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Gemlab Report * * By Ted Themelis @Ted_Themelis September 1, 1997 Issue #3
HEATING RUBIES & SAPPHIRES: TECHNICAL ASPECTS
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Heating corundum appears to be a fairly simple issue. Just
purchase a furnace, put some rubies/sapphires into the chambers,
heat to some temperature, for some time and the stones are ready,
right? ong! Although some results may be produced from the above
experiment, the issue is really quite complex, precisely because
no two rubies or sapphires exist in nature that have exactly the
same chem al composition. Therefore, when these corundums are heat
treated using the exact same parameters, the results will differ.
Temperature
Maximum operating temperature is of paramount importance. The
ability to apply uniform temperature in the heating chambers and
to maintain such temperature during any given step in the heating
proc s, depends upon the engineering and design of the furnace
system. An over-heating protection feature is highly desirable to
prevent a possible melt-down should something go wrong with the
temperatu controlling instruments. Verification of the true
temperature is performed using specialized instruments capable of
measuring values to within 0.001mV. Maintaining a given
temperature value, dep upon the type of furnace used. Gas furnaces
must maintain constant gas mixtures using appropriate valves and
other parts, while electric resistance furnaces must have proper
power distribution c ollers, preferably with a thyristor
phase-angle control to limit the current flow.
Atmosphere conditions
The type of environment in the heating chambers of the furnace is
another very important parameter. The amount of oxygen present
during heating, greatly influence the results of the process.
These “oxygen-rich” (oxidizing) or “oxygen-poor” (reducing)
atmosphere are external descriptions only, having different
meanings. Atmosphere conditions directly influence the operating
temperature of furnace and must be used very cautiously.
When an electric furnace is operated without an auxiliary
atmosphere control apparatus, the amount of oxygen in the
heating chambers is about 20.9% - the remainder is principally
nitrogen. If add nal oxygen is purged into the chambers via an
atmosphere control apparatus, then the amount of oxygen is
increased; creating what is often called “oxidizing conditions”.
Maximum oxidizing condition are obtained when the oxygen content
in the heating chambers is 100%. Practically, this is not possible
- there are no vendors selling bottles containing 100% oxygen.
However, technical-grade oxyge (around 99.999%) or lab-grade
oxygen (99.995%) may be purchased that will be sufficient for
heating rubies/sapphires.
Inversely, when the oxygen content in the heating chamber is
reduced (usually to less than 1%, with the remaining balance
being made up of mostly nitrogen), we produce what is usually
termed an “in t atmosphere” environment. Other noble gases such as
argon and helium are sometimes used. An inert atmosphere reduces
the maximum operating temperature in the furnace heating chamber
Reducing atmosphere
Reducing atmospheres are obtained by using various gas mixtures.
For instance, a 9% hydrogen atmosphere - with the balance being
made up of nitrogen, is known as a “forming gas”. Although the
perce age of hydrogen in the mixture is low, the effect of
hydrogen is considerable, especially with extended periods of
operation at elevated temperatures. Since hydrogen is traveling
very fast in cor m in relation to other atoms, the processing time
required for heat treating corundum is much less when compared to
other gas mixtures that do not contain hydrogen. An endogas gas
mixture (typic consisting of 40% hydrogen, 20% carbon monoxide,
and the remaining balance being made up of nitrogen) generally
produces a strong reducing atmosphere effect. The poisonous nature
of this mixture, educes the maximum operating temperature in the
furnace heating chambers. Application of endogas mixtures as
purgion gas when heating corundums is very dangerous, and is
generally avoided; when it needed, it should be used only by
experienced and knowledgeable technicians. Numerous other gas
mixtures consisting of carbon monoxide, carbon dioxide, hydrogen,
water vapor, or any combination re of, may be composed and used.
Each of them have different reducing atmosphere effects, and
should be used at different temperatures for each specific type of
corundu
Vacuum atmosphere
Heating rubies/sapphires may be performed in a vacuum
environment at a given temperature. A vD vacuum of up to 10-4 Torr
may be used at high temperatures using specialty furnace
Atmosphere conditions using gas furnace Gas furnaces can produce
various atmosphere conditions, while maintaining a fairly steady
temperature by controlling the carbon content. For example, if
natural gas is used in a gas furnace, oxyg oor conditions are
obtained by increasing the carbon content as follows: C3H8+ 2.5O2
-> 4H2O+CO+2C (ratio: 3 carbon to 5 oxygen). In this case, the
reducing agents are carbon monoxide and some free arbon. If the
carbon/oxygen is further reduced, the carbon becomes the main
reducing agent (in the form of soot). This is quite
characteristic of many “overcooked” blue sapphires. If the carbon/
en ratio is further reduced at ultra high temperatures (over
1900oC), the corundum gradually starts losing its oxide status and
eventually becomes carbonized alumin
Oxygen-rich conditions may be achieved in gas furnaces by
increasing the oxygen content in the gas mixture as follows:
C3H8+6O2 -> 4H2O+3CO2+O2. (ratio: 3 carbon to 12 oxygen). Most
Thai treaters gas furnaces to obtain a reducing atmosphere
condition when heating blue sapphires. For rubies, they generally
use electric furnaces. Of course, the precise amount of the oxygen
content cannot be actically determined unless a special gas
analyzer is used to monitor the gas mixture during the heating
process.
Atmosphere conditions using electric furnaces
The best method to apply in order to precisely and fully control
atmosphere conditions is to make use of an alumina muffle,
installed vertically in the furnace. The great advantage of using
this od is that the heating elements and the corundums, are
located in the same heating chambers in two different areas,
having two different atmosphere conditions: The heating elements
are installed ically near the wall of the furnace and exposed to
air, while the corundums are enclosed in the muffle,in to which
the selected gas mixture may be purged directly. Gas purging is
achieved externa via a separate atmosphere control system
consisting of flow meters, pressure gauges, gas valves, etc. Thus,
totally controllable, precise, and reproducible atmosphere
conditions may be obtained by using the muffle method.
Effects of atmosphere conditions - additives
Atmosphere conditions are greatly altered when additives, fluxes
are used in the heating process. For example, when heating rubies
in air from Mong-Hsu, Burma (and similar rubies from elsewhere),
results after heating are different when the same type of rubies
are heated under the same conditions using borax-based flux
additives. Another example is when heating dark Australian blue
sapph . When carbon-based fluxes are used, the results are much
different from those produced with hydrogen/nitrogen mixtures.
Fluxes may be used in any compatible atmosphere condition. The
results vary eatly, influenced by additional parameters. The
effect of the atmosphere conditions varies greatly with the gas
mixtures used, although the same operating temperature is used.
Thus, certain corun require an oxidizing atmosphere, other
corundums (usually rubies) may be heated satisfactory in air,
while certain types of sapphires require reducing atmosphere
condition
Time and Rate
Time is another important parameter when heating
rubies/sapphires. Time at the heat-up, processing, and cool-down
segments of the heating process, must be computed, according to
the load being he and type of furnace insulation. For instance, a
high temperature rate at heat-up time, say over 200oC/hr, will
shorten considerably the life expectancy of the ceramic muffle and
peripheral alumina arts (furniture); rapid heat-up rates (above
250oC/hr) will probably crack the alumina muffle, crucible and
other ceramic furniture installed in the chambers, due to unequal
thermal expansion. Conv sely, during cool-down segments of the
process, the rate should be controlled to avoid ceramic breakage.
Cool-down rates vary considerably according to the heating process
sought. In some instanc such as when heating corundums to
reduce/eliminate their silk, cooling rapidly (30oC/minute) from
1800oC to 1250oC is desirable to prevent re-crystallization of the
rutile silk needles. In other ances rapid cooling is not
desirable, even at higher temperatures.
Time required for heating specific types of ruby/sapphire at a
predetermined operating temperature, depends upon a) the average
size of the corundum and b) atmosphere conditions used in the
process The processing time must be computed using these
parameters.
Rubies/sapphires heated in order to reduce/eliminate their silk,
require that the diameter of the silk to be measured and
appropriate treatment time computed accordingly. Thin silk
requires less ti to be dissolved in the corundum substance, while
thick, stubby-type, silk requires a longer processing time.
Furnace Technology
Furnace technology is required to fabricate a furnace that is
capable of performing repeated, predetermined heating processes.
Here are the requirements of an ideal furnace
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A maximum operating temperature of 1800oC is highly
desirable, but not necessary. -
Insulation of a type and quality that is able to withstand
high-temperatures for long periods of time. -
An atmosphere control apparatus capable of mixing most
commercially available gases; along with an appropriate
apparatus to exhaust the gases out of the furnace safel -
An alumina muffle (preferably gas-tight) to insure proper
atmosphere control. -
A cool-down rate control mechanism that utilizes a
mechanical elevator or similar device. -
A programmable temperature controller.
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A temperature limit control of some type to prevent
melt-down conditions should the primary (programmable)
temperature control mechanism fail -
A method of cooling the cold sections of the heating
elements in order to prevent premature element failure. -
Sufficient ventilation to keep the electronic parts cool and
clean. -
A power controller, preferably with thyristor phase angle
control to limit the current. Some accommodation should be made
for unstable power supply conditions. In some countries, the power
s y varies greatly between daytime (below 208Vac) and nighttime
(over 240Vac).
Heat-Treatment technology
Heat treatment technology must be developed by the end-user.
Developing the proper heat treatment technology for a certain type
of corundum requires many years of practical experience and
constant experimentation. The following issues must be taken into
consideration with each type of corundum:
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Know the chemical contents of the type of corundum to be
heat-treated. -
Develop a cleaning technique that will not harm the
chromophoric impurities. -
Determine the atmosphere conditions to be used: type of gases
to be used; gas flow rate; gas pressure; purging time at any given
temperature, for each step of the heating process. -
Determine the number of heating segments (ramp, dwell) and
their rates (ToC/hr). -
Determine the operating temperature.
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Determine the processing time (soaking) at the operating
temperature. -
If additives are to be used, determine the type, mixture, and
ratio.
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