Ask A Scientist General Science Archive

 
name        Marti F.
status       other
age       50s

Question -   If I put piles of sawdust on the ground to control weed
growth, is there any danger of the sawdust spontaneously combusting?
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This is not a scientific response so much as some personal observations. I
once collected a lot of fresh grass clippings and left them in a pile 
about four
feet tall in the driveway. After a couple of days, the decomposition of the
fresh grass had generated enough heat to start it smoldering deep inside. 
Then
there was the time the neighbor's big round hay bales caught on fire,
presumably due to the same thing.
In your case, I doubt that you would have a deep enough pile of sawdust on
the ground to retain enough heat to support combustion.

Paul Mahoney, PhD
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In order for anything to combust, a little starting energy must be supplied.
Even substances that are flammable (gasoline, coal dust, paper) need some 
kind
of spark or small flame to get it started. This is true of sawdust as well. I
imagine if the sawdust is very dry that an errant flame or spark (such as 
from
a carelessly thrown cigarette butt or charcoal grill sparks) can ignite the
sawdust. This is about the same danger as dry grass fires. However, if the
sawdust is damp (as from morning dew), this danger will be reduced greatly.

As a rule, nothing really "spontaneously combusts". Some starting energy must
be supplied first.

Roberto Gregorious
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Unless you pile the sawdust several feet deep there is little chance for
"spontaneous" combustion. Air flow and water usually will keep the
temperature low enough that decomposition reactions are kept under control.

Vince Calder
====================================================
I do not know, from either experience or principle.
I am sure that in some locales it just gets damp and soft and gradually 
turns into soil itself, and is never a hazard.
But possibly some places are otherwise, so I'll speculate.

Unless it is very warm from some underlying composting activity, it will 
pick up fresh moisture every night or every rain.
And/or you will be watering it.  Being damp makes it harder to burn, of 
course.

You only mean piles an inch or so, right?
Deep piles might have more excuse for an occasional weird incident,
because they can trap heat very well inside.

If deep piles have been in place for a long time (i.e., it is dirty) there 
is more chance that foods and "rare" organisms
can build up enough to sustain a heat-generating biological activity,
and that waste products with low ignition temperature can be accumulated.
Wood barely browns at 350F, and organisms die and soon stop producing heat 
at 212F or so.
So how is the ignition temperature ever reached?
For spontaneous combustion to occur there must be some poorly-stable batch 
of chemicals created which
is vulnerable to slow but direct reaction with air at 250F, able to climb 
through that temperature gap with no further biological help.
Living organisms can make such chemicals, and presumably sometimes do.
They also make catalytic enzymes which may survive to 300F, and some 
inorganic substances can be catalytic too.
None of which are usually there when the sawdust is new.

So it would be responsible to decide in advance, how often will you remove 
and replace this sawdust?
I would assume that two years is too long.

If you are in a very dry place, you may generally not have the chance to 
tell the difference between spontaneous combustion
and some small ignition source.
For that reason it would be a fire hazard there.
I can imagine occasionally checking the humidity by burning a bit of 
sawdust from your garden,
but I'd be speculating as to what procedure is both indicative and safe.

Your surroundings matter.
If someone's garden is in a northern woods that usually cannot burn, of 
course they can relax about this.
If it is right next your old wooden house in the Mojave Desert, the risk 
is noticeably less advisable.

Jim Swenson
====================================================
Nope, will not burn unless it is being fluffed up while a match is being 
put to
it.  You would need to raise the temperature well over 400 degrees
Fahrenheit.

Now if I saw squirrels with matches, I would worry.

Martha Croll
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