There are two issues heRe: protection from thermal shock and the
danger of setting diamond on fire. Thermal shock: substances which
are vulnerable to thermal shock, are substances which do not
conduct heat well. That been the case, parts of such substance
become overheated, while other parts remain less heated; that causes
unequal expansion and it shutters.
While low thermal conductivity is an aspect of thermal shock
cracking it is not the only reason. There is also toughness and
thermal expansion coefficient. Fused quartz is a poor thermal
conductor but is very thermal shock resistant because of its low
thermal expansion coefficient and while it is moderately tough when
compared with diamond it is quite tough when compared to other
Regular glass belong to this class.
But borosilicate glass (Pyrex) is highly resistant to thermal shock
cracking and is not markedly different from regular “soft” glass in
its thermal conductivity. However it has a lower thermal expansion
and greater toughness than regular glass.
Diamonds are one of the best conductors of heat in existence.
Diamond heat conduction is so good, that one heat the diamond to red
heat and plunge it into liquid nitrogen without any danger of
Again very low thermal expansion and excellent toughness along with
the greatest thermal conductivity of any material and if it is a
colorless or near colorless F or IF stone or a clean lab created
diamond this would be no problem. But you try that with a SI1 and
you are liable to remove that diamond from the liquid nitrogen in
several pieces. Which is why it is not a very good idea to quench
that diamond ring after re-tipping.
Danger of burning diamond: diamonds do burn, but in order for
reaction to start, diamond has to reach temperature of 1562
degrees Fahrenheit. Due to high thermal conductivity of diamonds,
even if the piece itself would reach such temperature, the diamonds
would be few hundred of degrees cooler. In another words, the piece
been worked on, would melt before diamonds would reach required
temperature to trigger reaction of oxidation ( burning ).
While you might be able to set up such a scenario it is unlikely in
typical repair work that the diamond will be significantly cooler
than the surrounding metal. The main reason for this is that the
diamond is in thermal contact with the metal and will act as a very
good heat sink till the difference in temperature between the diamond
and the metal is fairly low. Also the boric acid will act to increase
the thermal contact between the stone and the metal lowering the
thermal impedance between the diamond and the metal due to the
increased surface area of contact. In the common job of re-tipping a
prong on say a.5 carat stone as long as the diamond is sucking away
the heat you will not be able to raise the prong tip to the point
where the solder will flow till the diamond is very near the flow
point of the solder because of this heat sinking.
Why then we do it? It is to protect the metal under the diamond
from oxidation. Diamonds themselves are quite save. Also, by
dipping it in boric acid - alcohol solution, the piece is degreased.
If there are grease on diamonds, such stones may require
repolishing if brought to high temperatures, but not due to diamond
burning. That is the true reason for the practice.
Since the majority of the time that one would be soldering around a
diamond is in repair operations so I am sure you rely on something
more than the alcohol in the fire coat to clean and degrease the
stone as this would be insufficient for anything more than
fingerprints or other very light soil. Because most of the diamonds I
see in repair jobs are filthy.
James Binnion Metal Arts