# How does the numbering system of welding glass work? **URL:** \ **Category:** Factual Questions\ **Created:** [August 21, 2017, 12:12am UTC](https://boards.straightdope.com/t/how-does-the-numbering-system-of-welding-glass-work/794239 "2017-08-21T00:12:16Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![G.A.Pster](https://avatars.discourse-cdn.com/v4/letter/g/ce7236/32.png) [@G.A.Pster](https://boards.straightdope.com/u/G.A.Pster)\ **Post date:** [August 21, 2017, 12:12am UTC](https://boards.straightdope.com/t/how-does-the-numbering-system-of-welding-glass-work/794239/1 "2017-08-21T00:12:16Z")
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? ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [August 21, 2017, 12:21am UTC](https://boards.straightdope.com/t/how-does-the-numbering-system-of-welding-glass-work/794239/2 "2017-08-21T00:21:10Z")
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. ---
**Author:** ![Ignatz](https://avatars.discourse-cdn.com/v4/letter/i/3be4f8/32.png) [@Ignatz](https://boards.straightdope.com/u/Ignatz)\ **Post date:** [August 21, 2017, 1:43am UTC](https://boards.straightdope.com/t/how-does-the-numbering-system-of-welding-glass-work/794239/3 "2017-08-21T01:43:40Z")
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. ---
**Author:** ![G.A.Pster](https://avatars.discourse-cdn.com/v4/letter/g/ce7236/32.png) [@G.A.Pster](https://boards.straightdope.com/u/G.A.Pster)\ **Post date:** [August 21, 2017, 7:46am UTC](https://boards.straightdope.com/t/how-does-the-numbering-system-of-welding-glass-work/794239/4 "2017-08-21T07:46:07Z")
> [@DPRK](#): > > 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. Thanks, that’s just what I was looking for. ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [August 23, 2017, 6:11pm UTC](https://boards.straightdope.com/t/how-does-the-numbering-system-of-welding-glass-work/794239/5 "2017-08-23T18:11:17Z")
> [@Ignatz](#): > > 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. NASA says, “[If it’s less than 12 (and it probably is), don’t even _think_ about using it to look at the Sun.](https://eclipse2017.nasa.gov/safety)” Are your eyes OK today? ---
**Author:** ![PoppaSan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/poppasan/32/189_2.png) [@PoppaSan](https://boards.straightdope.com/u/PoppaSan)\ **Post date:** [August 25, 2017, 1:09am UTC](https://boards.straightdope.com/t/how-does-the-numbering-system-of-welding-glass-work/794239/6 "2017-08-25T01:09:27Z")
> [@CookingWithGas](#): > > NASA says, “[If it’s less than 12 (and it probably is), don’t even _think_ about using it to look at the Sun.](https://eclipse2017.nasa.gov/safety)” Are your eyes OK today? 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.