"So I think that we can safely say that fulachta fiadh were not used in the way the mainstream archaeology suggest they were used: for cooking large amounts of meat in troughs full of water heated by hot stones. The Bronze Age people who built fulachta fiadh had much more efficient ways of cooking large quantities of meat at their disposal."
But what about the troughs? Every fulachta fiadh had a trough, so they must have been used for something. But if not for cooking, what were they used for?
In my next few posts I will like to propose what the troughs could have been used for.
In this post I would like to propose that one of the possible efficient (very important) uses of the Fulacht fiadh's troughs could have been acorn leaching.
In my two posts about Irish bullaun stones: "bullaun stones" and "new material about bullaun stones" I presented my theory that Bullaun stones from Ireland and the similar stones with large deep cup marks, were made to be used as mortars for grinding probably originally acorns, and then wild grain, grain, tubers and even ore...
You can find these articles here:
"Oaks", "Acorns in archaeology", "How did oaks repopulate Europe", "Eating acorns", "Christmas trees from garden of Eden", "Acorns in ancient texts".
But in order to eat acorns, they first have to be leached.
So what is acorn leaching?
There were two distinct types of oaks and acorns:
The white oaks whose acorns mature in 6 months and taste sweet or slightly bitter; The inside of the acorn shell is hairless. The bark is light in colour, gray to light gray. The leaves mostly lack a bristle on their lobe tips, which are usually rounded.
The red and black oaks whose acorns mature in 18 months and taste bitter to very bitter. The inside of the acorn's shell can be hairless but is in most cases woolly. The bark darker in colour. Its leaves typically have sharp lobe tips, with bristles at the lobe tip.
The acorn bitterness is caused by tannin or tanic acid. The concentration of tannin varies from species to species. This is why acorns from some oaks can be eaten raw and some are so bitter that they are inedible unless the tannins are removed. This process of removing tannins from acorns is called leaching. The tannins leached out of the acorns "tan", color the water.
These are the same tannins used in tanning leather...
Now there are many different ways in which you can leach acorns. In my post about eating acorns I wrote about the discovery and development of the acorn processing techniques and tools.
Most acorn leaching techniques involved soaking acorns in water as water dissolves tannin.
There are basically two main types of water leaching: active and passive.
Passive techniques involve storing whole shelled or unshelled acorns in baskets which are either submerged in running water or in waterlogged pits. Water slowly leaches tannin and eventually after several days, weeks (running water) or months, years (waterlogged pits) the acorns become edible. This technique requires very little work but takes time and ties the people to the location of the leaching baskets or pits.
Active leaching involves basically shelling and crushing acorns and then washing them in cold or hot water. This technique requires a lot of human work but is much quicker. Acorns leached like this become edible after several days to several hours depending on the temperature of the water used for leaching. So you could store your acorns dry into baskets, which is good if you want to carry them around from place to place, and then leach them when you need them.
It is this second type of water leaching, the active leaching, that I believe the fulacht fiadh's troughs could have been used for.
So to do this type of quick leaching you need to first shell the acorns. You can then crush them, because the smaller the pieces, the larger the contact surface area between the acorns and water, and the faster extraction of tannins, but you don't have to. Sometimes is more practical to leach acorns whole, as it is easier to store whole leached acorns than acorn mush....
Anyway, once the acorns are shelled (and optionally crushed) they can be leached by getting them into contact with water. This is done by submerging the acorns into a container containing fresh water in proportion 1 part acorn 3 parts water or more water. The acorns will sink to the bottom and start leaching straight away turning the water dark. You wait for a while for tannin to dissolve in water and then you pour out or scoop out the tanned water. You then pour in new fresh water and repeat the procedure until the water stops turning dark or until acorns stop tasting biter. Simple. If you can get large enough watertight containers near running water... Like large pots, pits, sand beds, or wood or stone lined troughs.
The cold water leaching process takes from from 8 hours to several days depending on the tannin content in acorns. You can speed up the leaching process by using hot water. Acorns submerged in hot water leach a lot faster, taking only two to three hours to lose their bitterness. What is interesting is that from the ethnographic data collected among the Native American tribes, we know that they used heated stones for heating water for acorn leaching. The stones used for water heating were carefully chosen so that they don't fracture during the continuous heating and cooling. The best stones for this purpose are basalt stones as the don't shatter under thermal pressure.
There are couple of things that need to know when using water for acorn leaching:
You have to use either only cold water or only hot water for leaching. If you mix cold and hot water or if you put acorns into cold water and then heat the water, the tannin in the acorns will be bound to the acorn meat permanently and you will not be able to remove it. The temperature of water with which you leach the acorns is very important. Heating water over 73 degrees Celsius precooks the starch in the acorn. Cold processing and low temperatures under 65 degrees Celsius does not cook the starch. Acorn meal that was leached in cold water thickens when cooked, hot-water leached acorn meal does not thicken when cooked. Also, when you leach the acorns in very hot water you also boil off the oil with the tannins, reducing acorn meal nutrition. So ideally you would want to leach acorns in water which is hot but not very hot.
This brings me back to what I said about why I believed that pit meat boiling was extremely unlikely usage for fulacht fiadh. Fulacht fiadh have much larger surface area compared to their dept and "the heat loss due to evaporation of water from a surface of an open tank is totally dominant at higher water temperatures". What this means is that at boiling temperature, it becomes extremely difficult to keep the water in the shallow trough with the large surface boiling using heated stones for long enough to actually cook meat. But the heat loss trough the surface is much smaller on lower temperatures and these temperatures can be maintained relatively easily using heated stones. Which means that fulacht fiadh could be efficiently used for hot water acorn leaching.
So how would the hot water acorn leaching be done in fulacht fiadh? First you would fill the trough with clean water. You would then heat the water using heated stones until it is hot but not boiling. You would then pour in whole or crashed acorns. You would occasionally add new heated stones to keep the water temperature high. You would also steer the water with a stick to help the leaching process. You would also from time to time scoop out some of the tanned hot water and replace it with some clean cold, followed by adding heated stones to keep the water hot.
This leaching procedure is very efficient not just because we are using hot water, but also because we are using stones heated in ashes for heating the water. Every time we drop a heated stone into the trough, hot wood ash which was stuck to the surface of the stone gets washed off the stone and dissolved in water. And believe or not we know that in addition to using hot water to speed up the process of dissolving the tannin, some cultures also used wood ash to induce chemical reactions which transform tannic acid into harmless chemicals. Early ethnobotanist Huron Smith (1923, pg 66) documented the Menominee method of processing various oak acorn species: "The hulls were flailed off after parching, and the acorn was boiled till almost cooked. The water was then thrown away. Then to fresh water, two cups of wood ash were added. The acorns were put into a net and were pulled out of the water after boiling in this. The third time, they were simmered to clear them of lye water. Then they are ground into meal with mortar and pestle, then sifted in a birch-bark sifter."
So it seems that acorn leaching is one of the possible uses for fulchta fiadh. Well at least for those fulacha fiadh which were cut into bedrock or into a clay rich soil next to a clean streams. However, as I already said in my post "Fulacht fiadh - a cooking pit?", one of the key feature of the most fulacht fiadh sites is elevated soil acidity. Basically most fulacht fiadh were located in marshy boggy areas where a hole dug into the ground would quickly fill with water. Acidic marshy water. A very very bad water for cooking food. And a very very bad water for leaching (extracting acid from) acorns. So even though fulachta fiadh could functionally have been efficiently used for acorn leaching, only the ones not located in marshy pit bogs could in reality have been used for this purpose.
I will continue exploring the possible uses of fulachta fiadh in my next few posts.