A continuation of the discussion elsewhere.
Some basic information...
There are always good reasons for filters in our daily lives. Grey tints and polarized filters to cut back the sunlight, UV filtered automobile windows to lessen the amount of damage to fabric and vinyl.
In the crafting of art glass, the most well-known is 'didymium'. This has become a generic term over the years for a sodium flare filter that removes the yellow ball of light that occurs when any type of glass that contains sodium (almost all of them) is heated.
The above picture is a bench-mounted art glass torch, burning oxygen-propane at approximately 3800 F. The glass being worked is borosilicate art glass (pyrex). The picture on the left is unfiltered, the picture on the right is filtered through a 3rd generation didymium-variant filter. As you can see, the yellow of the flame is totally removed, leaving only the blue-white flame plume. This allows the glass artist to see the work he's doing inside of the flame, which would otherwise be obscured.
Here's the visible light spectrum of the camera filter being used:
This filter, we've named AUR-40, or simply Dash 40, is a 3rd generation didymium-variant from Schott. Based off the previously available S-8817a ACE glass, this particular filter has a much deeper sodium flare notch, as well as a secondary green/yellow notch. These filters were originally designed for avionics to remove stray distracting colors in the cockpit of commercial and military aircraft.
Here's a color comparison of the previous AUR-92/ACE filter and the new Dash 40 filter:
The lens on the left is the Dash 40, the lens on the right is AUR-92/ACE.
Dash 40 and AUR-92/ACE are good filters for working soft glass (soda/lime). However, when the artist is working borosilicate glass, the amount of heat energy required, as well as the overall size differences in the pieces being worked requires different filtration requirements. To meet these needs, I designed several different filters combining both generic welding filters as well as specialized IR filters.
--more to follow
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