How does the numbering system of welding glass work?

Factual Questions
1

I want to look at the eclipse, and all I have is welding glass,
I read that 12 is the minimum, but 14 is better.

I’ve got two 10s, a 5 and an 8.

I read that just putting a 5 over a 10 would not make 15 because the numbering system is non linear.

So now it got me wondering How does the numbering system work with welding glass?

2

10 and 5 make 14 because the formula is, confusingly,

shade = 1 - (7/3) log[sub]10[/sub](transmission fraction)

Ie shade #1 (not 0) is transparent.

3

I used a 10 shade glass for the northwest-to-southeast eclipse in Old Orchard Beach, Maine, in 1963 and the same piece in the southwest-to-northeast one in Savannah, Georgia, in 1970. I just happened to be working in those places at the time. I, and some cow-orkers who couldn’t find any viewers locally, plan to use the same piece tomorrow, where I’m currently working.

4

Thanks, that’s just what I was looking for.

5

NASA says, “If it’s less than 12 (and it probably is), don’t even think about using it to look at the Sun.” Are your eyes OK today?

6

It’s what I did (used #10) and I am fine. Note we also had high overcast so we didn’t get the full effect. Anyway, I didn’t stare at the sun, just a short glance now and then. I also had my prescription sunglasses on until almost totality so those also added a slight benefit. They were included in my calculation. My kids without shades had the eclipse glasses.