From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: sci.physics
Subject: Re: Diamonds
Date: 23 Jan 1996 23:19:08 GMT

In article <4e378e$nkm@news.bu.edu>, jeffs@bu.edu (Jeff Suzuki) says:
>
>Ben Bullock (ben@theory4.kek.jp) wrote:
>
>: > Diamonds burn (they are carbon)
>
>: Really?  I don't know what temperature you need to ignite them, but it
>: must be extremely high since diamond drill bits would burst into
>: flames otherwise.  I remember seeing a diamond dropped in liquid
>: oxygen and burning once though.
>
>You can also get them burning with a magnifying glass, if you're
>patient.  (Actually, I don't think burning is the right word:  they
>will oxidize and "evaporate")

At about 1000 degC the surface of a discrete diamond will form a
microscopic layer of carbon. The carbon oxidizes to CO2 more 
readily than does diamond and thus a very hot diamond in oxygen
could indeed appear to "evaporate." However a diamond will not
support combustion in the conventional sense any more than 
solid graphite will because the vapor pressure is too low
except at ultra-high temperatures. Obviously, porous carbon in
the form of charcoal does support embrous deflagration because
of the high surface area and insulating properties of the 
fuel/air matrix. The same is undoubtedly true of diamod dust,
which is somewhat less reactive than graphite.

If you use an oxidation catalyst such as lead oxide blended in
with a mixture of diamond and graphite dust, the mix will
begin glowing gently in a furnace at a few hundred degC, the
brightness fluctuating with passing air currents. If the mixture
is regulated to just the edge of glowing, the air will 
preferentially oxidize the black carbon leaving pure diamond 
dust. If this mix gets a little more incandescent, the diamond 
also oxidizes to CO2. At higher temperatures it is nearly 
certain that a mass of diamond dust would continue to support
the embrous process in the absence of the catalyst.

Jerry (Ico)