The origins of didymium

[Mike_Aurelius]

Yes and No.

There are other elements in the old "didymium" and the new(er) ACE glass.

Didymium started out as a filter for the ladies who were sealing vacuum tubes back in the 20's and 30's as the 'radio era' exploded and all sorts and varieties of tubes were needed. It was through the name "glass blowers" filter that all sorts of deviations of the correct use of the filter came to be, as a misnomer some people still call it that and use it for traditional glassblowing (which it should not be used for btw).

I believe AO (American Optical) was the originator of the glass type, and it was first made in their facility in Southbridge MA.

Fast forward to the 1960's, Schott took over the manufacture of the product as AO phased out its glass making production lines and moved towards making frames exclusively.

Schott refined the material substantially from AO's formulation, and did large scale strip melts of the glass through the 1980's. A typical strip was about 7" across and 9-10 mm thick and melted in a continuous strip, cut to length during the annealing process.

The glass itself was melted in a continuous batch furnace and started life as clear crown glass, with an index of refraction of 1.523. Traces of arsenic, potassium, are in the glass to stabilize the index, while neo- and praeso- dymium are added (the exact mixture remains a protected "secret"). The strip was repressed into a variety of shapes by United Lens Company of Southbridge MA (across the street from the AO facility).

In the 1980's Schott developed an amethyst (blue) shaded glass filter for the aviation industry because of the increase in video screens in the cockpits. This filter was required to have several additional filter notches along with the classic sodium flare. Schott approached Aura in early 1991 to see if we were interested in the glass for ophthalmic applications, and did a short run test melt in several ophthalmic lens shapes. The rest, as they say is history.

The original ACE glass was a lanthinum based crown, with an index of refraction of 1.572. There are problems with lanthinum based glass lenses, most notably staining during processing. Around 2001, Schott reformulated the glass and moved it back to a clear crown base, but kept the index stabilized at 1.572. There is barium, potassium, and a few other "rare earths" in the glass mixture along with the neo- and praeso- dymium materials. Again, the exact composition is a trade secret.

Hoya makes a similar glass type, but it is available only in finished polished flats, 2 mm thick. Corning France used to make an "enhanced" version of the glass, but stopped production when the price of the raw materials (due to Chinese embargoes of the rare earths became too expensive).

Schott moved the production of all of their glass to Germany in the mid- 2000's, and then sold the production facility to an Italian company. The facility remains in Germany, with Schott overseeing the production, but all sales of all products manufactured in the facility are made through the Italian company.

The last batch of ACE glass was manufactured approximately 2007, and as far as I am aware, there are no current plans in place for the next melt. I am told that there is enough ACE stock to last for another couple of years at the current rate of usage. Typically, melt requirements for ACE were at least 5,000 pounds of glass minimum.

That's probably far more history than you wanted to know, and probably not enough chemical detail, but that's what happens when trade secrets are involved.

As far as making your own glass is concerned, the biggest problem will be keeping striae, inclusions and air bubbles out of the glass. Ophthalmic quality glass has a bubble spec of 1 bubble smaller than .1 mm per cubic inch of glass, with no inclusions or striae. Additionally, the glass has to be fine annealed so that it can be reprocessed into usable shapes.

This is a good topic for discussion, it lets folks see how things started and how the product has evolved to meet our current needs.

[smolder holder]

Thanks for the info and history lesson mike. I love learning more about the origins and materials we use. I'm interested in finding out how much UV boro emits while at working temps and what in the mix of glass is used to filter that as well as the sodium flare. Gonna look up what the wavelength of light is that sodium produces when combusted. I find it fascinating that we can produce a lens that is transparent to most wavelengths but blocks certain ones. Also, besides eye strain is there a way to tell how much UV protection your glasses are providing you while working?

[Jimi The Don]

Fascinating

[glassdocnc]

Thanks for the info Mike. Repped!

[Mike_Aurelius]

I've pulled the MSDS on the didymium glass types from Schott, here are the percentages as listed. Note that the OSHA does not require exact percentages, allowing for trade secrets, ranges are permitted.

S-8304 (medium green) didymium

Silica > 51%
Sodium Oxide 11 - 20%
Zinc Oxide 1 - 10%
Calcium Oxide 1 - 10%
Aluminum Oxide 1 - 10%
Arsenic Trioxide < 1%
Neodymium Oxide 1 - 10%

S-8305 (dark green) didymium

Silica > 51%
Sodium Oxide 11 - 20%
Zinc Oxide 1 - 10%
Aluminum Oxide 1 - 10%
Arsenic Trioxide < 1%
Neodymium Oxide 1 - 10%

S-8801 "rose" didymium

Silica > 51%
Sodium Oxide 11 -20%
Potassium Oxide 1 - 10%
Zinc Oxide < 1%
Aluminum Oxide 1 - 10%
Neodymium Oxide 1 - 10%

S-8807 A.C.E. (originial formulation)

Silica > 51%
Boron Oxide 1 - 10%
Sodium Oxide 11 - 20%
Zinc Oxide 1 - 10%
Neodymium Oxide 11 - 10%
Cerium Oxide < 1%
Copper Oxide < 1%

For comparison, here are the formulations for clear crown ophthalmic glass:

S-1 crown

Silica > 51%
Sodium Oxide 1 - 10%
Potassium Oxide 1 - 10%
Zinc Oxide 1 - 10%
Calcium Oxide 1 - 10%
Aluminum Oxide 1 - 10%
Titanium Oxide < 1%
Arsenic Trioxide < 1%
Antinomy Trioxide < 1%

S-3 crown (S-3 crown was developed to optimize the glass for chemical bath strengthening)

Silica > 51%
Boron Oxide 1 - 10%
Sodium Oxide 1 - 10%
Potassium Oxide 1 - 10%
Zinc Oxide 11 - 20%
Aluminum Oxide 1 - 10%
Titanium Oxide < 1%
Arsenic Trioxide < 1%
Antinomy Trioxide < 1%

[Mike_Aurelius]

Thanks for the info and history lesson mike. I love learning more about the origins and materials we use. I'm interested in finding out how much UV boro emits while at working temps and what in the mix of glass is used to filter that as well as the sodium flare. Gonna look up what the wavelength of light is that sodium produces when combusted. I find it fascinating that we can produce a lens that is transparent to most wavelengths but blocks certain ones. Also, besides eye strain is there a way to tell how much UV protection your glasses are providing you while working?

Actual UV generation from heat alone does not start until temperatures reach approximately 4500 degrees F. However, some metals and some minerals do generate UV, but the threshold has always been below that where serious protection is needed.

All of the eyewear sold that is made from glass protects the eye from the hazardous UV, which is generally accepted to be 320 nanometers and below. The range of 320 nm to 400 nm is still UV, but the amount of damage done there is far less than the lower range. Basic ACE glass stops filtering at about 350 nm. But, keep in mind that this is a soft glass filter, and at soft glass temperatures, the amount of UV generated has been shown to be negligible.

A borosilicate filter, such as our AGW-203, a shade 3 equivalent, starts passing UV at approximately 380 nm and at 400 nm passes about a 0.5%. Higher shade numbers pass about the same amount of UV.

The point here, that needs to be made, is that UV **IS NOT** and should not be your main concern. IR, infrared, heat energy is and always should be your concern.

Plain ACE glass alone passes approximately 45% of IR between 750 nm and 900 nm and nearly 85% of IR out beyond 900 nm, which is the hazardous portion of the IR spectrum. This is the main reason we have never recommended plain ACE glass for anyone working with borosilicate glass.

A borosilicate filter, such as our AGW-203, a shade 3 equivalent, passes an average of 2% of IR between 750 nm and 900 nm, and less than 0.25% beyond 900 nm. This is the minimum we recommend for working with color borosilicate glass.

In my opinion, the reason the UV is a concerning issue from the OD/MD standpoint is that there is documented proof that outdoors sun exposure does over time damage your eyes. However, you are not outside (usually) when working glass, so it really isn't the issue that it is made out to be in these circumstances.

IR damage is not really taught in OD/MD school, nor is specialized filtration requirements, so many times doctors and optical dispensers don't have the knowledge base to work off of for proper selection of lenses. This is how "didymium" came to be called the 'glass blowers' filter, and even after 30+ years of us trying to educate the optical profession, we still get dispensers and doctors trying to provide didymium lenses to people who do traditional glass blowing. "Well, it's called a glass blowers filter..." *sigh*

Any way, I've written several articles on my blog on the hazards to the eye from working with hot glass, both at the furnace/glory hole, and at the torch, so I'm not going to rewrite them, but I will post links here so that you can read them at your leisure. Feel free to print them out and share as long as you maintain the copyright notices from the blog.

(I will note that there are the occasional rants in there, just keep in mind a lot of this was written back in the day when some folks weren't quite as accepting of our data lol)

http://mikeaurelius.wordpress.com/20...e-glassworker/
http://mikeaurelius.wordpress.com/20...ilter-eyewear/
http://mikeaurelius.wordpress.com/20...pair-you-have/
http://mikeaurelius.wordpress.com/20...t-uvireyewear/
http://mikeaurelius.wordpress.com/20...o-communicate/
http://mikeaurelius.wordpress.com/20...-glassworkers/
http://mikeaurelius.wordpress.com/20...rom-the-field/

[Mike_Aurelius]

BTW, sodium generates a flare between 570 nm and 600 nm.

[kbinkster]

Great information, Mike. I'm glad to see you posting it.

[Mike_Aurelius]

I briefly touched on the internal specifications for bubbles and striae above in my first post, I've dug a bit deeper and found this on the Schott website:

http://www.schott.com/advanced_optic...r-2011-eng.pdf

Index of refraction variances: +/- .00002 % within a batch
Striae: Non visible (VS1)
Bubble/Inclusion class:
B0 EVB as follows: maximum cross section in square mm in 100 cubic millimeters: 0.006 mm
maximum allowable quantity per 100 cubic millimeters: 2
Stress birefringement (measurement of annealing) <10 nm per centimeter

The whole document makes for interesting reading if you are into technical stuff.

[Mike_Aurelius]

Data sheet for ACE: http://www.us.schott.com/advanced_op...8807_09_us.pdf (note this is for the original ACE glass, not the revised S-8807A)

[PyroChixRock]

Awesome. Repped.

[Pogo]

This is a great read!

[smolder holder]

Wish I could rep you again Mike. Thank you for sharing.

[Jason Lindquist]

I love hard data. Thank you, Mike, for your contributions. Your knowledge of optics and how to protect our eyeballs is unparalleled on this forum. Your ventilation primer is also unmatched. Kudos.

[PyroChixRock]

(I will note that there are the occasional rants in there, just keep in mind a lot of this was written back in the day when some folks weren't quite as accepting of our data lol)

For anyone speculating about the legitimacy of Mike's data, I have some inside info.

Years ago when Phillips started coming out with "boro" shades Mike warned us that the coverage was not up to par. a bunch of TMP members decided it would be a good idea to get them tested and see if what Mike was saying was true. We rounded up aura lens and Phillips and sent them to one of our members (leaving his name out unless he wants to jump in) who worked in an optical lab at the time.

The result charts showed mikes shades performed at the levels he was claiming or better. Charts also showed Phillips did not.

Some distributors pulled Phillips from the shelves.

Since it seems Phillips has reworked their design but as far as I know they have never been independently tested as we had done back then. Personally I won't wear them because my eyes always feel tired after I do, that's enough difference for me. Plus the yellow isn't fully filtered on any pair I've worn.

Anyway long story short, Mike's data is legit.

[Greymatter Glass]

Wow, good reading mike, thanks!

So are there any (that you know of) contract glass melting facilities that are able to melt smaller amounts of optical filter glass for our industry should the need arise? If Schott should ever become unwilling or unable to provide the materials needed have they ever been known to license their formulas to such contract melters?

Are there any comparable filter materials available to you should such a need arise?

Also, what is the current status of the green welding filter glass shortage, has that been resolved?

thanks, and I will rep again when I can!

-Doug

[Mike_Aurelius]

Just a quick comment, the welding filter material shortage has been resolved, we should have several thousand pieces here in about 2 weeks. I'll talk more on Monday.

Thanks all for the great comments!!!

[Mike_Aurelius]

Wow, good reading mike, thanks!

So are there any (that you know of) contract glass melting facilities that are able to melt smaller amounts of optical filter glass for our industry should the need arise? If Schott should ever become unwilling or unable to provide the materials needed have they ever been known to license their formulas to such contract melters?

Are there any comparable filter materials available to you should such a need arise?

Also, what is the current status of the green welding filter glass shortage, has that been resolved?

thanks, and I will rep again when I can!

-Doug

The very first run of ACE glass was done as a pot melt, and I'm fairly sure it could be done that way again. There are two drawbacks to pot melts: one, it tends to be a bit seedier than a continuous melter, and two, produces only strip, which would then have to be repressed into ophthalmic lens shapes. Pot melts are typically 500 to 1000 pounds, and the price per pound will be higher.

World wide, the number of glass manufacturers capable of manufacturing ophthalmic glass has dropped substantially since the 1980's. Schott in Germany, Hoya, Ohara, Asahi, and Isuzu in Japan, and Corning in France/Brazil are the only remaining companies that I'm currently familiar with. There are a number of manufacturers in China, however, chinese optical glass is still an evolving product, and personally, I wouldn't consider it fit for use as a spectacle lens. Maybe in about 10 years or so. Hoya is the only manufacturer with their own "ACE" product, however, they have shown no interest in making it in any other form apart from finished flat plate for specialized video monitor filters.

I work very closely with the current supplier of raw ACE glass, and like I said before, there is no indication of any shortage of ACE for at least the next couple of years, insofar as non-prescription lenses are concerned. There is a shortage of a couple of prescription thickness lens blanks, but we are using acceptable workarounds and it hasn't been much of a problem at all.