[TheForge] FW: Didymium? I say "no way."

[Larry Zoeller]

Original Message ----- 
From: Mike Porter 
To: zman59 at earthlink.net
Sent: 2/8/2006 3:43:52 PM 
Subject: Didymium? I say "no way."


Larry,
Would you please post the message bellow. I can receive mail from the
group, but can't send for some reason. This email, which took quite a while
to write, was returned to me with the following message:

The following addresses had delivery problems:

<theforge at mailman.qth.net>
Permanent Failure: Other address status
Delivery last attempted at Wed,  8 Feb 2006 02:32:39 -0000


I've done heavy research into vision enhancement for hot work over the last
year for one of the key chapters in my next book. Here are the main pieces
of information, you need to know. Didymium is added to optical glass in
order to block the yellow sodium line (589 nanometers). This is only of use
to some hot glass workers, who are running oxygen rich torch flames on soda
based glass. Soda on the glass surface flares up as it combines with the
free oxygen, creating an uncomfortable and obscuring glare. Some brazing
work, involving sodium/potassium based fluxes also must be shielded from so
called "sodium flare," which in that case is more complicated (glare from
about 580nm through 620nm mainly, and rapidly tapering off above these
light bands). Most glass, along with polycarbonate plastic, stops
ultraviolet light (UV) in the UVA and UVB bands, which is good because
didymium can't.

Unless you're running an oxy-hydrogen flame, an electric arc, or playing in
the dessert sun, ultraviolet isn't a problem. Infrared is a problem anytime
you are creating much heat. The heat from a coal or gas forge certainly
qualifies as "much heat." Didymium does not stop infrared (IR). There are
ways to reduce or totally block IR. Number one is with iron oxide, found in
ANSI rated green filter shades and heat-absorbing glass. However, seeing
worth spit and stopping IR with ANSI rated filters don't go very well
together.

There are dichroic UV/IR Hot Mirrors that do a great job of stopping UV and
also stop IR up to 1100 nanometers. That's only about one-half the IR band
of ocular concern (780nm to 2000nm by federal code), but it is also the
heavy duty half. They can be combined with heat-absorbing glass--not the
window variety;that's pretty weak. Lens grade heat-absorbing glass of
various kinds (Hoya #30 or #50, as well as Shott KG-3 and KG-5 are very
nice, but plenty expensive), or you can compromise and buy photocopier
heat-absorbing lenses on eBay (supposedly Shott KG glass, but grade
unknown). There are a few on-line photography suppliers who sell new and
used heat-absorbing glass for photocopiers at about one-third what you'll
pay on eBay. You'll have to cut the glass to size afterward. Or, you can
combine a Rosco #38000 dichroic hot-mirror with an ANSI shade #1 green
filter, and do very well without the hassle. However, all the
aforementioned solutions are really set up to work with torch goggles.

I've got a feeling that you're looking for "the solution to all your
troubles" in one convenient place, right? The 'ultimate' solution is a set
of Auralens AGW-300 spectacles. They are the very latest thing: 

http://www.auralens.net/e_gw_agw300_announce.cfm

If your wallet isn't up to the hit (or you're worried about being seen
sobbing in public after they get broken), then you should consider buying a
flip up burner goggle, and using it with the filters mentioned above until
after you become rich and infamous :-) On the bright side, once you have
the basic filtering elements in place, you can start playing with various
colored filter gels, and do really cool hot work viewing tricks in red,
green, & blue. Larry Zoeller has been sweet talked into merchandizing
everything but the heat-absorbing glass. As Shott KG-5 filters in 2" x 4
1/4" rectangles are a minimum of $98.30 (with no less than twenty filters
per order), I just can't see him stocking them during the first half of
this century. Eventually, lower priced dichroic filters will force the cost
of KG-5 glass down from the stratosphere--but not anytime soon. 

Michael Porter (AKA Mikey, AKA Dr. Frankenburner)