Different woods certainly do burn faster than others, and the burning
rate depends on quite a large number of variables. Some of these (and I
can't possibly hope to get them all!) are
How much water is in the wood. Freshly cut trees will have a lot
of water in their woods. If you let wood dry for several months, it
will be lighter and burn faster.
How porous the wood is. A very solid, hard, dense wood will not spread the fire as fast as wood with lots of holes in it.
How much oil is in the wood. Eucalyptus trees burn like torches because they store oils inside them that burn rapidly.
For your questions of "what does fire consist of?" There are two
components to a burning wood fire: the chemical reactions in the wood
itself -- you see these as "hot coals", and the flames that leap above
the burning wood. Oxygen in the air combines with the starches, sugars,
and cellulose and proteins in the wood to make carbon dioxide, water
vapor, and a variety of other compounds, depending on what the original
wood was made of. Of these remainders, those that do not burn show up
after the fire has burnt out as ash and soot, or creosote (although
soot is only partly burned -- it can collect in a chimney and burn
later if it is not swept away). The flame is the "afterburner" of the
fire -- gases released by the burning wood which are still burning
continue to do so as they rise above the wood. If you extinguish the
flame but leave the coals burning, often a dense cloud of smoke rises
above the coals. This is the stuff that the flames are finishing off.
Flames can be used to "light" unburnt wood, and depending on how
easy it is to burn the wood, the fire will spread quickly or slowly. If
a fire has completely burned the wood, leaving only ashes, these will
not burn, and there will be no more fire. It is important to think of a
fire as something the wood is doing, and not an object in its own right
-- it doesn't really "disappear" or "go somewhere", but rather the wood
just stops making flames when the wood runs out of chemical energy.
Some people complain that partially burnt wood, that is, charcoal,
does not spread fire as quickly as they would like. These complaints
arise from two factors: 1) charcoal burns with very little flame and so
does not catch nearby charcoal on fire very quickly, and 2) people are
usually hungry and impatient while waiting for the fire to spread on
(published on 10/22/2007)