From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: rec.pyrotechnics,sci.chem
Subject: Re: low temp flames?
Date: 22 Jan 1996 21:29:11 GMT

In article <4drtj2$6pc@ccnet3.ccnet.com>, berniew@ccnet.com (Bernie
Willis) says:

>Michael Freiert wrote:
>> i know some liquids burn at low temperatures (ie 70-110 F of
>> around 30 C for you metric people)
>
>At what point do we draw the line between burning and simply oxidizing?

If it isn't hot enough to be capable of producing black body
visible light, it isn't burning or deflagrating. Dull red light
emission begins around 500 deg C.

Now try the more difficult task of defining the difference between
simple oxidation and combustion. I would say that combustion
(which includes burning and deflagration as a subset) usually but
not always connotes the emission of light, but not necessarily in
the form of black body radiation.  Cool flame combustion may be
luminous below 300 degC. Combustion appears also to be a useable
term where there is no (visible) light emission but the combusting
substance disappears as a result of the reaction. A hypothetical
example of such a reaction would be one which otherwise had
the properties of cool flame combustion but lacked the specific
electron transitions resulting in luminosity. All forms of combustion
require thermal feedback from the energy of their reactions to 
sustain them in the absence of insulation and usually in the presence 
of substantial gas flow or reaction zone movement.

Simple oxidation may be thought of as occurring at the
temperature of the surrounding environment without feedback from the 
heat of reaction raising the temperature significantly above 
that of the environment..

The reaction of unsaturated oils in a pile of rags is a form of
oxidation which does rely reaction energy feedback without
producing visible radiation until the rags burst into flame.  We
call the entire process autocombustion, but the initial reaction
is virtually the same as the drying of oil-base paint, which
hardly qualifies as "combustion." We can at best now invoke the
fact that the substrate is not "consumed" in the initial reaction
to avoid the "combustion" classification.

There are also numerous cases of catalytic oxidation which
may occur with or without light emission. In describing these
reactions, the distinction between oxidation and combustion
seems extremely arbitrary since it depends often on merely
how far we turn the feed valve - enough to make the catalyst
glow or not?  It might be argued that the dividing line in this
sort of process is set by whether or not the energy feedback
is a primary agent in determining the reaction rate.

I guess it boils down to the frequently encountered principle
that we know it when we see it, but it's hard to define
unambiguously.

Please do not succumb to the temptation to invoke Webster
as the FINAL authority in this discussion. Dictionary definitions
are rarely adequate to describe technical shades of gray.
Words have a way of  gathering different connotations in different
areas of the arts and sciences. We sometimes need to listen to
hear what they seem to mean in our art, and then sometimes
create new conceptual fine structure  by mutual  agreement.

I have not defined "combustion," and the related terms above.
I have merely tried to describe what the terms mean to me,
along with some of the ambiguities. Others will surely have a
different perspective.  The initial authoritative tone above
was adopted to ensure some combative spirit in the replies :)

Jerry (Ico)