Heat Treatment of Sapphires

Heat Treatment

HEAT TREATMENT OF SAPPHIRE

( A layman's simplified explanation of the various levels of treatment)

Sapphire is formed deep down below the earth's surface, some 65 kilometres down, and comes to the surface in a volcanic explosion in a 'formation tube' or vent. The sapphire comes up at the forefront of a volcanic eruption and is expelled with the pyroclast, or volcanic ash. This is typical for all Australian sapphire occurrences.

If the temperature in this formation tube is hot enough, and the residence time is long enough, the sapphire is clarified, on its journey to the surface. The main contaminant in sapphire is rutile (Iron - Titanium Oxide) and if the temperature/time relationship is sufficient this is volatilised and expelled from the sapphire or it is reabsorbed add reconverted within the crystal.

However, if the temperature is too low, or the rate of travel of the sapphire and pyroclast up the tube too rapid, the sapphire may be left with inclusions such as the "silk" veils which often cloud natural sapphire crystals.

The Thais developed the practice of commercial heat treatment many years ago to remove these silk veils to produce stones of improved clarity and brilliance This process has always been considered quite acceptable as no chemicals were added, and the holding of the sapphire at high temperatures to bring about this improvement was essentially only mimicking nature and was just finishing off what nature had started.

However, the heat treatment technology has been greatly extended in man's quest to turn lower quality material into brighter stone in the endless search for more profit - and some quite dubious if not actually false practices have resulted in recent years!!

A simplified explanation of the levels of heat treatment is as follows :-

a. Natural Untreated Sapphire.

This may be totally clear or may have small inclusions or silk bands. Whilst it was once the accepted practice to heat treat to remove all "silk', this is now not always done as a small quantity of silk is considered acceptable, if not actually desirable since untreated good quality material usually commands a premium price. We try not to heat treat wherever possible.

b. Simple Heat Treatment

This is considered quite acceptable in the jewellery trade as no chemicals are added and the sapphire is not changed chemically apart from the removal or conversion of the rutile and contaminant bonds.

This is a simple process brought about by a one-off heat treatment, and the resultant clarification of the sapphire is permanent and irreversible.

No radiation, chemicals or health risks are involved in this process.

EVERY SAPPHIRE WHICH ORIGINATES FROM THAILAND OR IS CUT IN THAILAND MUST BE CONSIDERED TO HAVE BEEN SUBJECT TO AT LEAST THIS SIMPLE HEAT TREATMENT AS NORMAL PRACTICE.

c. Multiple Heat Treatment - Colour Generation

In certain cases, the amount of included material is too great to be removed by the simple one-off heat treatment process, So a system was developed of multiple heat treatments, sometimes in the presence of fluxing agents, which eventually lead to a clearer or brighter stone of acceptable colour.

Much of the "Ceylon" sapphire on the market comes from Australia or from Madagascar and may have been subject to a simple treatment process, but a portion of it started off life as Geuda Stone from Sri Lanka. This very highly included stone looks like a "moldy potato" before heat treatment.

The geuda stone is heat treated as many as six or eight times in order to clarify it and produce the pale "Ceylon Blue" which is sold at quite high prices - but is essentially a heat generated colour.

d. Bulk Chemical Diffusion - Beryllium Treatment.

The chemical diffusion process is carried out by multiple heat treatments of the sapphire in the presence of artificially introduced elements like beryllium or titanium, which penetrate the structure of the sapphire.

The beryllium treatment is carried out to produce golden, yellow, orange, apricot, padparadschas and similar colours from lower grade rough sapphire or from sapphire of less desirable colours. The beryllium can penetrate the sapphire to a considerable depth and can be difficult to detect.

The chemically modified stone may have good colour and brilliance, but it is no longer a 'natural sapphire' and cannot be sold without proper and adequate disclosure of the treatment it has suffered. It is essentially a "fake", albeit sometimes a very good one!!

The titanium treatment is carried out to enhance the colour and brilliance of blue sapphires, but the depth of penetration is far less than that for beryllium treatment and consequently repolishing of a stone may remove part of the chemically imparted colour.

Concerns have been raised as to the possible health risks arising from the beryllium treatment process, and these concerns are very real for the operators of heat treatment furnaces as the beryllium vapour is highly toxic.

Beryllium treated sapphire has only been present on the world markets - in the last two years or so, and it is still too soon to know whether the process may result in any long term health problems for the wearer of such chemically treated stones.

Persons selling beryllium treated sapphires are required to disclose the fact clearly and adequately to purchasers. However, since some sellers have been known not to make the requited disclosure, buyers are recommended to seek a guarantee or statement of authenticity for their purchases.

Anybody who wishes to gain more technically correct or detailed information on the treatment process can do so by visiting the website of Gemlab Inc., whose principal, Mr. Ted Themelis has been a world leader in such technology testing. His website is www.themelis.com.

Coolamon Mining Pty. Ltd guarantees that no chemical treatment of any kind is used in the production of our sapphires, and that all information is disclosed.

The above article was extracted from a talk given by Mr. Jim Elliot, Coolamon Mining, at the Central Queensland GEMFEST - August 2003

genuine sapphire - made by nature

An Information Resource on the Australian Sapphire Industry

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