We lit a fire whose fuel was old timber wood.
Is the word whose referring to fire, an inanimate object, correct in this sentence? Or is there a more appropriate word?
The word "whose" is used in several different grammatical ways. For some of these (see my original answer below), it has been grammatical to use it for inanimate objects, at least since the days of Shakespeare. For others (see my update), it is only used for people or animals.
Many people seem to believe that you cannot use whose for inanimate objects, but I don't believe this was ever proscribed except by out-of-control grammarians. Consider the following quotes from Shakespeare (selected from many more quotes where whose refers to an inanimate object) and more recent authors:
I could a tale unfold whose lightest word
Would harrow up thy soul, freeze thy young blood,
Two Gentlemen of Verona, III.ii
By wailful sonnets, whose composed rhymes
Should be full-fraught with serviceable vows.
Timon of Athens IV.iii
The sea's a thief, whose liquid surge resolves
The moon into salt tears:
Jane Austen also used whose to refer to inanimate objects:
Pride and Prejudice (1813)
On reaching the house, they were shown through the hall into the saloon, whose northern aspect rendered it delightful for summer.
Also F. Scott Fitzgerald:
The Great Gatsby (1925)
I walked out the back way ... and ran for a huge black knotted tree whose massed leaves made a fabric against the rain.
Not to mention Pat Conroy:
South of Broad (2010)
... as I walk down Church Street, whose palmetto trees are rattling and whose oaks shake with the ancient grief of storm.
UPDATE: I just realized that whose is used in several different grammatical ways. In some of these ways, I would never use whose for anything but a person or animal. In particular, one of whose's uses is as an interrogative pronoun, as in:
Whose shoes are these?
Whose are these shoes?
If you had some leaves, and were asking which tree they fell off of, you cannot say:
*Whose leaves are these?
*Whose are these leaves?
You have to say something like
Which tree's leaves are these?
But when it is a relative pronoun that immediately follows its antecedant, whose can be used for inanimate objects:
The tree whose leaves look like hands ....
This may be part of the cause of the confusion about whether whose can only be used for people or animals.
English whose is somewhat like Latin cuius or Spanish cuyo in that it is strictly a function word. It is just fine for anything at all. You cannot use which there.
However, it does make a difference whether you use whose as a relative pronoun or as an interrogative pronoun. This one is ok:
But this question:
is soliciting an answer of a person, not of a fire. To get the other answer, you have to say:
You cannot say:
And get back a list of fires instead of a list of persons.
"Whose" is also used to refer to a component of something inanimate and your original is perfectly grammatical. I wonder if you're inventing a problem where there isn't one. If you think using "whose" sounds a bit too formal, you could say e.g. "A camera with wires going through the wall", which would maybe be what people would say in everyday spontaneous speech. It's not clear to me that "a camera of which the wires..." is more common or natural than "a camera whose wires...".
As far as grammatical category is concerned, it really depends on your model and, depending on your model, exactly what type of analysis you're doing. As a fairly broad label, it's probably fair to group it with other "wh-words" ("when", "which" etc.).
Wiktionary lists such usage as "formerly proscribed". Presumably this means some would still proscribe it, and others consider it correct.
Personally I have no problem with it, though I would probably say the following:
We lit a fire for which the fuel was old timber wood.
Or, better yet:
We lit a fire, using old timber wood for fuel.