The same leaching procedure can be used for extracting tannins from barks. The tanned water which results from this procedure can be scooped out of the fulacht fiadh trough and collected in large clay vessels. It can then be boiled and concentrated. The concentrated tannin rich “tea” can then be poured back into the trough and used as tanning solution for animal skins.
Soaking of the skins in tanning solution should be done first in a relatively weak solution and then in progressively stronger solutions. It is very important to use a very weak solution for your first bath. If the hide is put into a strong tannin bath, the outside gets tanned and shrinks. This inhibits the tannins from penetrating to the center of the hide, leaving the inner parts raw. This is called “dead tanning” or “case hardening”. The ideal bath to start with is one that has already been used for another hide. That way all the large tannin particles have already been used up. This is known as a “spent liquor”. There is another advantage to spent liquors. In an old bark liquor, the bark sugars have fermented, forming lactic and acetic acid, which help remove any traces of lime as well as help preserve the hide.
The skins should then be left in the strong solution for as long as it takes for the solution to penetrate all the way to the center of the skin. But how long that is depends on the thickness of the skin and can go from few weeks to few months. Here you can see skins submerged in the tanning pit and the pile of oak bark next to the pit, used to make tanning liquid. Fulacht fiadh trough uses for skin tanning would look very similar to this.
During the soaking period, fulacht fiadh trough containing skins submerged in tanning solution can be covered with planks or split branches and then covered with hide and soil or peat. From time to time it can be checked on and more tanning concentrate or raw bark can be added to strengthen the tanning solution.