Shabbat 28 - Wick made of wood
Any material that comes from a tree should not a be used to make a wick for the Shabbat lamp, since it does not draw oil well - with the exception of flax. Incidentally, materials coming from a tree, if used to construct a roof over a dead body, do not themselves become ritually impure - again, with the exception of flax.
This terse rule can be explained as follows. Ordinarily, flax is not considered wood, but it is called "wood" once, in the book of Joshua, " And she brought them up to the roof and hid them in the trees of flax... ". Since flax can be used for a wick, and since it does accept ritual impurity, it had to be mentioned and excluded.
The law of the wooden roof not accepting ritual impurity of the dead - although it transmits this impurity to other objects - is derived from the laws of the covering of the Tabernacle. A related question is the skin of an animal called "tachash," also translated as "blue processed skin" - was it a kosher animal or not. It had one horn, and therefore should have been kosher, since we learned in the tractate Niddah that this is a sign of a kosher animal. Besides, the ox that Adam brought as a sacrifice, also had one horn, and it definitely was kosher. However, it could be that the unicorn called tachash was a special creature that appeared just once, to provide skins for the Tabernacle (it was multicolored, " joyous in his colors" in Aramaic), but then was hidden again, and its kosher status remains in doubt.
Art: Raphael - The Woman with the Unicorn