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GEMLAB REPORT:
by: Ted Themelis
themelis@mail.otenet.gr
In This Edition:
- Rutile Silk and Flux Inclusions in Ruby
// --Rutile Silk and Flux Inclusions in Ruby –\
Nowadays gem dealers and members of the jewelry industry need to
be extra careful regarding some new types of inclusions in ruby.
These can significantly alter the appearance of the stones,
particularly rubies, thus making their correct identification
more difficult.
For example, a small parcel of rather expensive rubies was
recently submitted to our heat-treatment laboratory. The goal
of treatmeny was to remove/reduce the rutile silk inclusions.
The stones were faceted in the familiar “Bangkok-look” style, and
averaged just under one carat per piece. They had exceptional
lustre and excellent color. The obvious rutile needle
inclusions, in the form of “silk”, were observed along with other
tiny inclusions, appearing as droplets. The rubies were examined
using a specially-designed microscope to determine the diameter
and size of these rutile needles. From this, the proper heating
time, at the soaking temperature, couldl be computed, leading to
the determination of correct heat treatment parameters.
Chemically pure rutile melts at about 1825oC, passing several
tranformation stages, starting at about 1250oC (experimental
work is currently in progress by the author determining the
limits of alpha-TiO2, beta-TiO2 and gamma-TiO2). When rutile is
combined with two atoms of oxygen, under favorable conditions, it
forms TiO2, which comes in several configurations, one of them
being the so-called “silk” found in some corundum. These rutile
needles (silk) are formed parallel to the faces of the
first-order hexagonal prism in corundum. Silk in natural corundum
may be hair-thin and long (usually from Sri-Lanka), or thick and
stubby (usually Burmese). Rutile needles are crossing each other
at 60o/120o angles.
In addition to other causes, various impurities, in the form of
mineral inclusions, can alter the nature of pure rutile,
lowering it’s melting point to as low as 1500oC or below, under
ideal conditions. Excess amounts of silk produces hazyness, which
reduces the transparency of the corundum, and detracts from its
overall appearance. Heat treatment reduces or removes the silk
(depending the heating parameters used in the process), so the
stones appear more transparent (and more saleable). However,
there is the problem of the re-crystallization of the silk at
about 1250oC, if the cooling process is not perfomed properly.
The stones must be cooled quickly (at about 40oC/minute) to avoid
the re-crystallization of the rutile needles, that produces the
familiar “dotted-line”, sign of incomplete melting of the TiO2.
In these rubies, we observe that the silk needles were coarse,
thick, stubby and rather short. They were somewhat irregularly
distributed throughout the ruby, as oppposed to being found in
specific zones. We computed that to dissolve this type of rutile
silk, treatment for 120 minutes at a minimum of 1780oC was
necessary.
With all this in mind, we did not bother to investigate the
nature of the tiny droplet inclusions that appear irregularly in
almost all of these rubies. We kept some control samples, and
heat-treated the remaining rubies in a batch process at 1780oC
for 120 minutes, with cooling rate of 40oC/minute to 1250oC, in
attempt to remove these inclusions as well as the rutile silk.
When we opened the crucible at room temperature, we were
surprised to find fluffy clouds of some other strange inclusions
throughout the rubies. Rushing to the microscope, we were amused
(and relieved!!) to discover that these tiny droplets were
actually melted flux. They now appeared as blackish droplets,
probably combined with hematite (Fe2O3), to form this very
unusual and confusing configuration. No evidence of silk was
seen; it had melted almost completely.
Since the silk was found all over the interior in the host ruby
prior to our heating, it is very unlikely that the silk was
diffused over or just below its surface after its original growth
process. More likely, the silk was developed in the host
synthetic ruby during its crystal growth process. But, how?
Merrs.J.N. Burdick and J.W.Glenn, in their patent, described the
development of potential silk and asterism in the synthetic
flame-fusion corundums. It is logical to assume that the process
is analogous if flux-grown corundums are used in lieu of
flame-fusion corundums.
The potential development of rutile silk is achieved by heating
the material between 1100oC and 1500oC, under oxidizing
conditions. Below 1100oC no silk is produced, and above 1500oC
no precipitation of the TiO2 can occur. The typical heat
treatment of inducing TiO2 into the host corundum takes about 2
hours. The solubility of TiO2, in natural or synthetic corundum,
decreases strongly with the temperature. That is, if sufficient
amounts of TiO2 are in the host crystal during its growth
process, rapid cooling keeps it in solution in a supersaturated
state. When the host crystal is now held at a lower temperature
for extended periods of time, the excess TiO2 precipitates out as
rutile needles in the form of silk. Subsequently, the rutile will
have enough time in this specific environment, to orient itself
properly and produce the asterism effect desired.
Flux-rubies are grown at between 1300-1350oC, depending on the
manufacturer, methods, flux ingredients and dosodology used.
When the synthetic corundum is “matured”, and chemically pure, at
the completion of the synthesis, it’s melting point is around
2025oC. Crystallized defects, such as flux inclusions, lower the
melting point of the synthetic crystal. Melting points are not
affected, however, as much as those of their natural counterparts
containing natural impurities in the form of mineral inclusions.
We feel It is entirely possible that, under suitable conditions,
rutile may be induced into the host synthetic crystal forming
silk.
We are still not sure what kind of synthetic rubies we were
working with, but we have narrowed it down to several candidates:
Kashan, Ramaura, Chatham, Douros, the Russian group, the Japanese
group, the newly formed Chinese group, or possibly others. Your
guesses are good as mine.
Ted Themelis
Any Comments should be posted to @Service or directly
to the author at: themelis@mail.otenet.gr