I would always use a #14 shade lens for air arcing or technically it's called air carbon arc gouging.
Depending on the carbon rod size, you would use 250 to 350 amps and that would cause a very bright light during the process.
I also used a #14 shade lens for large flux core wire that ran over 250 amps.
Generally, the shades of lenses are matched by the amperes of the welding process. (Or more specifically the brightness of the light created from it)
For example, gas welding with an oxy acetylene torch or cutting steel with a torch you would use a #5 lens.
Arc welding light sheet metal up to say 3/8'' you would use a #10 shade and between 80 to 100 amps.
Most medium welding of say 125 to 175 amps using a 1/8'' rod i always used a #11 shade lens.
As a rule of thumb, you would go up one shade higher for the approximate amperage when you were Tig welding because the purity of the flame.
So, why not use two #5 lenses stacked on each other to arc weld at say 100 amps?
It would look as dark as the #10 shade while welding, but you would only get the U.V. protection of the highest shade lens because of how they are designed.
DON'T DO IT!
I don't doubt anything you say. I just don't know.
Everyone is talking about a total eclipse being the only time it is "safe" to look at the sun with no protection.
They are saying the sun's atmosphere around the dark spot is awesome.
At this point, I'm just more curious than anything else.
I wish someone could provide a simply explanation of what is probably a very technical topic.
I've got dark safety glasses that claim to block 99.99 percent of UV.
(Wiki says the UV is the radiation that causes sunburn).
I always wear those UV glasses to prevent cataracts too.
But I sure would not try and look at the sun with them.
They say even clear lens can block 99.99 UV.
So what you have said about the welding lenses confuses me.
In other words, do any of them block 99.99 percent of the UV like my sunglasses claim to do?
It doesn't make sense to me.
For instance why make a number 7 or 8 or 9 lens that only block part of the UV when we are welding?
Does a 14 block all of it then?
Maybe we should research IR radiation on the back of the eye ball?
It may get hot without dark enough glass.