Confused and looking for an answer. Heating = more or less prone to damage?

[jvLin]

I''m a bit confused regarding the subject of sapphire heating. If I remember correctly, heating removes a few inclusions from the gem. Fewer inclusions means the stone is less likely to chip, correct? I''ve also heard that heating increases likelihood of chipping, although this was from an unreliable source. Perhaps heating removes the smaller inclusions, but makes the larger ones larger?

Can anyone clarify this for me? Please list your sources whenever possible. Thanks!

 

[oldmancoyote]

"Gentle" heating (a few hundred degrees C) dissolves small rutile crystals and will enhance clarity and possibly colour - however more aggressive heating risks damaging the crystalline structure and creating fragile zones, especially if it is followed by relatively rapid (less than a few years...) cooling

 

[jvLin]

Then then does gentle heating increase the durability, or are there only aesthetic benefits?

 

[oldmancoyote]

Purely aestethic - as far as I know. Rutile that can be dissolved by gentle heating is not the type of inclusion that would be detrimental to durability anyway. In fact, if there''s enough rutile and it''s well-oriented, it becomes valued in its own right since it generates asterism. Star sapphires and rubies are as durable as the transparent varieties (or more, since they aren''t faceted)

 

[Edward Bristol]

This is discussed often and mostly ends with "There is no scientific data".
(I hope you don’t refer to that as “unreliable”)

However, many agree:

1. A heated sapphire is more brittle than an unheated one
2. The more heat, the greater the potential damage
3. The harsher the cool-down the greater the potential damage
4. The more heating runs, the greater the potential damage

Sure, somebody will now screech: “You don’t know that! We have no data on it! You just say that because of your business interests”. Yes, yes, …

I am not saying that a carefully heated sapphire is not durable, not at all. It will be just fine. No problem.
However, if you have ever overcooked a sapphire you will see that it gets so brittle that you can’t use it for jewelry at all. Talk to cutters or pro-burners in private and you will get the same opinion.

It just is a taboo topic raised only by people who have absolutely nothing to do with gem treatment. Even the Natural Sapphire Company has heated stones in their stock. Nobody wants to devalue his own stock or anger his customers (up and down stream).

Gem-treaters in Thailand (or elsewhere) have been hiding from and lying to the public for decades now, and they still do. Unless somebody scientifically proofs otherwise I will not believe them now.

 

[oldmancoyote]

Edward (and others),

Given the low cost of synthetic corundum and its relatively high quality/simlarity to "wild" gems (pun intended), would you hazard a guess as to why no-one has ever done a "proper" study on the effect of heating?

I''m not trying to be provocative - just curious.

 

[Edward Bristol]

Because of the result?

 

[Harriet]

Wouldn''t that align with the interests of those who purvey unheated stones, then? (No swipe intended at you, Ed.)

 

[Edward Bristol]

Yes, exactly.

If some of the big fish would like to proof that heat does not cause brittleness (which even Ted Themalis says everybody knows it does) they could make a study on synthetics and finished. They got the lobby and the cash. But they don''t do it. Or they did but don''t publish the results. As always the topic is shrouded in mystery.

There is enough money for the development of new treatments every month but none for consumer concerns.

(Though I also don''t believe any health studies paid for by the pharma industry. Do you? Does anybody? I think that clever we have become already.)

 

[jvLin]

But if money is where the unheated stones are, wouldn''t it make sense for dealers of unheated stones to conduct studies as proof?

As the value of a gem is somewhat relevant to its hardness/durability, this should be a controversial topic of paramount importance. Even a falsified experiment(much like the ones cigarette companies endorse) would receive immense media coverage. But it appears that there has been no such experiment conducted.

Edward, I understand that your livelihood widely rests upon this notion that unheated gems are superior to heated in every aspect. But as a scientist, I''ve learned never to justify arguments with intuition, which is what your website seems to do. Do you have any sources I could see please? Thanks.

 

[oldmancoyote]

Date: 1/9/2009 4:39:34 PM
Author: jvLin
But if money is where the unheated stones are, wouldn''t it make sense for dealers of unheated stones to conduct studies as proof?

As the value of a gem is somewhat relevant to its hardness/durability, this should be a controversial topic of paramount importance. Even a falsified experiment(much like the ones cigarette companies endorse) would receive immense media coverage. But it appears that there has been no such experiment conducted.

Edward, I understand that your livelihood widely rests upon this notion that unheated gems are superior to heated in every aspect. But as a scientist, I''ve learned never to justify arguments with intuition, which is what your website seems to do. Do you have any sources I could see please? Thanks.

Ah, but that is rather the problem. Money is where the heated stones are - largely because of volumes. Having said that, your question is my question - given the cost of such a study would be fairly limited (synthetic corundum is cheap; energy and time requirements relatively limited; equipment required is simple and easily available), why is it not done?

From everything I know of materials science, Edward''s four theses make sense from a theoretical point of view, and there are plenty of analogue cases where they are certainly true - but there is no experimental proof for corundum (at least, none I could find).

 

[Richard M.]

Date: 1/9/2009 5:08:34 PM
Author: oldmancoyote

From everything I know of materials science, Edward''s four theses make sense from a theoretical point of view, and there are plenty of analogue cases where they are certainly true - but there is no experimental proof for corundum (at least, none I could find).

Here''s some.

“Heated rubies and sapphires may be very beautiful. Their durability and stability under normal wear is indisputed and confirmed to be identical with natural (non-heated) corundums,” gem treatment expert Ted Themelis wrote in the first edition of his book, “The Heat Treatment of Ruby and Sapphire.”

Edward brought up Themelis’s name on this thread, saying “If some of the big fish would like to proof that heat does not cause brittleness (which even Ted Themalis says everybody knows it does) they could make a study on synthetics and finished,” etc.”

That comment seemed at odds with Themelis’s earlier writings, so I contacted him to ask if his views had changed since his book was written.

He responded: “Truly gem quality and better grades of Sri-Lankan, Madagascar and Mogok corundums are very durable. In his Victorian classic book the British writer Tennent wrote that the kings of Kandy (Sri-Lanka) valued the heated corundums more than their unheated counterparts. Is that an issue of durability? Who knows...”

“Thai gem cutters in Chantaburi are telling me that the heat-treated corundums feel a ''little harder'' during the lapidary process, implying that the heat-treated gems are more durable. In general, the durability of untreated corundums may be increased if they are heat-treated, hence the statement in my book. The durability issue is also discussed in my book "Flux-Enhanced Rubies & Sapphires" http://themelis.com/P-Book-Flux.htm

“The durability of corundums depends on how their atoms are cited in their lattice in conjunction with the nature of their cracks/fissures/inclusions (if present). Porous gem materials (including corundums) are generally less durable, that''s one of the reasons they are treated, to increase their durability.

“The durability of gems is decreased as their porosity is increased. Thus, low-quality, "mica-rubies" and similar types of corundums are not durable and must be treated with additives. Untreated Mong-Hsu [Burma rubies from Mong-Hsu, not the Mogok Tract] are less durable and nearly all are heat-treated [and flux-healed] to increase their durability, diaphaneity, clarity and develop color. [The same is true of most rubies from Madagascar which are filled with lead glass to conceal cracks and fissures].

“Rubies-sapphires may become brittle after heating, it all depends how the fissures/cracks will respond to heat-treatment and how the treatment is performed, especially during the heat-up and cooling stage of the heating process.

“In my view, only untreated gems are qualified to be called "gems" and for many years I have advocated this issue to change the terminology of ‘what is a gem.’”

Ted Themelis

So like many questions involving gems, there''s no single simple answer. High quality heated sapphires are widely accepted in the gem trade because without them there would be very few natural mined gems to sell. There are various degrees/types of heating. As with computers, "garbage in, garbage out" seems to be the basic rule. The addition of beryllium or other elements not natural to the gem decreases prices in the current market.

Note to OMC: Gems can be cracked by too-rapid heating and cooling. Heat must be ramped up very slowly, perhaps 50 or 100 degrees F. per hour, until the desired treatment temperature is reached. Cooling is usually overnight, not years as you commented. The furnace is not opened to outside air temperatures which could thermally shock the stones inside until it has completely cooled.

Richard M.

 

[Edward Bristol]

Good that this discussion leads somewhere rational for a change.

1. I was always referring to brittleness as in “chipping” or "surface sensibility”, not overall “durability”.

2. Sure, low grade corundum will be of higher durability after fillings and heat. Absolutely. Many rubies and emeralds would probably fall into pieces without additional substances.

3. Ted Themalis in a discussion forum in 2006:
What is your opinion about the discussion regarding an increased surface brittleness in heat treated sapphire? Do you agree that treated stones are likely to have a higher surface brittleness?
edward: yes, it always has been the case.”
(I may be misunderstanding that answer but it seems conclusive, so I took it for a “yes”)

4. Unheated gems are a niche with a probably low single-digit market share in the gem market and a minute share of the total jewelry market. It is not where the money is.

5. A serious 3rd party scientific study will still need tens of Ks. I don’t see anybody in this niche to pay for it.

jvLin, sorry, but my livelihood rests on service level and quality, nothing else.
If my website seems not purely scientific to you, yes, that is because it is a commercial website not a scientific one. To run a business one must be allowed to do marketing. (However, I don’t pay doctors to say smoking is healthy or double the sugar content in snacks and then call it “bio”.)

 

[oldmancoyote]

Date: 1/9/2009 10:06:41 PM
Author: Richard M.
[snip]

Note to OMC: Gems can be cracked by too-rapid heating and cooling. Heat must be ramped up very slowly, perhaps 50 or 100 degrees F. per hour, until the desired treatment temperature is reached. Cooling is usually overnight, not years as you commented. The furnace is not opened to outside air temperatures which could thermally shock the stones inside until it has completely cooled.

Richard M.

Richard - thanks for the post; it''s great to have this type of insight. On the cooling period - apologies; I should have put a smilie there.

 

[Richard M.]

Old Man Coyote:

Sorry, I missed your humor.

I thought maybe you had some other treatment process like irradiation in mind.

 

[cofor]

My experience as a (formerly) professional gem cutter, has shown me at many occasions that when it comes to recutting sapphires (or any corundum) the heated stones are notably more brittle and prone to chipping or even scratching in certain planes of their crystallographic structure. It is easier to get a perfect polish on the facets of a non heated corundum. At least that is my experience.

Elaboration: I talk about stones with no to few inclusions, heated to remove silk or improve colour and not the flux healed, crack filled junk coming out from many localities which I have never cut.