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[Gemlab] Heat Treating Sapphires from The Anakie district, Australia



                   July 1998 - Issue #13

                       By: Ted Themelis

In This Edition:

Heat Treating Sapphires from The Anakie district, Australia.

Heat Treating Sapphires from The Anakie district, Australia.Sapphires in Australia
were first reported to occur in N.S.W. by Strutchbury in 1851, as a result of gold
prospecting operations. Large commercial mechanized operations and smaller
concerns are involved in mining the sapphires from alluvial deposits widely
distributed in Rubyvale, Sapphire, Willows-Glenlave, Tomahawk Creek, and other
areas in the Anakie District.

The practice of heating the Australian sapphires for the purpose of enhancing their
clarity/color is well known for a long time. It is said that, the heat treatment of the
Australian sapphires originated in Germany several decades ago. In Thailand,
since the 1970s, numerous homemade furnaces have been used exclusively for
the heat treatment of the Australian sapphires. Today most of the heat treatment is
performed in Thailand using numerous heat treatment equipment and methods.
Other heat treatment facilities are located in the USA, Sri Lanka, Japan, Germany
and elsewhere.

Description of the sapphires used in this project
About 20,000 carats of commercial mine-run rough sapphires from the Anakie
District were used in this study. The weight of each rough specimen range from 0.15
carat to 15+ carats. The lot consisted of yellow, yellow/green, green/blue, blue,
blue/green, and their intermediate colors with light, medium, and dark tones. Some
stones were characterized with brownish spots against a greenish/blue or bluish
"color body", while others were parti-color (yellow/green and blue, blue and
colorless, etc.). Most of the sapphires displayed prominent color zoning in an
alternating pattern. The color zoning was found primarily parallel to the basal
pinacoid and hexagonal prism faces. Many sapphires appeared brownish in
reflected light, while others appeared milky-bluish. Some stones showed
yellow/orange stains at the surface and/in the cracks. Most were translucent to
transparent, strongly dichroic, and with a silky appearance.

Preparation for heat treatment
Prior to heat treatment, the sapphires were placed inside the rubber drum-type
tumbling apparatus and soaked overnight in hydrofluoric acid mixed with water.
The purpose of cleaning was to a) spot and remove from the lot the non-
corundums, which some were turned opaque and chalky-white and b) to remove
the yellow/orange iron oxide stains/spots that may have located at the surface or
entrapped in cracks on the sapphires.

Heat treatment methods and procedures

Method A: Heating by type.
This method requires proper classification of the sapphires into their types, based
on common characteristics observed under the same lighting conditions. Heating
the sapphires based on their classified types provides direct comparison of the
results obtained, thus determined the best heating process for each of the
classified types. Proper classification into their designated type is more important
than setting the heating parameters. Wrong classification would mean wrong
parameters setting, thus producing different results.

The Anakie District sapphires described in this study were classified using a
sorting table equipped with transmitted fluorescent light. It should be mentioned that
not all varieties are encountered in a specific mining area; neither are or found in a
given lot of sapphires. Furthermore, the classification used in this article does not
represent all possible types found in the Anakie District. For convenience, the
sapphires were grouped into types of similar appearance. Based on the author’s
experience the tolerance of error is about 10% to 12%.

Type 1: Medium blue/grayish with or without greenish tinge
Type 2: Dark blue/grayish
Type 3: Brownish spot on bluish or greenish "body-color"
Type 4: GREEN/blue
Type 5: BLUE/green
Type 6: Parti-color
Type 7: Dark blue/green to dark green/blue
Type 8: Milky
Type 9: Yellow, yellow/green and greenish
Method B: Heating by batch.
Using this method, the sapphires were heated in a batch, (without type
classification) as received from the supplier, with predetermined parameters. After
the initial heating, sapphires deemed to be suitable for cutting were removed from
the lot (medium blue and parti-color). The remaining sapphires were heated again
by their type (dark blue and light blue), using different heating parameters; after
heating, additional sapphires deemed to be suitable for cutting (medium blue)
were removed from the lot. The remaining sapphires were re-heated till the entire
lot was exhausted

Heat Treatment Equipment
All above described heating processes were performed in a special vertical muffle
top/bottom loaded furnace with actual maximum operating temperature for
continuous use to 1800oC, engineered and fabricated by the author. The furnace
consisted of three modules mounted on wheels and integrated to each other:

A) power supply and controls
B) furnace and
C) Water-cooling. Additional auxiliary/monitoring equipment employed. Selected
gases were mixed in a atmosphere control apparatus specially-designed by the
author, equipped with flow-meters, pressure gauges, and other parts, that produce
precise oxygen fugacity ranging from 10o to 10-10. The final gas mixture was
purged into the open-loop muffle system during heating. All processes were
performed utilizing different heating parameters under fully controllable
atmospheric conditions.
Summary and conclusions
Carefully controlled heat-up and cooling rates (from room temperature to about
900oC) prevented the cracking of the stones and the associated ceramic 'furniture’
residing in the furnace.

Careful controlled cooling rate from 1800oC to about 1200oC prevented the re-
crystallization of the rutile, regardless of the heating parameters used. All the
specimens appeared with increased transparency.

Milky/silky (Type 8) sapphires when heated over 1400oC turned blue, regardless
the atmosphere conditions.

Heating processes performed under strong reducing atmosphere, using hydrogen-
based gas mixtures produced undesirable results. In almost all types, the stones
turned “muddy-green” or black, not suitable for cutting. It is proven that if these
strong reducing atmosphere conditions would be further increased, the sapphires
would gradually lose their oxygen content and turn blackish.

It was observed than several types (Type 1, 4 and 5) contained silk. The diameter
of the silk needle was measured carefully by the author ranged from 0.02 mm (“thin
needle”) to 0.08 mm (“thick needle”). After heating at 1650oC, the “thin silk” was
dissolved and the stones appeared with increased transparency. The “thick silk"
was partially dissolved and appeared as “dotted” or “interrupted” line. This
demonstrates the relationship of the heating time at a given operating temperature
and the diameter of the needle silk: “thick silk” takes longer to be dissolved, while
"thin silk” takes less time.

After heating (even at 1800oC), the sapphires were easily removed from the
crucible without being “glued” together or with the crucible itself. This demonstrates
the importance of cleaning the stones prior to heating. In all processes it was
observed that the final color of the sapphires was established 1-2 days after
Sapphires heated using Method A yielded slightly better results than sapphires
heated using Method B. It is the author’s opinion, that sapphires of high quality
should be heated using Method A; commercial and lower quality sapphires should
be heated using Method B, provided that proper methodology has been

Method A. Best yields and processes on Anakie District sapphires, heated by type

Type 1 Blue/grayish 60% medium blue
Type 2 Dark blue/gray 100% lighter blue
Type 3 Brown spot 100% blue (dark, medium light)
Type 4 Green/blue 90% bluish (dark, medium, light)
Type 5 Blue/green 100% lighter blue
Type 6 Parti-color 56% blue (dark, medium, light)
Type 7 RDark blue/green 80% lighter blue/green
Type 8 Milky 100% blue (dark, medium, light)
Type 9 Yellow/green no color improvement

Method B. Summary of yields obtained from batch processes after heating.
Population sample consisted of 2,080 pieces of mix-types, commercial, mine-run
sapphires from the Anakie District.

		No. 		%)

Dark blue 575 27.60
Medium blue 1,262 60.70
Light blue 58 2.80
Parti-color 104 5.0
"Muddy-green" 81 3.90
TOTALS 2,080 100.0

Disclaimer notice
The mixture of gases used in the above-described heating processes are
poisonous and/or highly explosive, especially when are used in conjunction with
high temperature during heating. The author discourages the use of these gases
and practices, unless qualified technical personnel under suitable laboratory
conditions perform the heating processes.

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