A ? for Color Experts - heat treatment and durability

[Garry H (Cut Nut)]

Twice i have asked GIA (Jim Shigley and Shane McClure) about heat treatment effect on corundum durability.
Both said "yeah, interesting, cant''t answer, we should do a study".
Neither ever replied.
No one to my knowledge has done a proper study, although there is plenty of myth that suggests durability suffers.

Any ideas guys?

 

[strmrdr]

interest question it will be kewl to hear what the Richards have to say and Michael.

 

[elmo]

Wouldn''t fracture healing potentially improve durability?

 

[Richard M.]

Garry,

I don''t know of any formal study done on the topic and I''d like to see one. I hope Richard Hughes and Vincent Pardieu check in on this. I hear frequent speculations from amateur faceters that heat treatment somehow degrades the structure of corundum. I doubt they''re based on any truly scientific observations.

Ted Themelis is actively involved in corundum-heating research in Bangkok. In his book "The Heat Treatment of Ruby and Sapphire" he comments:

"Beauty, durability and rarity are the most important attributes that determine the value of rubies and sapphires. Heated rubies and sapphires may be very beautiful. Their durability and stability under normal wear is undisputed and confirmed to be identical with their [sic] natural (non-heated) corundums." He may be in a better position than many to make such a statement since he works with such stones daily.

Richard M.

 

[Michael_E]

Gary,
I cut a lot of sapphire. Most of the lighter stones have been heat treated very close to their source and most of the deeper blues are from Australia and have not been heat treated. When cutting with the coarse to medium diamond grits there is no perceptible difference in the way that they cut, or the size of the "pitting" that occurs. I would think that if heat treating had an affect on the strength of sapphire that it would show up during the cutting operation. This is kind of relative, however, since there''s just no way to measure the cutting forces used, at least with my meager technology.

I think that the easiest way to get a feel for this would be to throw a buch of treated and untreated faceted stones into a rotating tumbler and give them a spin. If the untreated stones had less facet junction chipping, then it would be a very good indication that they were stronger. Perhaps we could talk some of the ladies here into giving up their stones for a test ?

 

[Garry H (Cut Nut)]

Date: 1/7/2006 1:31:20 PM
Author: Michael_E
Gary,
I cut a lot of sapphire. Most of the lighter stones have been heat treated very close to their source and most of the deeper blues are from Australia and have not been heat treated Michael i am not an expert in this field but as an aussie gemmo I can say that we here believe that most of our sapp''s get heat treated to lighten the colour and remove secondary green.. When cutting with the coarse to medium diamond grits there is no perceptible difference in the way that they cut, or the size of the ''pitting'' that occurs. I would think that if heat treating had an affect on the strength of sapphire that it would show up during the cutting operation. This is kind of relative, however, since there''s just no way to measure the cutting forces used, at least with my meager technology.

I think that the easiest way to get a feel for this would be to throw a buch of treated and untreated faceted stones into a rotating tumbler and give them a spin. If the untreated stones had less facet junction chipping, then it would be a very good indication that they were stronger. this is exactly the type of test that i would execpect GIA to have considered in their "foremost" brief. We should all put a bit of peer pressure on them. Perhaps we could talk some of the ladies here into giving up their stones for a test ?

 

[Garry H (Cut Nut)]

Date: 1/7/2006 11:34:08 AM
Author: Richard M.
Garry,

I don't know of any formal study done on the topic and I'd like to see one. I hope Richard Hughes and Vincent Pardieu check in on this. I hear frequent speculations from amateur faceters that heat treatment somehow degrades the structure of corundum. I doubt they're based on any truly scientific observations.

Ted Themelis is actively involved in corundum-heating research in Bangkok. In his book 'The Heat Treatment of Ruby and Sapphire' he comments:

'Beauty, durability and rarity are the most important attributes that determine the value of rubies and sapphires. Heated rubies and sapphires may be very beautiful. Their durability and stability under normal wear is undisputed and confirmed to be identical with their [sic] natural (non-heated) corundums.' He may be in a better position than many to make such a statement since he works with such stones daily.

Richard M.

Thanks for the feed back Richard. I am sure Ted is very knowledgable - but I always prefer to see a biblography in a serious text on a serious topic where such claims are backed by empirical research and data.
Would anyone who knows him please ask how he feels that statement is supported.

BTW - Argyle diamonds are harder on the wheel, yet easier to break. so wheel hardness may not be an indicator of durability?

 

[Richard M.]

Date: 1/7/2006 9:18:49 PM
Author: Garry H (Cut Nut)
I am sure ted is very knowledgable - but I always prefer to see a biblography in a serious text on a serious topic where such claims are backed by empirical research and data. Would anyone who knows him please ask how he feels that statement is supported.

Why not cut out the middle-man? His email is [email protected]. I''ve corresponded with him at some length in the past and have always found him very helpful and responsive.

Richard M.

 

[mogok]

Hello,

Heat treatment is performed to improve color, clarity but also durability in some cases.
I explain: One of the most serious thread to the durability of a gem is fissures. Some gem materials a re too fissures to be able to be faceted properly: they chip easily.
When a ruby is heated using some borax of flux agent, a healing process will occur and the fissure will be closed by dissolution and then re-crystalisation of the corundum. Technically, you have then in the fissure some trapped glassy substance (remaining of flux) and some synthetic corundum. Such fissure is not dangerous anymore for the durability of the stone and as a result the stone durability has greatly improved.
Richard Hughes has written an excellent study of this aspect. it is available on his website:

The fracture healing of ruby, by Richard W. Hughes

This is one of the reason that explain why such heat treatment with flux additives are performed before cutting in most of the cases with the ruby material from Mong hsu in Burma.
The same process happen with the lead glass treatment but in this case as the fissures are filled with glass only and as there is no re-crystalisation (the temperatures involved for this treatment are usually under 1000 degres) the durability is improve a little at least enough to limitiate the chipping of these stones during the cutting process.

Lead glass ruby treatment

Now regarding to the heat treatment of rubies and sapphire without fissures and without inclusions, most of the chanthaburi burners told me that after heat treatment the stones are usually a little bit more brittle. I guess they did some tests and I''m Ok to believe them as these people have a real experience on these matters... Then I also dont have the money to begin to make some destructive tests on gems... But nevertheless such a heated sapphire will still have a durability which will be probably much higher than most of the other gemstones. But Shane Mc Clure and James Shigley are righ, proper studies should be performed in order to bring some definitive answer.

All the best,

 

[Garry H (Cut Nut)]

Thanks Vincent.
I emaild Ted Richard

 

[Edward Bristol]

I feel the (newest) rumor that treated stones are less durable comes from endbuyers who find that treated stones are more often chipped or scratched than natural ones.

The reason for this seems not to be the treatment but the bad handeling of treated stones. Most (smaller) treated stones are transported rather careless and tend to come in bigger parcels. Thus they arrive more often chipped or hurt at the end of the supply chain.
On the other side, a trader who appreciates a natural stone is more likely to invest into single boxes or paper handeling (even if the effort is much higher). It is the $/carat ratio, not the treatment that leads to defects and flaws while cutting and transport.

Edward Bristol
www.wildfishgems.com

 

[mogok]

Hi Edward,

My info are not coming from endbuyers but from a Thai ruby and sapphire burner from Chanthaburi with more than 30 years experience in ruby ans sapphire heat treatment. I''m currently collaborating with him and some other leading Thai burners about an article about ruby and sapphire heat treatment in Thailand.
As you know I''m very fond of history besides sicentific gemology and I''m currently meeting many burners in Thailand in order to issue an article for the "Revue Francaise de Gemmologie AFG" about this subject as in fact most of the people in France have very few information about what is really heat treatment and about the Thai Burner in general.

All the best,

 

[Edward Bristol]

Vincent,

sorry, I didn''t fully understand the last part of your text or did not read properly.

You are saying the burners in Thailand do confirm that heating makes sapphires more brittle !!!???

I am so surprised, because the traders/burners in Sri Lanka strictly repulse this idea. I had a few stones burned for experience sake last year and had a similar feeling, but was laughed at when I mentioned it.

As I repeatingly read from people (endbuyers) saying treated stones are chiped more often, I thought it is only because of transport and handeling.

Edward Bristol
www.wildfishgems.com

 

[mogok]

Hi Edward,

Yes, a very experienced Thai burner told me that, but it does not means that the stone become brittle... They are still rubies and sapphires which are one of the toughest material available on earth.
It is just that they are as a matter of fact a little bit more succeptible to suffer from abbration especially when they are put together in the same plastic bag and scratch each other.
On the other hand, in the case of heat treatment with flux as the fissures are closed, the global durability of the stone greatly improve as these fissures are not anymore a danger for the stone. So as a result, heat treatment with additives like flux is performed to improve, color, clarity and durability of the rough material. On the other hand for material without fissures, heat treatment make the stones a little bit less tough and make them a little bit more subject to abbrasion.

As a matter of fact when you check sapphire and ruby parcels in Chanthaburi most of the time you will see that all the culets are broken as the stones are put together is plastic bags. Most of them show then a small "broken culet window".
My impression after having look at heated sapphire parcels in Chanthaburi and unheated parcels in Burma is that heated stones are more likely to get abbraded than unheated ones. I so agree on this issue with the Thai burner who told me that, but just understand that it does not mean that they become brittle...

I still think that a heated sapphire is much more durable than a diamond as an example because sapphire heated or not has no cliveage and is much more resistant to blows and heat compared to a diamond.

All the best,

 

[Garry H (Cut Nut)]

Date: 1/9/2006 5:35:26 AM
Author: Vincent Pardieu

I still think that a heated sapphire is much more durable than a diamond as an example because sapphire heated or not has no cliveage and is much more resistant to blows and heat compared to a diamond.

All the best,

Dear Vincent,

On this point I am afraid I must disagree.
I have been behind the counter booking in retips and repairs to fine jewellery for +30 years. We reploish the crown facet abrasions from every ruby and sapphire engagement ring when the claws / prongs are retiped and settings rebuilt (usually between 10 and 30 years every day wear). It is a standard practice because they all need it.

Conversly about 1 in 50 diamonds show some chipping of the girdle and these are almost always very thin girdled stones, often with very shallow crown angles, set in only 4 pronged settings.

When ever I have seen scratches or abrasion on the crown of diamonds (maybe 10 times in my life) I can identify a very high diamond ring with exposed girdle that is worn on the finger beside- it that has been rubbing on the table of the other ring.

(In Australia rings are much lower than in USA - this phenonama would be more common in USA)

 

[WinkHPD]

At the Tucson Gem Show some years ago I attended a speach by Cap Beasly who was quite emphatic that heat treating does make the sapphires more brittle. I know that he has done a lot of research into being able to identify the origin of a sapphire or ruby by its inclusions, but do not know what research he might have done into proving or disproving his statement.

Wink

 

[Richard M.]

Date: 1/10/2006 6:44:49 AM
Author: Garry H (Cut Nut)
We reploish the crown facet abrasions from every ruby and sapphire engagement ring when the claws / prongs are retiped and settings rebuilt (usually between 10 and 30 years every day wear). It is a standard practice because they all need it.

Conversly about 1 in 50 diamonds show some chipping of the girdle and these are almost always very thin girdled stones, often with very shallow crown angles, set in only 4 pronged settings.

I think the difference in hardness corundum vs. diamond would explain the corundum abrasion, especially if the rings are worn daily. Diamonds are very difficult to scratch but thin girdles, the pointed corners on Princess cuts, etc. are areas of extreme weakness subject to easy breakage. Diamond''s perfect cleavage has caused most of the problems I''ve seen (combined with owners who refuse to remove E-rings when digging with their hands in rocky soil, etc.)

Are tension sets popular in Oz? If so I predict you''ll probably be seeing more damaged diamonds than in the past. Every time I look at my daughter''s Steven Kretchmer E-ring setting I shudder a little, thinking of all that exposed girdle. The story Eric Bruton tells of the experienced diamond cleaver who claimed he could split a diamond with only his thumbnail comes to mind. But I''ve hammered jewelry care into her head over the years.

As for corundum, I have questions about brittleness I''d love to see settled by a well-designed experiment. But I wonder if a truly accurate species-wide test could be designed due to so many variations in "burning" techniques: temperatures, introduced gases, cycle durations, etc. Probably the most important consideration is the chemical nature of the corundum being "burned." There are huge variations from source to source.

For instance Themelis publishes a graph in his book that compares treatments on several colors of Umba sapphires. Temperatures ranged from 1000C to 1850C in air, oxidizing and reducing atmospheres. Each of the 33 combinations produced different results. Now that''s corundum of a given type from just one location! And consider that each color from that location will have differing chemistry and properties. Yikes! Maybe that''s why the experiment we want hasn''t been done.

 

[mogok]

Hi Gary,
Well no problem: diamond is much harder than corundum, it is clear that you will see less scratches on a diamond after some normal weir. Now durability is something else far beyond just hardness:

Put a 0.5ct diamond between 2 coins, take a hammer and blow the coins... see what happen.
Take a 0,6ct ruby and do the same... See the difference.
Take your diamond ring, and a similar ruby nor sapphire ring and drop them into a camp fire, wait all night. Then in the morning see the difference.
Globally if diamond is harder, it is also more britle, and somewhere if your house is burning you will still have your ruby ring but your diamond will be gone.

I''m thinking about this aspect as I was recently scouting Chanthaburi to meet old Thai burners to get their story about how they discovered heat treatment. I was reported among others that one major event was the fact that in Chanthaburi, the central market was destroyed by a fire that lasted 3 days in 1968. After the disaster the owners of the jewelry shop in the market went back to see what they could gather from their shop. Looking at their remaining jewelry several people got the evidence that fire could change the coloration of rubies and sapphires. Diamonds were gone, rubies and sapphire were even better than before... For the small story then in 1969 Samuang Kaewen has discovered that milky light colored bluish or yellowish translucent sapphires could be turned into beautiful blue transparent stones... And guess what, the first stone he told me he had succes in was Australian. he bought 3 stones for 300 bath and one week later sold the best one for 45,000 baht. Then his Australian sapphire supplier refused to sell him anymore gems and he turned to pailin and Sri lanka where such stones became famous as "Geuda". This person was the first Thai to discover by himself heat treatment using reducing condition, but some chanthaburi people think that some other people knew about it before in geneva and were operating more secretly than them. In chanthaburi the secret spread in the city and soon you had many burners. The guys who became rich were not the guys discovering the way, it was the guys that had huge stocks and that were able with Samuang discovery to make some huge profit...
The company that had the monopoly in Australian sapphire and took a great advantage of Samuang discovery was named I was told King Star or something like that.

An important ruby burner was using this event to tell me that ruby heated with borax as it has no crack anymore and as its color is now definitive is much more durable than an unheated stone that might change color with fire or get destroyed as it has a fissure which can be a deadly thread for the stone.

Lets take 2000 ruby and 2000 diamond rings. How ruby rings will still be after lets say 200 years or 2000 years?
If all the stones have the same quality, meaning, no inclusions and no fissures. Then in my opinion there will be more rubies than diamonds, and than also probably the unheated rubies will be less abbraded than the heated ones. But possibly the surviving diamonds will have less scratches as they are harder...

To my knowledge the only stone more durable than ruby or sapphire is probably nephrite jade, but jade does not like too much temperature as it is an aggregate. But Richard M. is globally right, to clarify all that some serious test should be done...

Does somebody has some 1 carat diamond, and a 1 carat ruby with excellent clarity to blow, burn, drop, through to the walls, and so on? I would be interested to make such tests if i was provided the gems... LOL

All the best,

 

[Edward Bristol]

Vincent,

again your story surprises me. A fire in Chanthaburi in ''68??
Haven''t we always been told that heat treatment is as old as the gemtrade?

Edward Bristol
www.wildfishgems.com

p.s. I doubt we will find anybody owning enough treated sapphire to donate it for testing and thus (again) devaluate the rest of his treated stock, if heated stones in fact turn out to be statistically less durable.

 

[mogok]

Date: 1/10/2006 10:57:23 PM
Author: Edward Bristol
Vincent,
again your story surprises me. A fire in Chanthaburi in ''68??
Haven''t we always been told that heat treatment is as old as the gemtrade?

Edward Bristol

Hi Edward,

Heat treatment under oxidising condition is far much older, I think as old as gem mining... In XII centuries some arab writters are reporting the way Sri Lanka were performing blow pipe style heat treatment in order to remove the purplish overtone of some rubies and to make some dark blue sapphires a little bit less dark.
But Thailand did not had such long traditions in gem mining so blow pipe was never common here.
The thing is that up to their invention of the blue sapphire heat treatment under reducing conditions nobody was able to do anything with the milky silky light colored blue or yellow translucent sapphires that some people report to have been used as gravels to make the roads in Sri Lanka... These stones that are currently called "geuda" and are now selling at very high prices especially in Newitigala (sri lanka), Ilakaka (Madagascar), Tunduru (Tanzania) as I could see last summer visiting these areas.

All the best,