Ah, just found other translation - by Janet Hinson:
If you want to make good tinder to light a fire with (flint and) steel, take walnut bark (or possibly flowers?) past its prime, and put in a pot of very strong lye, either whole or in pieces the size of two fingers, whichever you wish, and boil continually for at least two days and a night. And if you have no lye, take good ashes and mix with the water and make it like "charree" (the thick mixture of ashes and water left at the bottom of the washtub after you pour off the lye), then put your bark on to boil in it for the time mentioned above and add liquid as needed while it boils. If you boil it in lye, add lye; if you boil it in charree, add water; and all the time it is boiling, if you can provide clean animal urine to add to it, so much the better. And when it is boiled enough, press it, and then wash in good clean water to soak it, then dry in the sun or in the hearth, away from the fire, so it does not burn, for it should dry gradually and gently; and when it is dry, if someone will help you , beat it with a mallet or a stick, until it gets spongy. And when someone wants to light a fire, let him take a piece about the size of a pea and put it on his flintstone, and he will soon have a fire; he only needs a sulphured spill, and he can light the candle. And you must keep it very clean and dry.
I think there maybe something here too:
http://www.anglo-norman.net/cgi-bin/xpr ... ;target=26
but medieval French is beyond me and I may have got completely the wrong idea about what this is about!
As Jorge suggests, linen fluff does work, as indeed do dry dead holly leaves and tinder fungus.