Fulacht fiadh - salt extraction facility?

There is more salt in animal tissues such as meat, blood and milk, than there is in plant tissues. Nomads who subsist on their flocks and herds do not eat salt with their food, but agriculturalists, feeding mainly on cereals and vegetable matter, need to supplement their diet with salt. However the primary attraction of salt in history and prehistory is its use as a preservative. The application of salt to organic material absorbs moisture, inhibiting the growth of bacteria and mold. That fact allowed societies to mass produce food and store it for lean times--a crucial piece of social engineering that made long-term survival through winters and droughts a possibility.

As population grew and the need for better and longer food preservation grew, the value of salt grew too. Eventually, salt became one of the world's main trading commodities. It is not surprising then that people were willing to put considerable effort into obtaining salt and that salt production was one of the first human activities performed on industrial scale.

You can read more about the role salt played in history in "Archaeology of salt - approaching an invisible past".

On an industrial scale salt is produced in one of two principal ways: by mining rock salt and by extracting of salt through evaporation of salty water (brine).

Mining rock salt

Before the advent of the internal combustion engine and earth moving equipment, mining salt was one of the most expensive and dangerous of operations, due to rapid dehydration caused by constant contact with the salt (both in the mine passages and scattered in the air as salt dust), among other problems borne of accidental excessive sodium intake. While salt is now plentiful, until the Industrial Revolution it was difficult to come by, and salt mining was often done by slave or prison labor and life expectancy among those sentenced was low. Even as recently as the 20th century, salt mining as a form of punishment was enforced in the Soviet Union and Nazi Germany.

The title "the world's oldest known salt mine", is currently held by Hallstatt in the Salzkammergut region of Upper Austria. This is based on the discovery of a pick, made of stag horn and dated to 5000 BC, which was presumably used to mine salt. However we really have no proof of any mining going on in the area until about 1500 BC. This is when we find the oldest concrete proof of an organised mining operation. The salt has been mined in the Hallstatt region ever since.

The actual oldest salt mine in the world is actually located in Azerbaijan. Archeologists have recently shown that the Duzdagi salt deposits, situated in the Araxes Valley in Azerbaijan, were already being exploited from the second half of the 5th millennium BC. This is the most ancient exploitation of rock salt attested to date. And, it seems that intensive salt production was carried out in this mine at least as early as 3500 BC.

Salt extraction from salty water through evaporation

Evaporation can either be solar evaporation or evaporation through boiling.

Solar evaporation

In the correct climate (one for which the ratio of evaporation to rainfall is suitably high) it is possible to use solar evaporation of sea water to produce salt. Brine is evaporated in a linked set of ponds until the solution is sufficiently concentrated by the final pond that the salt crystallizes on the pond's floor.

Evaporation through boiling

One of the traditional methods of salt production in more temperate climates is using boiling in open pans. In open pan salt works, brine is heated in large, shallow open pans. Brine was poured into the pans, and concentrated by the heat of the fire burning underneath. As crystals of salt formed these would be raked out and more brine added. Earliest examples date back to prehistoric times and the pans were made of ceramics known as briquetage, or lead. Later examples were made from iron. This change coincided with a change from wood to coal for the purpose of heating the brine. The below pic is from a great article about salt extraction in East Timor by Juergen Freund. 

In addition to the above means of extraction, in many parts of the world salt was also extracted from plants.

In North and South America and West Africa salt was extracted from palms through burning.

In Japan, moshio salt which is beige in color and has a nice, rich taste, is made from seaweed. The very earliest method of obtaining salt from seawheed was probably burning seaweed and using the resulting ashes for their salt content. The first challenge in salt-making through boiling sea water has always been to find a way to concentrate sea water, which contains only 3 percent salt. So another method of extracting salt from seaweed seems to have involved collecting seaweed and allowing it to dry in the sun until salt crystals formed. The crystals were then washed off into vats of sea water, creating a concentrated brine that could be boiled down to yield salt.

In modern New Guinea, salt is extracted from plants which have been steeped in brine from salt springs. After soaking plant leaves for three or four days, firewood is used to build a square pyre and is set alight. The soaked leaves are stacked on the pyre, and the fire is kept going from the afternoon until the morning of the next day. The result is a large pile of charcoal, ashes, and salt concretions. The salt is picked out and placed on a large wooden platter, then pressed into a rectangular mold, kneaded with brine, compressed, and wrapped. The salt is further dried over a hearth for a week, which forms a “salt stone” or compact block of salt.

In the article "Extracting Salt from Distichlis spicata (seashore saltgrass): Continued Investigations into Methods of. Salt Extraction and Salt Utilization in Prehistoric California" we can read that:

Distichlis spicata

Many California tribes extracted salt from plants.

Various plants such as Distichlis spicata (seashore saltgrass), Petasites frigidus (sweet coltsfoot), Umbelliferae (Celery) were burned to create salty ashes which were then used as salt.
In some regions salt grass was burned on a grating of hardwood sticks which was laid over a pit full of hot coals. The salty sap oozed out of the plants and dropped on the coals, forming lumps which were extracted from ashes after the pit was cooled.
 Another way of extracting salt from salt grass was by drying it on flat rocks and pounding it in mortar holes. The crushed bits were then winnowed using a circular tray which separated the salt from the grass. The resulting salt was then dampened and pressed into balls. The balls were broken as needed for use.
In some cases, the salty plants were were eaten raw.
Sometimes non saline grass was soaked in brackish water and then burned.

In the article "Evidence for medieval salt-making by burning Eel-grass (Zostera marina L.) in the Netherlands" we read that during the medieval time in the area of Zealand, the peat from reclaimed land was cut and used for fuel, but due to the high salinity of the peat was also used for salt-making. For the purpose of salt-making, at low tides the clay layer was dug away and the salt-impregnated peat layers were cut into bricks. These were then dried in the wind and burned on the spot. Next, the ashes were gathered and brought to salt sheds (boiling huts), mostly located near towns or villages. In the sheds, the ash was plunged into large drums, preferably filled up with salt water to increase the salt content of the brine, and subsequently heated to evaporate the water. When the peat deposits were exhausted, eel grass was use instead of salty peat for concentration of brine used in salt making.

Marine eelgrass

In the article "London Gateway: Iron Age and Roman Salt Making in the Thames Estuary" we can read that Stanford Wharf Nature Reserve area in Themes valley was during the middle Iron Age (c 400-100 BC) a very important center of salt making. Excavation across the north-western corner of Area A uncovered the remains of red hills, a characteristic feature of long-term salt production on the Essex coast. Other evidence relating to salt production included pits, hearths and briquetage, a coarse ceramic used for making salt-processing equipment, such as cylindrical moulds, troughs, pedestals and firebars. The site continued to be used until late Roman period. The analysis of the content of the red hills revealed that they consisted of fuel ash derived from burnt salt marsh plants and sediment (peat). The plants, harvested still with marsh sediment adhering, were dried and burnt as fuel for hearths, above which brine was evaporated to crystallise salt. A by-product of the fuel burning was a salt-rich ash, which when mixed with seawater, was turned into a highly saline solution. This was filtered, and the resulting brine was then also evaporated above salt marsh plant-fuelled hearths. It was the residue from hearths and filtering that was dumped to create low mounds or red hills.

Strabo reports further methods of brine concentration and salt extraction by European tribes to include  'flinging' or 'dowsing' salt brine over hot stones and then collecting and scraping the salt crusts from the stone. "A surprisingly quick process..." The stones were certainly fractured due to the heat gradient in such methods as were the crude sherds found at the red hills.

Another method, used in Gaul and Germany, included dripping brine on to glowing charcoals and using the resulting ash as salt. You can read more about this in "Studies in ancient technology volume III" By R. J. Forbse. The same method was recorded in Philippines and you can read about it in the article "Documenting Bohol's traditional method of salt production and the importance of salt in the region's early economy".

Knowing how valuable the salt was it is also not surprising that large communities grew around sources of salt, and along the salt trading routes. 

The oldest industrial scale salt-works operation in Europe has been discovered at the Poiana Slatinei archaeological site next to a salt spring in Lunca, Neamț County, Romania. Evidence indicates that Neolithic people of the Precucuteni Culture were boiling the salt-laden spring water through the process of briquetage to extract the salt as far back as 6050 BC. The salt extracted from this operation may have had a direct correlation to the rapid growth of this society's population soon after its initial production began. The remnants of this salt extraction facility today are giant burned mounds consisting of broken salt extraction pottery and ash.

Salt production was very lucrative. No wonder then that what is now thought to have been the first city in Europe Solnitsata, in Bulgaria, was built next to a salt mine, which provided the area now known as the Balkans with salt since 5400 BC. Even the name Solnisata means "salt works".

In the article "Neolithic flat-based pots from the Carnac Mounds in the light of Cycladic ‘frying pans’", we can read about a rare type of pottery, found in four single graves under earthen mounds in the Carnac region of Brittany, and dated to 4600–4200 BC. These vessels are circular, flat-based and with a near-vertical wall. The authors of the article suggest that based on the salt extraction equipment known from elsewhere in the world, it is possible that these dishes too were used for extraction of salt from sea water. The results of experiments using replicas of these dishes demonstrated that these  vessels could have been heated and used to boil brine in the same manner as their ethnographic counterparts.

The same method of salt extraction from sea water was used In England during Bronze Age, Iron Age and into the Roman period. The remnants of these salt works are today known as Red hills.

Red Hill is an archaeological term for a small mound with a reddish colour found in the coastal and tidal river areas of East Anglia and Essex. Red Hills are formed as a result of generations of salt making, deriving their colour from the rubble of clay vessels made from briquetage and used in the salt-making process that have been scorched red by fires used to evaporate sea water to make salt cakes. Briquetage is also known as Very Coarse Pottery or VCP.

These sites contained large water settling pits, where water was left to rest for a period of time to allow impurities to settle to the bottom. The water was then poured into clay pans which were heated over the fire until the water became concentrated enough for salt crystals to start forming. The concentrated salty water (brine) was then poured int crystallization cups. These were then also heated over fire. As the water evaporated the salt crystals would form and settle at the bottom of the vessel. Concentrated salted water (brine) was added to the cups continuously and evaporated until the salt crystals filled most of the vessel. The remaining water was then evaporated and the salt in the cup was dried until it became rock hard. The cup was then broken and the containing "salt cake" was taken out.

You can read more about this in the book "THE RED HILLS OF ESSEX: Salt-Making in Antiquity" by Fawn, A J, Evans, K A, McMaster, I: Davies, G M R

The same or very similar way of extracting salt from sea water was still used in Britain in the 17th century. In her book "Through England on a side saddle" Celia Fiennes describes salt works she saw near Limington.

"The seawater they draw into trenches and so into several ponds that are secured in the bottom to retain it, and it stands in the sun to exhale the watery part of it, and if it prove a dry summer they make the best and most salt, for the rain spoils the ponds by weakening the salt.

When they think its fit to boil they draw off the water from the ponds by pipes which conveys it into a house full of large square iron and copper pans; they are shallow but they are a yard or two if not more square, these are fixed in rows one by another it may be twenty on a side, in a house under which is the furnace that burns fiercely to keep these pans boiling apace, and as it candy's about the edges or bottom so they shovel it up and fill it in great baskets and so the thinner part runs through on moulds they set to catch it, which they call salt cakes'.

The rest in the baskets dry and is very good salt and as fast as they shovel the boiling salt out of the pans they do replenish it with more of their salt water in their pipes. They told me when the season was dry and so the salt water in its prime they could make 60 quarters of salt in one of those pans which they constantly attend night and day all the while the fire is in the furnace, because it would burn to waste and spoil the pans which by their constant use wants often to be repaired. They leave off saturday night and let out the fire and so begin and kindle their fire Monday morning, its a pretty charge to light the fire.

Their season for making salt is not above 4 or 5 months in the year and that is only in a dry summer. These houses have above 20 some 30 more of these pans in them, they are made of copper.

They are very careful to keep their ponds well secured and mended by good clay and gravel in the bottom and sides and so by sluices they fill them out of the sea at high-tides and so conveyed from pond to pond till fit to boil".

Now what about Ireland? How did ancient Irish get their salt? 

Well we have no idea what the Bronze Age and Iron Age Irish relationship with salt was, but we can presume that that like everyone else in the world, they also used and highly appreciated salt. The situation is a bit better when we come to the medieval time as there are a lot of mentions of salt in Irish medieval manuscripts.

In the "A History of Irish Cuisine (Before and After the Potato)", we can read that the medieval Irish used salt for seasoning food but also for preserving meat and fish.

The Senchus Mór mentions salt as one of the important articles in the house of a brewy, on which the glossator remarks that it is "an article of necessity at all times, a thing which everyone desires." It was kept in lumps or in coarse grains; and at dinner each person was served with as much as he needed.

The Críth Gablach mentions salt meat as part of the sick maintenance given to a noble if he is unlawfully injured by another party. You can read more about this on this page of the "Early Medieval Irish Túath" web site.

In the Bretha Crólige, the druid is considered as having the same sick-maintenance as a bóaire, which entitles them to salt meat on his dish every Sunday, as well as extra if they have more property. You can read more about this on this page of the "Early Medieval Irish Túath" web site.

In the "Papers read for the Royal Irish Academy by MacNeill, John, 1867-1945" we can read that the 8th century text Conall Corc and the Corco Luigde records that the Aran Islands paid a tribute of salt to the King of Cashel, their overload.

In the "Irish Hagiographies as Tools for Conversion" we can read that in Saint Columba’s hagiography which was written in the 7th century, Adomnán tells of an instance where Columba had blessed a salt block and given it to a family, who in turn hung it on their wall. The village later burned down, destroying everything except the small section of wall from which the block hung.

And so on and so forth...

But even though we know that the medieval Irish loved and valued salt, we have no clue how they obtained it. Now I can hear people saying: Ireland is an ISLAND in a SALTY sea...Wouldn't it be logical that they got their salt from the sea?

Well sea water was probably the source of the salt used in Ireland. Even though Ireland has huge rock salt resources they were not discovered until 19th century. These rock salt deposits were discovered by people who were exploring for coal and metals and were located deep under ground, quite out of reach of people with primitive mining equipment. One location where there has been (and remains) a large volume of rock salt is at Kilroot, near Carrickfergus in Co. Antrim. In the 1850s, a surveyor searching for precious metals discovered a thick layer of rock salt under about 600ft of rock. The salt was initially removed by pumping water down shafts and pumping the resulting brine mixture back up, before evaporating the water to leave salt crystals behind. This is not something Medieval, Iron age and Bronze age Irish could have done. So the sea was the only source of salt for the ancient Irish. But how did they extract the salt from the sea water?

We know that there are two ways for extracting salt from sea water: by solar evaporation and by evaporation through boiling. Now in order to extract the salt from sea water through solar evaporation you need to have "the correct climate, one for which the ratio of evaporation to rainfall is suitably high". Basically you need to have dry sunny hot climate, or at least dry sunny hot summers. Now I have been living in Ireland for over 20 years now, and I can tell you that Ireland does not have "the correct climate"... This is what Irish summer is like:

What Irish people mean when they say summer

What Irish summer looks like on satellite images

What an Irish summer day is very likely to look like at least half of the time (Picture taken in July)

You are not going to be extracting too much salt through solar evaporation in this climate.

And yet there are numerous place names along the Irish coast which have the word salt as their root, indicating that these places were salt production areas. For instance Lough Salt in Co. Donegal (which may have had salt pits or deposits nearby, rather than being a salt-water lake); Salt Island on Strangford Lough; the Saltee Islands off the coast of Wexford; Salters Grange in Armagh; Salthill in Co. Galway; and Saltpans townland in Co. Donegal, among many others. Based on this there is a widespread belief that the medieval Ireland was a big producer and exporter of salt. In "A smaller social history of ancient Ireland" we read that in 1300, salt was one of the commodities sent from Ireland to Scotland to supply the army of Edward I. This seems to confirm that Ireland was indeed large producer and exporter of salt.

But in "The Archaeology of Medieval Ireland" by Terry B. Barry we can read that at the same time when salt was sent from Ireland to Edward I, salt together with iron was imported into Ireland from France through ports such as Drogheda...So was the salt sent to Edward a tribute in rare commodity rather than export?

Similarly in the "Ireland before the Normans" by Donnchadh Ó Corráin we can read that the medieval Irish laws have mentions of vessels loaded with cargoes of iron, salt, hides, nuts, honey and wine, and they lay down certain provisions for the landing of cargo ships and the liabilities which they may incur. There are, in addition, numerous references in the literature to the import of wine, salt and iron and to the export of hides and wool.

In "The Economy of Early Medieval Ireland" we can read that  although salt is mentioned in the early eighth-century Críth Gablach, there is no evidence for native salt production prior to the late twelfth century, and all Irish place-names associated with salt production are ultimately English in derivation.  An example of one of the earliest references to "salt pans" in Ireland is found in the Calendar of Documents Relating to Ireland for June 1258, where a land dispute is mentioned between John de Verdon and the Abbot of Mellifont over "three carucates of land in Mulygadaveran and Thulachalyni, and five carucates xcepting three acres used in salt works".

Now this is very strange indeed. You saw the pictures of what the Irish summer looks like. How the hell is it then possible that the medieval Irish were able to use solar salt pans for the extraction of salt from sea water? Well the answer is something called "The Medieval Warm Period (MWP), Medieval Climate Optimum, or Medieval Climatic Anomaly". This was a time of warm climate in the North Atlantic region lasting from about 950 to 1250. It was followed by a cooler period in the North Atlantic and elsewhere termed the Little Ice Age. Temperatures in some regions matched or exceeded recent temperatures in these regions, and it is believed that temperatures in the north of Europe were on average 2 degrees higher than they are today. But were summers in Ireland during that period sunnier, less rainy? Very likeley. And if so that would explain the confusion about Ireland being described as salt exporter and salt importer, and all the English place names with salt as their root. Basically prior to 900 the climate in Ireland was too damp to extract salt from sea water using solar evaporation. So salt was imported into Ireland and this was recorded in all the early Irish records. The start of the change of the Irish climate into warmer and drier climate coincided with the Viking and then Norman invasion of Ireland. During that time we see emergence of salt pans along the coast with place names with salt as their root, and salt production and export. Then before 1300 the Irish climate starts changing for the worst and starts getting colder and wetter. And the salt production industry disappear from Ireland and the salt is imported into Ireland once again which was recorded in the late medieval records. Then the climate starts warming again in the 18th century and hey presto salt works appear again along the Irish coast. 

Now during the Medieval Warm Period the climate in Ireland was probably the same like the climate we find today in England. Summers were probably much warmer and drier. That allowed the salt to be extracted from salt water using the same method of partial solar evaporation and partial evaporation through boiling, which was until recently still used in England, and which I already described above. Sea water was concentrated in open salt pans and maybe, depending on how severe the climate change during the Medieval Warm Period was. If the water could not have been evaporated completely using solar evaporation it was then boiled in metal pots, which have by this time replaced coarse clay pots (biquetage).

But what about salt production in Ireland before this time? Was climate in Ireland ever as good as during the Medieval Warm Period thus making salt extraction possible? Well yes it was. It seems that the climate in the North Atlantic region oscillates quite a bit, and has extreme high temperature peaks roughly every 1500 years with smaller high temperature peeks in between. At least this is what the available data for last 5000 years is showing us.

The Greenland Ice Cores provide a temperature record for the last 5,000 years. Clearly manifest are three temperature peaks which correspond with the archaeologically and historically documented Warm Periods in the North Atlantic region: Minoan Warm Period 1450–1300 BC, a Roman Warm Period 250 BC – 0 AD, the Medieval Warm Period 800–1100 AD. On the chart you can also clearly see the well documented extreme cold period known as the little Ice Age 1350 to 1850 AD.

In "The Bronze Age climate and environment of Britain" by Tony Brown we read that there was an abrupt climate change around 900 BC which resulted in much colder and wetter climate. This climate change ended what is known as "The Bronze Age Optimum (1500—900 (800) year BC)",  the period of warm and dry weather in north Atlantic region. This period fallowed The Middle Bronze Age Cold Epoch which was a period of unusually cold climate in the North Atlantic region, lasting from about 1800 BC to about 1500 BC.

Now on the above chart you can see that "The Bronze Age Optimum" starts with the sudden sharp rise in temperature during the Minoan Warm Period which started right about 1500 BC. How warm was Atlantic northern Europe during the Minoan Warm Period can be discerned from the fact that during the Minoan warm period, millet was grown in southern Scandinavia. Today Millet is grown in tropical and subtropical regions, it is an important crop in Asia, Africa and in the southern U.S.. The average annual temperature in Mississippi and Alabama where millet is grown today is about 10 degrees, which should be compared with today's average annual temperature in Denmark, which is 8 degrees.

The temperature after the Minoan Warm Period drops and has another minimum around 1200 BC rising to another maximum around 1000 BC. After that it oscillates around relatively stable low value until it suddenly starts to rise around 250 BC. This is the beginning of the Roman Warm Period

“The Roman warm period started quite suddenly around 250 BC. Some studies in a bog in Penido Vello in Spain have shown that in Roman times it was around 2-2.5 degrees warmer than in the present. The Roman warm period is amply documented by numerous analyses of sediments, tree rings, ice cores and pollen – especially from the northern hemisphere. Studies from China, North America, Venezuela, South Africa, Iceland, Greenland and the Sargasso Sea have all demonstrated the Roman Warm Period. Additionally, it has been documented by ancient authors and historical events.

How warm was Northern Europe during the Roman Warm Period can be seen by the fact that during the culmination of the Roman warm period olive trees grew in the Rhine Valley in Germany. Citrus trees and grapes were cultivated in England as far north as near Hadrian’s Wall near Newcastle.

The temperature then has a sudden drop during the first century AD but it then rises as suddenly and stays stable high until the end of the fourth century AD when it suddenly drops to an extreme low level. It then suddenly rises to extreme high during the Medieval Warm Period....

You can read more about this in "History of Earth's Climate 7. - Cenozoic IV - Holocene".

Now what is very interesting is that the oldest red hills (burned mounds consisting of briquetage and ash) salt extraction sites found in England date to right about the beginning of the Minoan Warm period.

In "The English Coast: A History and a Prospect" by Peter Murphy we read that the early evidence for salt production in east England comes from a Red Hill Bronze age site in Fenn Creek, Essex, dated to 1412 - 1130 calibrated BC. The site continued to be used through Iron age and was abandoned during the late Roman period. Translated the last sentence means that the site was used during warm peek periods between the beginning of the Minoan Warm Period and the end of the Roman Warm period when we have such cold and wet climate in England that the solar extraction stopped.

Now there are no red hill type burned mounds in Ireland. But guess what appears in Ireland right about the time of the Minoan Warm Period: our old friends fulachtai fiadh burned mounds. Radiocarbon dating indicates that the majority of fulachtaí fiadh were constructed during the mid to late Bronze Age (1500 - 500 BC), though some Neolithic examples are known. However, some were still in use up to medieval times. This is very very interesting, don't you think? Were fulachtai fiadh used for salt extraction? Well I believe, like most other researchers today believe, that fulachtai fiadh were multi purpose facilities used for many different things involving liquids and heat. But I definitely believe that some of them were used for salt extraction. 

So how would you use fulacht fiadh for salt extraction? 

Well the simplest way would be to build your fulach fiadh anywhere where the forest grows all the way down to the the seashore. That way you will have easy access to sea water and wood. You would then need to collect lots of round hard rock pebbles from river beds and beaches. You would fill the through with seawater, and heat the stoned in the fire. Then you could proceed in one of two ways.

Do you remember what Strabo reported about methods of brine concentration and salt extraction used by "European tribes" which included  'flinging' or 'dowsing' salt brine over hot stones and then collecting and scraping the salt crusts from the stone. "A surprisingly quick process..." Salty water would be kept in the through from where it would be scooped using cups and then poured over a pile of super heated stones, in the same way water is poured over sauna hot stones. Once the stones have cooled down, the salt would be scraped from their surface and the stones would be put back into the fire to be reheated. A new pile of hot stones would be made and the process would be repeated. The stones were certainly fractured due to the heat gradient in such methods as were the cracked stones found at fulach fiadh sites. 

Another possible way in which fulachtai fiadh could have been used for salt production was as a salt pan for evaporating salty water through boiling inside the through heated with super heated stones. You would fill the through with salty water, heat the rocks in the fire and drop them into the through. They would heat the water to the boiling point and eventually after many reheated stones were dropped into the through all the water would be evaporated and you would be left with the layer of salt covering the bottom of the through. 

Believe or not, this exact method of salt extraction through boiling salty water in pits heated by super heated stones was used by Native Americans in the Eastern USA. In the article Historic importance of salt on the website of the Louisiana department of culture, recreation and tourism, we can read that Native Americans in Eastern North America used salt as a condiment and highly regarded it. The DeSoto expedition which explored today's South Eastern USA in the 15th century observed four ways in which salt was produced. It was gathered in a free state (rock salt),  extracted from the ashes of plants and salt-impregnated sand, but it was most commonly extracted from brine water at salines. The principal artifact identified with the prehistoric salt industry in Eastern North America is the salt pan which was used for extracting salt from the brine. Fragments of these shell-tempered vessels have been found at most salines. This is a complete vessel from Bone Bank site at  Posey County, Indiana.

In the article "Methods for calculating brine evaporation rates during salt production" we can read the detailed description of how these salt pans were used:

"Thick-walled ceramic ‘‘pans,’’ most in association with salines, have been found with capacities ranging from 40 to 400 L. The enormous size and weight of these vessels when filled with brine would have made them practically immovable and suspension over a fire seems equally unlikely. It is suspected that these salt pans were placed in basin-shaped ground depressions (pits) and heated stones from nearby fires were dropped into the pan to facilitate evaporation. The lack of exterior discoloration from fires on many pans and the occasional find of stones inside pans lend support to the conclusion that stone boiling was at least one method utilized by Native Americans to evaporate brine."

Now this is very interesting. In North America we have clay lined pits heated with hot stones. In Ireland we have stone or wood lined pits heated with hot stones. The capacity of North American pits is 40 - 400 L. This means that the dimensions of the biggest ones are 2 meters long by half a meter wide and half a meter in depth. Guess what. The size of fulacht fiadh throughs varies a great deal from site to site, from rather small pits lined with stones to pools approximately a meter wide by 2 meters long and maybe half a meter or more in depth.

The article "Methods for calculating brine evaporation rates during salt production" then explains how the hot stones were used to evaporate the water from from the salt pans:

"When the hot stones are dropped into the pan, the heat transferred from the stones raises the temperature of the brine up to its boiling point. Any additional heat released by the stone serves to evaporate water and concentrate brine. After about 2 minutes the brine is concentrated to a maximum value of about 29.0 wt% at its boiling point with further evaporation resulting in the formation of salt crystals. The salt crystals forming continues until the stone and brine reach thermal equilibrium. Placing additional hot stones into the pan would continue the evaporation process. Depending on the size of the pan and the amount of hot stones used, the evaporation process can take up to several hours.

From the short timescales involved to obtain salt, it seems this method would clearly be effective in evaporating brine. Unlike suspension of a pan over a fire, stone boiling releases all of its internal heat directly into the brine. However, there may be practical limitations for manipulating large volumes of very hot stones. Stones heated to high temperatures often shatter and large stones would be difficult to transport from a fire to the brine pan. Smaller stones would make handling easier, but would require more repeated firings to achieve the same evaporation as large stones. The smallest salt pan found at the Kimmswick site near St. Louis had a volume of approximately 40 L. Assuming a scenario similar to that outlined above, emplacement of 25 vol% stone into the salt pan would translate to 26 kg of extremely hot stone(s) that would have been manipulated. This seems to suggest that stone boiling may actually require a tremendous amount of human labor to achieve significant quantities of salt..." 

But it seems that Native Americans thought that this effort was worthwhile because salt was such a valued commodity. Considering that the people world over had the same opinion about salt, I would suggest that the ancient Irish would probably also think that the effort involved in extracting salt using fulacht fiadh was worthwhile. 

Interestingly in "The A to Z of Ancient Mesoamerica" by Joel W. Palka we read that in Yucatan and Guatemala the process of evaporating and refining salt was also carried out by pouring salt water into wooden throughs which were slowly drained, leaving the salt behind. An Irish fulacht fiadh trough dug into as well drained, particularly sandy soil and lined with wooden planks, would be even more efficient salt extraction vessel than the clay lined North American salt pit. This is because water would be removed from the through both through evaporation and draining. 

On average sea water contains about 3% salt. This means that if you evaporate 100 liters of seawater you would be left with 3 kilos of salt. Several hours of hard work for 3 kilos of "white gold"? Definitely worth it. Now there is a way to make this process even more worthwhile.  

To do that you would need to built your fulach fiadh next to a salt marsh or a salty shallow lagoon which would fill only during the very high tides. The sea water caught inside the marsh or a lagoon would slowly evaporate over time and get more and more concentrated and salty. You would then boil this partially concentrated salty water in your fulacht fiadh. 

If the weather was particularly sunny and windy the water in these lagoons could get so concentrated (29%) that salt crystals would start forming in the water on their own. But even if the concentration of salt was increased to 6 percent that would mean that the salt yield for the same amount of work would double. 

This is exactly the process that was used in Bronze Age salt extraction facilities in England now known as red hills burned mounds. 

But there is a way to increase the salt concentration in salty water processed in fulacht fiadh even more. By using seaweed. I already mentioned that in Japan people used seaweed to increase the salinity of water which was boiled for salt. They collected and dried seaweed in the sun and wind until salt crystals formed. They then washed the crystals off into vats of sea water, creating a concentrated brine that could be boiled down to yield salt. Irish coastal waters are extremely rich seaweed growing grounds and you can see that this method of increasing the salinity of seawater could easily have been used in fulach fiadh throughs.  I also mentioned another method of increasing the concentration of salt in seawater using seaweed. In England and Netherlands they used ashes of dried seaweed which was burned to heat the salt pans. The ashes which contained a high concentration of salt were mixed into seawater thus greatly increasing its salinity. Now in Irish fulachai fiadh it was the stones which were heated, but it is entirely possible to heat the stones using dry seaweed or a combination of wood and dry seaweed. The resulting ashes could then be mixed into the seawater to increase its salinity. 

That seaweed was used in Ireland in salt production process was first suggested based on linguistic evidence. In "Celtic culture: a historical encyclopedia. Vol. 1;Volume 2" we can read that in Old Irish one of the terms used for salt was "murluaith" =  "muir + luaith" which means "sea ash". Now the meaning "sea ash" was taken as proof that originally it was seaweed ash which was used as salt. That is possible as we have seen that Native Americans used salt grass ash as salt. But if seaweed ash was used in the process of salt production in ancient Ireland it was most likely used just like it was used in England, for concentrating seawater into brine before boiling. Interestingly the word "luaith" from "murluaith" has another meaning in Old Irish. It means simply "dust". The picture below shows a rock pool which was filled with seawater during a very high tide or storm and in which subsequently all the water had evaporated, leaving behind white "sea dust", salt. Collecting salt from this type of rock pools could be the oldest way ancient Irish obtained salt. Hence the name "muir luaith" = "sea dust" could be the oldest word for salt in Irish, the current word "salann" being later import. 

So....

What do you think? I think that the Bronze Age Irish used and highly valued salt, just like the Bronze Age English. And I can't see any reason why they couldn't have used a method of extracting it by boiling a concentrated brine solution in a very similar way the Bronze Age English did it. Or by just dripping the salty water on hot stones, a "very effective method" which Strabo reports was used by the "Europen tribes". Now I am not saying that all the fulachtai fiadh were used for salt extraction. A fulacht fiadh built on top of the hill in the center of Ireland certainly wasn't. But those built next to the seashore could have been used as very cost effective salt extraction facilities...

The salt extracted in these fulachai fiadh was then used as condiment, for meat and fish curing and for another very important process: wool dying and hide tanning... And guess what, fulacht fiadh could also have been used in these two processes. But I will talk about this in one of my next posts.