local source. Amongst the individuals working on the problem were a number of Protestant settlers from France and the Low Countries who arrived in England during the reign of Elizabeth.

The first interest in developing a copperas industry at Tankerton is recorded in a letter written by Matthew Parker, the Archbishop of Canterbury, to the Lord Treasurer, Sir William Cecil in 1569. In this letter Parker alludes to a ‘stranger’ who claimed to be able to manufacture ‘brimstone’ using raw materials from Whitstable. This individual seems likely to have been Cornelius de Vos who was also involved in the opening of a copperas works in Dorset.

The success of this venture encouraged others to investigate the potential offered by the south coast and by the 17th century a whole series of plants had been established around the coast and in the Thames estuary. That the industry was a profitable one is attested by remarks in private letters and by the accounts of the various individuals involved. It appears from legal records and inventories that the copperas industry came to be controlled by a group of families with strong social links who intermarried and maintained close connections. The Tankerton industry was thus closely connected with those in Essex, Queenborough and Deptford.

The copperas works in southern England prospered for about two hundred years and by 1746 England was amongst the largest producers in Europe. During the 18th century however, more efficient methods of production were developed in northern England and in Scotland. In 1787 a means of producing sulphuric acid without using copperas was discovered and this further depressed the industry. Such developments spelled the end of the southern English industry and the Tankerton and Deptford works ceased production in 1828, while others merged with other local chemical manufacturers and were eventually superseded.

The copperas industry at Tankerton

As mentioned above, Cornelius de Vos appears to have recognised the potential of Tankerton as a location for copperas production in the mid 16th century. The nature of the production process, which required long term investment in plant and raw material, meant that considerable capital was required to cover the initial period of processing. It appears that de Vos lacked such capital and, despite receiving the patent to produce copperas in 1565, he does not appear in the Whitstable lay subsidies until 1588. It is probably no coincidence that this was the date when the lease on his Dorset works (initially funded by Lord Mountjoy) ran out.

Once established, the profitable Tankerton industry became the subject of a series of legal disputes which involved the theft of equipment and leases, the assumption of false identities and other forms of chicanery. Law suits are well documented and it may be no coincidence that early inventories from the Brightlingsea and Walton works list muskets, swords and bayonets amongst their equipment. The legal papers also provide important insights into the organisation of the industry and the employment of the workforce.

Both documentary and archaeological sources indicate that the two earliest copperas works were situated on the coastal plain to the north of Tankerton Slopes and were lost to marine encroachment within fifty years. Charts dating to the early 18th century show later buildings located above the Slopes. Cartographic evidence can also be used to follow the rise and decline of the industry. In 1770 ten buildings were shown on the maps, but by 1835 only one survived.

The final years of the copperas industry are a story of decline in the face of competition from the more efficient northern English and Scottish manufacturers. By the end of the 18th century the various concerns centred on Tankerton had been acquired by Charles Pearson who seems to have set out to become a copperas ‘magnate’. In addition to Tankerton he owned the Deptford works and controlled the rights to collection at Kirkby Thorpe and Walton-le-Soken. By 1808 he controlled 30% of the copperas production in the south of England. The investment was unwise. During the 1820s he was forced to sell the Tankerton copperas works and his home, Tankerton Towers, to his wealthy cousin Wynne Ellis. Ellis, evidently a shrewder businessman than his cousin, did not continue copperas production. In 1835 Pearson was declared bankrupt.

Documentary sources for the history of the copperas industry

The primary documentary evidence falls into three categories:

• Legal documents, including grants of patents, indentures, leases, mortgages, deeds of sale, inventories, cargo manifests, wills, and ownership documents.
• Contemporary accounts of the manufacturing process and descriptions of associated activities, including technical accounts and private letters.
• 18th and 19th century maps and charts showing the positions of copperas houses and associated structures.

Secondary sources include a number of histories of the English and European chemical industries.

The archaeological investigation

Tankerton, nr. Whitstable, Kent. : Copperas industry.

The archaeological site lies to the north of the concrete sea wall and promenade at Tankerton, some 9 km north of Canterbury. The site lies beneath a low cliff of London Clay, the origin of which has been ascribed to marine erosion, although the archaeological evidence has cast doubt on this suggestion.

Following recent erosion which exposed a variety of timber structures and a substantial poured mortar floor on the Tankerton foreshore, a proposal for archaeological investigation was put forward by the Canterbury Archaeological Trust. The proposal centred around a survey of the exposed timbers and other features and an evaluative excavation of two areas (6.6% of the total) within the inter-tidal zone.

Initial excavation was undertaken using a mechanical excavator which removed a substantial quantity of shingle and sand which was then used to construct a barrage to protect the site from the effects of wave action. In spite of this, the site was inundated at each high tide. The lower levels of sand and shingle were removed by hand.

Evaluation area 1

Area 1 contained a series of floors, floor beddings and levellings laid down over a protracted period of time. Finds included iron and copper nails and fragments of scrap iron. The basal layer consisted of yellow powdery dumped deposits, most probably the sulphur-rich by-products of copperas production. Similar material was found underlying the poured mortar floors and seems to have been regularly employed as a levelling material.

Timber structures set into these floors appear to have included windbreaks and groyne-like structures, together with two abutting triangular structures of undetermined purpose. Although resembling groynes in their form, the structures were too light to have withstood heavy seas, and other evidence indicates that the area was not subject to inundation during the early 18th century, the period to which they can be most plausibly ascribed.

Evaluation area 2

Evaluation area 2 included an area of London Clay, redeposited as alluvium, overlying a layer of peat which contained fragments of tile suggesting that it had accumulated during the medieval or post-medieval period. Further excavation revealed a spread of chalk rubble and limestone blocks associated with a row of upright timbers on its north-western edge and a second row intersecting the rubble feature on a south-eastwards alignment. The area defined by the two linear features was covered with flint cobbles and stained red to a depth of 5 cm. This suggested that the feature was the site of a copperas bed, used for the initial weathering of the nodules. The adjacent flint cobbling included numerous fragments of brick, iron nails and metal objects, including a 15th / 16th century cloth seal.

Two rows of timbers set in a triangular pattern were also exposed and seemed to be part of the similar arrangement seen in area 1.

Later features in the area appeared to indicate the existence of a wharf or similar structure, constructed of limestone fragments from the Thanet Beds which do not occur naturally in the area. A further timber structure, adjacent to the modern sea wall, appear to be the remains of a raised walkway or long wharf.

Interpretation

Without the contribution of documentary records it would be extremely difficult to interpret the structures discovered on the Tankerton foreshore. Fortunately a number of accounts give precise details of the industrial process and these can be used to interpret the archaeological features. An account written in 1677 by D. Colwall describes the structure of the copperas beds at Deptford:

"They ram the Bed very well, first with strong clay, and then with the rubbish of Chalk, whereby the Liquor, which drains out of the Dissolution of the Stones is conveighed into a Wooden shallow Trough ..." (Colwall 1677: 1057)

This immediately brings to mind the foundation-like chalk and limestone spread found in evaluation area 2, with the residue of the drained out ‘liquor’ comprising the red-brown stain. Similarly Colwall’s description of the construction of the bed for the boiler used to concentrate the liquor prior to crystallisation suggests that the cobbled area in Area 2, with its numerous fragments of brick, two iron bars and fragments of lead represents the remains of a demolished copperas boiler. If this is the case, then Area 2 would appear to be the site of one of the earliest Tankerton copperas plants. The dating is confirmed by the find of a 15th / 16th century cloth seal which did not appear to have been redeposited.

The triangular timber structures appeared to be parts of the structures shown schematically on a plan dating to 1725. The function of these is not immediately apparent, but their clear association with the poured mortar floor in Area 1 indicates that they were originally dry-land structures. It may be suggested that they represent the remains of copperas beds post-dating those described above. It may be that the use of mortar was a design improvement intended to speed up the weathering of the nodules, the most time consuming element of the manufacturing process.

The groyne-like rows of upright timbers, which were interpreted as windbreaks, are not shown on the 1725 map, but were clearly associated with the triangular structures and appear to be contemporary with them. The triangular structures had vanished by 1770, when a map shows the area to have been occupied by thirteen rack-like structures of unknown function.

A survey dating to c 1770 makes specific mention of defensive works carried out on the Tankerton foreshore, including groynes and quantities of stone, laid down to stabilise the area against erosion by the sea. These can be identified with the large amounts of redeposited limestone visible in the area. The ‘long wharfing’, specifically mentioned in the survey, seems to refer to the linear plank and post structure exposed in the southern part of Area 2. A further feature shown on the 1770 map, a timber fence running east to west, can be confidently identified with a linear structure mapped during the EDM survey of the site. It probably marked the northern limit of the wharfing, landpiling and other works described in the 1770 survey.

The significance of these connections between the documentary and archaeological evidence lies in the fact that the copperas industry has been largely forgotten and, where this is not the case, has been misunderstood. The Tankerton excavations are unique in providing the only body of integrated archaeological and documentary evidence presently available for this important industry.

The artefacts

The artefacts recovered from the site range from the cloth seal of 15th / 16th century date to two Edwardian coins. The mixed nature of the assemblage is a result of the continual reworking of the shingle overlying the site through wave action. It is clear however, that, in addition to the intrusive artefacts, a number were contemporary with the cobbled surfaces within which they were found. This would appear to include the cloth seal, fragments of brick and tile and many of the iron and copper alloy nails. The lead fragments and waste scraps were probably associated with the maintenance of the lead boilers. The scrap iron, including many of the nails, may have formed part of the large quantities of iron required during the boiling of the copperas liquor. A considerable part of the artefact assemblage can thus be linked directly with the copperas industry.

The potential of the documentary and archaeological material

The excavation and documentary research carried out during the Tankerton Slopes project offers considerable potential for the understanding of the archaeology of the copperas industry. The documentary sources are sufficient to;

• provide the basis for a detailed history of the copperas industry in the south of England from its establishment in the 16th century until its collapse in the early 19th century,
• provide a detailed description of the industrial process from the gathering of the raw material to the transport and sale of processed copperas,
• provide the basis for an interpretation of the archaeological remains on the Tankerton foreshore.

The archaeological information constitutes a body of data valuable in the recognition of copperas sites elsewhere on the coast. Together with the documentary material, the two bodies of data are sufficient to form the basis of a comprehensive report describing the establishment, development and decline of an important early industrial process and its contribution to the early modern economy of southern England. The combination of specialised knowledge held by Huguenot immigrants, the role of early capitalists and the technical developments within the industry all represent an excellent case study in early industrialisation. It is proposed that the report, upon which this summary is based, should form the core of an occasional paper dealing with the copperas industry of southern England.

Bibliography

Colwall, D. 1677, An account of the way of making English Green Copperas. Philosophical Transactions of the Royal Society XII.