George wrote:

Yep, on record as saying that, absent the volatile organics which are the
source of the ignition, you have wet sawdust, which won't burn on its own or
with a flame until the water is expelled. Fresh sawdust and fresh hay
outgas the same.

That's quite a brash statement (that they outgas the same). But I like
the way you make brash (or ambiguous) statements and then mock everyone
who disagrees. It makes the internet worth while.

Anyway, to the matter at hand...a cursory search of the internet, which
as we know never provides false information, shows many examples of
spontaneously combusting sawdust, albeit usually at the bottom of a 20'
mound. I did, however, find an example of a fire caused by a "pile of
sawdust under a sawbench." How big a pile? I dunno'. See:

http://www.news-mail.com.au/news/0603/280603_2.html

Likely, the dust was from green wood.

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Now, because of the nature of the danger, this problem has actually been
studied (believe it or not, we need not concern ourselves with hay
fires!). The following absolute truth was also found on the internet:

From http://www.ciwm.co.uk/mediastore/FILES/10867.pdf:

"As with compost and rubber tyres, wood chips and sawdust material can
undergo self-heating reactions, leading to spontaneous combustion, under
certain storage conditions. Factors that will play a part in whether
self-heating leading to combustion will occur are the size of the wood
chips, the moisture content, the presence of other flammable waste
materials within the stockpile that are easily ignitable, and the
natural oil content of the wood. Sawdust has been tested extensively[1]
over any years and it has been found that the greater the proportion of
oil present in the wood, the lower the critical ignition temperature
(the temperature at which a runaway reaction occurs)."

1. Bowes, P C. Self-heating: evaluating and controlling the hazards.
Building Research Establishment. Department of the Environment. 1984. HMSO.

It seems this Bowes fella' made a career out of this stuff. Even though
he died in 2001, he was nice enough to write it all down in a book.
Sadly, now that I'm no longer a student, I can't get my hands on it
easily. But maybe if one of you could find it, it could settle the
matter of whether or not s.c. is possible in this case. It might also
answer the more immediate question of who can **** farther.

In the above, I note the following:

1) Oil content is critical. How much oil is in sawdust from green wood
vs. dust from air dried vs. kilned, I don't know. Anyone?

2) Note that several other factors, including other oil sources are
important. This certainly has bearing on the original poster's question.
Does, for example, it make a difference if he cuts plywood or any
other source of organics? What if he hacks up an old project that was
treated with linseed oil?

I suspect that if one read the book, you'd be hard pressed to make a
case for s.c. at this small a scale (especially since the water content
must be just right); nevertheless, I bet you _could_ come up with a
scenario, albeit unlikely. Would I worry about it? No. But I'm not silly
enough to put water in my DC system...

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As for hay, I found this incontrovertible evidence (of course, because
it's on the internet):

From http://www.cdc.gov/nasd/docs/d000701-d000800/d000758/d000758.pdf

"Hay fires usually occur within six weeks of baling, but they may occur
in hay several years old. Fire can occur in loose hay, small bales,
large bales or in stacks. The fires can occur in hay stored inside as
well as in hay stored outside. Regardless of when or where the fires
occur, the most common cause is excessive moisture."

Note that "old" hay is also subject to s.c., the point being that once
dried, s.c. is rare because, well, it's dry. But if re-wetted, I see no
reason why bacterial growth can't restart. Of course the C:N ratio of
wood is so high that it's hard to get a lot of bacterial growth and heat
production, but that obviously happened in the s.c. examples above. I
have certainly felt warmth in sawdust piles, but they were outside where
heat/organics couldn't build up enough for s.c.

Anyway, the point is that if you want to claim that hay and sawdust
outgas the same, then if old hay can s.c., then surely you agree that
old sawdust can, too. Yes?

The wonderful thing about science is that there is a cause - effect,
reproducible outcome.

Well, of course there is. But if we don't understand _all_ of the
factors involved, then changing the input (cause) may lead to an
unexpected result (effect). That is why scientists have jobs -- because
not everything is understood.

Enjoy,

Greg