Hello!
I was at first hesitant to comment on this, but I feel it should be stressed again:
It is correct and good to inform people about the Wiki. However, it is not necessary nor good for the forum atmosphere to be aggressive about it.
The original link to the Wiki was presented in a very provocative tone and seemed to imply frustration about the OP not having found the answer to the question in the Wiki before asking here.
If people actually go to the trouble of looking up the correct link and then share it, why don't they want to invest those two additional minutes to be nice about it? After all, by being all agressive and things, you do not really make people like you, and when you need help/information/cheering, it becomes less likely that you will get it as people may remember how you spurned them. On the other hand, if you give a friendly reply and include the link to the Wiki in it, you have both the effect that the person is encouraged to use the Wiki AND the person is thankful for you doing so, increasing your status.
At least that is the way I see the situation and the way I felt about the reply in question.
As for the original question, even without the Wiki, I would probably have suggested making clear glass - I got called names by the Elven traders once because I wanted to sell them clear glass, so the association between clear glass and wood was very fresh. In addition, when you create clear glass, you go through basically all forms of ash (ashes -> potash -> pearlash), so should it be some special ash after all, it would appear during the process and get claimed as needed.
I do not mean to criticize the OP, but I think that this burning wood riddle is actually a very nice riddle Toady One devised, and at least I was reminded that before we refer to the Wiki or ask people here, we may also approach obstacles in this game as riddles. With all the unexpectedness and complexity of DF, we may begin to mystify it, but in the end, it is a game simulating a consistent world. So there is a lot we as the players can actually figure out. For me, this question has revitalized the vigor of the intellectual challenge of DF, and for this awareness raising, I am actually thankful to the OP.
Deathworks