The aim today was twofold- to burn beech wood and bracken, in order to get two kinds of ashes for the production of sal alkali, or in other words alkaline salts, usually of potassium. Beech wood is recommended by Biringuccio and others, and bracken is known to contain large amounts of potassium salts, and is recommended in other books.
Raw materials, bracken and wood:
So, the first thing I did was build my furnace. I shall explain it and its genesis in a later post. Here is the inside of it:
On the right hand side is the lower section, on the left the higher section. I forgot to re-orientate the photo before posting it, so it is on its side rather than being vertical. It should be clear that there is a gap between some firebricks between the lower and upper sections. The lower section is open at the sides, and contained the beech wood. The upper section was open at the bottom and the top. This is essentially an attempt at a reverberatory furnace, where the heat and combustible material from the lower chamber rises up into the upper chamber, and heats whatever is in there.
This can sort of be seen here:
Here you can see flames from the wood rising up through the upper chamber. You have to use wood for this, as far as I understand it, so that you get lots of nice volatiles distilling off and burning as they rise.
The basic work was easy, the thing that took a long time was putting the furnace together, and covering gaps with clay, that took nearly an hour. Actually burning all the wood took only 45 minutes. I simply stuffed the upper compartment with bracken, and the lower one with fine twigs and newspaper, and lit the newspaper. After that the natural chimney effect took over, and the fire got going ok, although a little slowly. I just had to feed more bracken in the top as the previous stuff burnt.
I checked the temp half way through using my thermocouple. The temp in the wood fire was 680 to over 700 when the wind blew, but only a bit over 300 in the bracken compartment.
Results and discussion
The end results were a small amount of beech ash, and a small amount of bracken ash, with some unburnt stems. The problem was that I had not dried the bracken properly, and so the root stems, which were thick and wet did not catch light very easily. The wood used was a smidgen damp, which did not help, but burnt well enough, the problem was the lack of draft. Doing it in my tiny drying area, in the shade of a fence, on a not very windy day, was not helpful. I had to use the bellows to really get it going. The problem is that a reverbertory furnace works by wind, and if there is no wind, it wont work well. Also the wood used should be totally dry, and not too thick, I think that anything more than a cm in diameter is not much use, since it will not burn fast enough. Remember, burnt wood is an insulator, that is why they use it on fire doors. I also didn’t have enough wood at all, I could easily have kept feeding it for ages, with enough wood available.
But now I have some ashes to play with. See my next post for what I shall do with them.
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