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Salt from Seaweed? An Experimental Archaeology Perspective on Salt in Early Medieval Ireland

Salt has been recorded as an essential ingredient and preservative in the diet of the Irish early medieval period (c. 400-1100AD), although its addition to food is likely to have been widespread prior to this. The archaeological record of this period has produced evidence for foods requiring salt for preservation, such as dairy, meat and fish. The contemporary literature features salt heavily as an ingredient and a preservative. However, there is very little information regarding the procurement of the salt during this period. In this article, I suggest that seaweed could have been a raw source of salt during the early medieval period. Using experimental archaeology, salt was produced by burning dried seaweed, soaking the ashes and allowing the water to evaporate. The experiment was successful in the production of sodium chloride and it was found to be a very feasible method of producing salt. This experiment was carried out as part of the author’s PhD, entitled Early Medieval agriculture, food production and consumption, which is currently being carried out in the School of Archaeology, University College Dublin. It is funded by a Teagasc Walsh Fellowship.

Source material about food in early medieval Ireland

There is an extensive amount of literary and archaeological information available with which to understand the food and diet of early medieval Ireland and the role of salt in that diet. However, the richness of this evidence is in stark contrast to the lack of information on the sources of salt in this period.

Ireland holds one of the richest collections in Europe of early medieval vernacular literature regarding the contemporary society. The early medieval texts include law tracts and legal glosses, ecclesiastical legislation and penitential, as well as annals, wisdom-texts, sagas and poetry. These documentary sources are invaluable for reconstructing food practices as they provide a great deal of information about food preparation and consumption, as well as cultural preferences, tastes and taboos regarding such.

Extensive information about the early medieval period has also been provided by archaeological evidence, which includes both the archaeological sites and the artefacts and ecofacts or environmental remains provided by the latter. This material has shed light on the production and processing of food, such as livestock management and cereal cultivation. Modern scholars have sought to provide syntheses of the information available at the time of their publication. A.T. Lucas and his seminal paper1 Irish Food before the Potato focused on the foods of the early and post-medieval diet before the potato was introduced. His work was recently revisited by Downey and Stuijts,2 who combined evidence from a range of early medieval, late medieval and early modern historical sources to identify the basic characteristics of the Irish diet before the famine. Fergus Kelly’s book3 on farming in early medieval Ireland incorporates evidence from early medieval texts and the known archaeological evidence at the time. Regina Sexton’s recent work4 on cereal products is also an extremely informative text. Most recently, the archaeological excavations carried out by the National Roads Authority, since incorporated into Transport Infrastructure Ireland (TII), have provided massive resources for reconstructing the past. These bodies have published a series of monographs and conference proceedings based on the results of excavations carried out prior to the construction of national roads and motorways (, within which there are reports and papers relating to food remains and associated artefacts (e.g. Carlin et al., 2008)5. The greatest attempt to synthesize food-related data generated from the hundreds of excavations carried out in association with the Celtic Tiger construction-boom has been in the form of research carried out by The Early Medieval Archaeology Project (EMAP). This was funded by the Heritage Council through the Irish National Strategic Archaeological Research Programme (INSTAR) and it sought to investigate and understand early medieval Ireland through the analysis of excavated sites and subsequent publications and reports.6

Diet in early medieval Ireland

Deductions from these sources indicate that cereals and milk were 'the mainstay of the Irish diet from the prehistoric period until at least the seventeenth century'.7 However, meat, fish, eggs, fruit, nuts, vegetables and honey were also consumed in varying ways. Salt was an important ingredient and preservative in much of the daily fare. It was also considered a luxury item offered only as a condiment to the upper grades of commoners/free landowning non-noble farmers when hospitality was being provided.8

Dairy products (banbiadh) were heavily consumed and cattle were generally not slaughtered until they were unable to give milk or were at the very ends of their lives.9  The term báinne was used as a collective noun for drinks such as leamhnacht (new milk), draumce (thickened and soured skimmed milk) and báinne clabhair (thickened milk).10 Descriptions of such variety are listed in the eleventh century tale or parody, Aislinge Mheic Conglinne (The Vision of Conglinne). The 'world under potential gluttony'11 outlines various forms of foodstuffs and serving methods.  

Cheese was also important in the early medieval period and there appeared to be a large amount of technicality and skill involved at this time which has subsequently been lost. There were a great number of cheeses which Ó Sé12 subdivided into categories of pressed and unpressed. They were made of cows’, goats’ and sheep’s milk, as well as whey and buttermilk, using animal and plant rennets. They varied from being small pocket-sized to those as large as buttocks!13 Salt or a brine bath would have been necessary to preserve the cheeses.14 Butter was also produced and preserved with accounts of fresh, lightly salted and heavily salted butter. The latter was known as gruiten; it was stored in firkins and required slicing and washing afterwards to remove the salt.15 As O’ Sé16 recorded, the law tracts denoted that salt butter was to be provided to the children of inferior social grades while in fosterage; fresh butter was supplied to the children of chieftains. The production and use of butter involved incantations and the ‘magical’ transformation of milk into butter is a good example of how pagan and Christian beliefs became fused. A thirteenth century manuscript (St. Gall MS 1395)17 contained ninth century healing charms, known as the St. Gall Incantations. One of these is spoken either when producing butter or to remove a thorn and it comprises a mixture of an Early Christian prayer and a dedication to a pagan god; Goibniu.18

Nothing is higher than heaven, nothing is deeper than the sea. By the holy words that Christ spake from His Cross remove from me the thorn, a thorn..... very sharp is Goibniu’s science, let Goibniu’s goad go out before Goibniu’s goad! This charm is laid in butter which goes not into water and (some) of it is smeared all round the thorn and it (the butter) goes not on the point nor on the wound, and if the thorn be not there one of the two teeth in the front of his head will fall out….

Seven grains were listed in the eighth-century Irish law tract Bretha Déin Chécht19 and it is believed that this also functioned as a list of the prestige value of each grain. However, the four main grains which are noted in the archaeobotanical assemblages are hulled barley, oats, naked wheat and rye – in that order of descending importance.20 Grain was consumed in different ways – in risen breads, flat-breads, savoury flat cakes, porridges, pastes and alcohol. A piece of oatcake made with whey was found in a sixth century ringfort, providing definitive evidence for its consumption.21 Sexton22 has carried out experimental work on breads and she suggests that, as salted foods were typically eaten as condiments, it was likely that salt was not an ingredient in bread. However, it may have been added to grain products to ensure storage.

Meat appeared to be predominantly from pigs, although beef was probably consumed during special occasions. Domesticated fowl, wild birds and seabirds, boar, deer, freshwater and salt water fish and shellfish were also consumed. However, it would appear that much of the meat eaten was in a processed and salted form.23 Pork products included bacon flitches, sausages and puddings and the flitch was often used as food-rent.24 Meat was also boiled in a gruel and cooked over a spit with honey and salt. Mutton was rarely consumed.

Other foods consumed included eggs, fruit, nuts, herbs, pulses, vegetables and honey. Much of the foods noted in the texts are eaten today such as onion, cabbage and chives25 although wild plants such as brooklime would not be seen on a contemporary dinner table.

Generally, it would appear that the main meal (proind) was eaten in the afternoon or early evening, with light meals or snacks consumed at other times of the day.26 There is often a distinction made between the daytime meal (díthat) and the evening meal (feis); with the latter often being consumed with beer. Another term (lón) suggests a snack which is taken on a journey.27 The supplementary food was most commonly known as tarsunn; although there is a distinction made between tarsann and annlann in some texts. It would appear that the former comprised of a wider variety than the latter; butter, dairy products, salted meat, seaweed, suet and cabbage.28 Another less commonly used word in place of tarsann and annlann is sercol, as noted in Críth Gabhlach, for seaweed, onions and salt. 

It is clear that there was a wide variety of food available in the early medieval period; however seasonality and personal preference dictated that salt was an important part of the diet both in relation to preserving food and as a taste enhancer. It is, therefore, unusual that there is very limited evidence as to where and how salt was sourced and produced.

Sourcing salt in early medieval Ireland

Despite the apparent importance of salt in early medieval Ireland that there are very few references and scant physical evidence of how salt was obtained or produced.  There are fifty-three references to salt (salann) in The Electronic Dictionary of the Irish Language (; however, none note the actual origin of salt.

There are obscure references, such as the eighth century note of a salt tax in the Aran Islands.29 In the seventh century, Adomnán refers to rock salt which was blessed by Saint Columb Cille.30 However, the first mention of the actual procurement of salt is in the twelfth century when there is a reference to salann Saxanach, (Anglo-Saxon salt) in the Aislinge Mheic Conglinne suggesting its fine nature;31

And he called for juicy old bacon, and tender corned-beef, and full-fleshed wether, and honey in the comb, and English salt on a beautiful polished dish of white silver.

The majority of salt is made from evaporating saline water and the method varies depending upon the local climate. In coastal areas where the air is dry and there is sufficient wind, sea water evaporates naturally allowing the harvesting of the salt which remains behind. The water can be directed into pits where the salt remains after the water is gone.  Evidence of such pits in Ireland date to the post-medieval period and were recorded as part of The Archaeology of Salt Project.32 However, there is no recorded early medieval evidence to date.

Another salt-production method which leaves archaeological traces consists of boiling seawater over fires to force evaporation in pottery known as briquetage. This pottery is then broken open to remove the salt. There is very little reference to or finds of such material in Ireland; fragments of a possible pedestal were present on a Bronze Age site in Co. Meath),33 but there does not appear to be comparable material.


An interesting reference to salt is made in the law tract Uraicecht Becc, where the texts rules that amongst other sacks of food produce, a bóaire should always have a sack of salt (sea-ash [murluaith]) 'against the cutting up of joints of his cattle' .34 The term sea-ash is intriguing as salt can be made from burning seaweed or sea plants, as is a common reference in historical literature throughout the world (see below).  Kelly suggests that it in maritime areas where evaporation or salt-mining is not possible,35

salt may be obtainable by burning seaweed or shore vegetation, and boiling up the resultant ash with water.

There is evidence for charred seaweed associated with a small hut site in Doonloughan, Co. Clare.36 Charred seaweed was also found to be associated with structures at the early medieval site of Deerpark Farms, Co. Antrim, and it is suggested that it was burnt to possibly produce lye.37 

Burning seaweed to make ‘black salt’ for human consumption and cattle licks has been historically recorded along the fringes of the damp North Atlantic region. Mooney38 has noted that the practice of producing black salt was recorded in twelfth century Denmark39 and in the medieval Norwegian Gulaþing law.40 Rosie Bishop noted charred seaweed in deposits excavated from a Viking longhouse in Iceland and surmised that the seaweed had been burned to produce black salt (pers. comm). Geochemical analysis of the house deposits indicated a high concentration of salts in a corner of the structure where there was also a large number of charred seaweed fragments.41 Post-medieval evidence in this region includes references to 'the ashes of burnt sea-ware' (seaweed) used to cure meat and fish in the Scottish Western Isles at the close of the 17th century.42 Lucas43 suggests that this manner of salt production corroborates the references made in Irish documentary sources to the sack of sea ashes (murluaith) required to be kept by the boaire.44 In the northern Netherlands archaeological evidence was uncovered for the burning of eel-grass (Zostera marina) in pits.45 The archaeologists theorised that the traditional method of salt extraction from peat in this area was shifted to salt extraction from eel-grass with the loss of peat due to environmental and anthropogenic activity.  Japanese moshio salt has been produced from at least the first millennium,46 by burning seaweed and mixing the ashes with seawater.

It is therefore proposed that some of the salt in early medieval Ireland was produced from seaweed, by drying it, burning it, soaking the ashes in water, removing the charcoal and boiling or allowing the water to evaporate until salt is produced.

Experimental salt-production

An experiment was devised to burn seaweed and filter the ashes in order to test out whether this was a feasible method of salt extraction during the early medieval period. Three large sacks of seaweed were gathered and were left to dry out for a week. A pit measuring c. 1m wide, c. 1m long and 0.40m deep was excavated in the Centre for Experimental Archaeology and Material Culture (CEAMC) University College Dublin. The seaweed was burned in the pit alongside various types of firewood. It took several hours for the seaweed to turn to white ash.

The contents of the fire were then placed in metal buckets and allowed to cool for two days. The ashes were subsequently soaked in water and the charcoal was sieved off. The water was then divided into two portions, with one amount of the fluid cleaned through a simple filtration process. This comprised pouring the water through filter paper twice. The filtered water was then divided into two amounts. One portion of the filtered water was burnt on a hot plate for a few hours to produce salt crystals and the second portion was allowed to naturally evaporate in order to form salt crystals. The remaining unfiltered water was allowed to evaporate naturally to form salt crystals.

A total of 184.73g of salt was produced; 22g of filtered white salt heated with hotplate; 97g of filtered naturally evaporated salt and 65.73g of unfiltered naturally evaporated salt. The three types of salt produced varied in size and colour. The salts were tested in the UCD School of Chemistry by Dr. Aaron Martin, who used silver nitrate to ascertain if they were composed of sodium chloride and therefore edible. This test comprised the addition of water to the salt to form a solution; silver nitrate was then poured into the solution. It reacted by forming a precipitate of silver chloride, which presented as a cloudy liquid. The chemical reaction indicated that sodium chloride had been formed during the experiment at the CEAMC.

Screen Shot 2020 01 20 At 10 22 53

Burning seaweed at UCD and testing for salt content with silver nitrate.


The results showed that the process of producing salt by burning dried seaweed, soaking the ashes and boiling the water was carried out with minimal effort. As such, the experiment is a reasonable interpretation of charred seaweed found on inland early medieval sites and it could therefore have been a very suitable method of procuring salt in this period. Small seashells which were attached to the seaweed were noted close to the fire in the CEAMC and they could therefore be indicators on future excavated sites that seaweed was imported onto the site. Other experiments associated with the use of the produced salt could be carried out; the firepit could be excavated to determine what remains would be left after burning seaweed; the different sizes of salt could be used to preserve various foodstuffs.

The experiments results therefore indicate that documentary and archaeological sources which suggest that seaweed can be burned to produce black salt are easily verified and the experiment can be used to inform future archaeological excavations when charred seaweed is present on an given site.

End Notes

Go to footnote reference 1.

A. T. Lucas, (1960) 'Irish Food Before The Potato', Gwerin: A Half-Yearly Journal of Folk Life, 3(2), pp. 8-43.

Go to footnote reference 2.

L. Downey and I. Stuijts, (2013) 'Overview of historical Irish food products - A.T. Lucas (1960-2) revisited', The Journal of Irish Archaeology, 22, pp.111-126.

Go to footnote reference 3.

F. Kelly, (2000) Early Irish farming : a study based mainly on the law-texts of the 7th and 8th centuries AD. Early Irish law series Dublin: School of Celtic Studies, Dublin Institute for Advanced Studies.

Go to footnote reference 4.

R. Sexton, (1998) 'Porridges, Gruels and Breads; The Cereal Foodstuffs of Early Medieval Ireland', in Monk, M.A. and Sheehan, J. (eds.) Early Medieval Munster. Archaeology, History and Archaeology. Cork: Cork University Press, pp. P76-86.

Go to footnote reference 5.

N. Carlin, L. Clarke, F. Walsh, and National Roads Authority. The archaeology of life and death in the Boyne floodplain: the linear landscape of the M4, Kinnegad-Enfield-Kilcock motorway. NRA scheme monographs Dublin: National Roads Authority 2008.

Go to footnote reference 6.

F. McCormick, T. Kerr, M, McClatchie and A. O'Sulllivan. (2011) 'The Archaeology of Livestock and Cereal Production in Early Medieval Ireland, AD 400-1100.', Irish National Strategic Archaeological Research Programme (INSTAR); F. McCormick, T. Kerr and A.O'Sulllivan. (2013) 'The Economy of Early Medieval Ireland. The Early Medieval Archaeology Project (EMAP): Irish National Strategic Archaeological Research (INSTAR) Programme' funded by The Heritage Council; M. McClatchie, F. McCormick, T.R. Kerr and A. O’Sullivan, (2015) 'Early medieval farming and food production: a review of the archaeobotanical evidence from archaeological excavations in Ireland', Vegetation History and Archaeobotany, 24(1), pp. 179-186.

Go to footnote reference 7.

Lucas, p.8.

Go to footnote reference 8.

C. N. Peters, '‘He is not entitled to butter’: the diet of peasants and commoners in early medieval Ireland', in Fitzpatrick, E.a.K., J. (ed.) Food and Drink in Medieval Ireland. Dublin: Royal Irish Academy 2016, pp.89.

Go to footnote reference 9.

Lucas, p.14.

Go to footnote reference 10.

M. Ó Sé, (1948) 'Old Irish cheeses and other milk products', Journal of the Cork Historical and Archaeological Society, 53(178), p.86.

Go to footnote reference 11.

W. Sayers, (1994) 'Diet and fantasy in eleventh‐century Ireland: The vision of Mac Con Glinne', Food and Foodways, 6(1), p.7.

Go to footnote reference 12.

Ó Sé.

Go to footnote reference 13.

Lucas, p.26.

Go to footnote reference 14.

A. Hagen, Anglo-Saxon food and drink: production, processing, distribution and consumption. Cambridge: Anglo-Saxon Books 2010, p.264.

Go to footnote reference 15.

Ó Sé, M. (1949) 'Old Irish butter making', Journal of the Cork Historical and Archaeological Society, 54, p.64.

Go to footnote reference 16.

ibid. p.64.

Go to footnote reference 17.

Go to footnote reference 18.

I. Tuomi, (2013) 'Parchment', Praxis and Performance of Charms in Early Medieval Ireland. Incantatio.

Go to footnote reference 19.

Kelly, Early Irish Farming, p.219.

Go to footnote reference 20.

McClatchie, McCormick, Kerr and O’Sullivan, 'Early medieval farming and food production'.

Go to footnote reference 21.

F. McLaren, M. Monk and R. Sexton, (2004) ''Burning the Biscuit': Evidence from the Lisleagh Excavations Reveals New Secrets Twenty Years on!', Archaeology Ireland, 18(3), pp.18-20.

Go to footnote reference 22.

Sexton, 'Porridges, Gruels and Breads'.

Go to footnote reference 23.


Go to footnote reference 24.

Kelly, Early Irish Farming, p.336.,

Go to footnote reference 25.


Go to footnote reference 26.

E. J. Gwynn and W.J. Purton (1911) 'The Monastery of Tallaght', Proceedings of the Royal Irish Academy. Section C: Archaeology, Celtic Studies, History, Linguistics, Literature, 29, pp.115-179.

Go to footnote reference 27.

Kelly, Early Irish Farming, p.317.

Go to footnote reference 28.

ibid; Gwynn and Purton.

Go to footnote reference 29.

Kelly, Early Irish Farming, p.342.

Go to footnote reference 30.

A. O. Anderson, and M. O. Anderson, Adomnan's life of Columba. Oxford: Oxford University Press, 1991.

Go to footnote reference 31.

H. Jackson, Kenneth (1990) Aislinge Meic Con Glinne.

Go to footnote reference 32.

R. McConkey and C. Breen, (2017) 'SALT OF THE EARTH, SALT OF THE SEA', Archaeology Ireland, 31(3), pp.12-16.

Go to footnote reference 33.

C. Ní Lionáin, (2008) 'Final Excavation Report, Site of Hotel Development, Stamullin, Co. Meath', Unpublished Report, Arch-Tech Ltd.

Go to footnote reference 34.

E. MacNeill, (1921) 'Ancient Irish Law. The Law of Status or Franchise', Proceedings of the Royal Irish Academy. Section C: Archaeology, Celtic Studies, History, Linguistics, Literature, 36, p.291.

Go to footnote reference 35.

Kelly, Early Irish Farming, p.341.

Go to footnote reference 36.

E. Murray, F. McCormick, C. Newman, S. Hamilton-Dyer and G. Plunkett, (2012) 'Doonloughan: a seasonal settlement site on the Connemara coast', Proceedings of the Royal Irish Academy. Section C: Archaeology, Celtic Studies, History, Linguistics, Literature, pp.95-146.

Go to footnote reference 37.

H. Kenward, A. Hall, E. Allison and J. Carrott, (2000) 'Environment, activity and living conditions at Deer Park Farms: evidence from plant and invertebrate remains', Reports from the Environmental Archaeology Unit, York, 57. p.520

Go to footnote reference 38.

D. E. Mooney, (2018) 'Charred Fucus-Type Seaweed in the North Atlantic: A Survey of Finds and Potential Uses', Environmental Archaeology, pp.1-13.

Go to footnote reference 39.

N. Hybel and B. Poulsen, The Danish resources c. 1000-1550: growth and recession. Netherlands: Brill, 2007.

Go to footnote reference 40.

A. Clément, (1914) 'Sort Salt', Danske Studier, 1914, pp.104-118.

Go to footnote reference 41.

K. B. Milek, and H. M. Roberts, (2013) 'Integrated geoarchaeological methods for the determination of site activity areas: a study of a Viking Age house in Reykjavik, Iceland', Journal of archaeological science, 40(4), pp.1845-1865.

Go to footnote reference 42.

M. Martin, A description of the Western Islands of Scotland. London: Andrew Bell 1703, p.129.

Go to footnote reference 43.

A. Lucas, (1965) 'Washing and bathing in ancient Ireland', The Journal of the Royal Society of Antiquaries of Ireland, 95(1/2), pp.65-114.

Go to footnote reference 44.

MacNeill, p.291.

Go to footnote reference 45.

B. van Geel and G. J. Borger,  (2005) 'Evidence for medieval salt-making by burning Eel-grass (Zostera marina L.) in the Netherlands', Netherlands Journal of Geosciences - Geologie en Mijnbouw, 84(1), pp.43-49.

Go to footnote reference 46.

T. Ohta, Y. Mahara, T. Kubota, Y. Saito, S. Fukutani, T. Fujii, A, Ando, E. Nakata, T. Nakano and Y. Abe, (2010) 'Radionuclides in ancient relics obtained from the matsusaki site and the hirohata shellmound on the Pacific coast of Japan', Radiocarbon, 52(2), pp.526-533.

Nikolah Gilligan

Nikolah Gilligan is a self-employed archaeologist and archaeobotanist. She is currently on leave from her PhD in UCD School of Archaeology, which is focused on research into early medieval food production, processing and consumption.