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.

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

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.

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

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.

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.