I was checking into glass painting on float glass and discovered that it's important to know which side your painting on. There is the air side and the tin side. The liquified window glass is floated on a bed of molten tin during the manufacturing process. This means that the tin side will have a different reaction to paints than the air side. So it is important to know which side is which before you start painting.
Unfortunately, it isn't all that easy to identify the tin side. Some people suggest licking the glass, to try and detect a metallic taste from the tin side! It was said that you need amalgam fillings for this to work, although putting a small piece of foil in your mouth first might work if you don't have the "silver" fillings.
Another method is using a water droplet. It is suggested that if you let a drop or two of water hit the glass it will react differently for the air side / tin side. Apparently, on the air side it should run down the glass with little beading action, while on the tin side it should tend to bead up more due to friction and more surface tension.
The best way to determine the tin side of the glass is to use a short wave UV light. Shining this type of light onto the tin side will cause a discernable glow. It may be difficult to see, depending on the light you use and how much light is in the room, but the glow will be visible.
Most people aren't highly informed about UV lights, so they may get a light but not get the results they expect. There are different types of lights and you need the correct type for this to work. Long wave UV or UVA is in the 365 nanometer range. That's much like sunlight and it won't work with the tin side of the glass. Mid wave UV or UVB is 302 nanometers, which is still too high in the spectrum. To get a good tin detector, you need Short wave, or UVC light, which is 254 nanometers.
This one is about $100 and comes with a UVA or, longwave, bulb. You can use that wavelength for curing UV adhesives. For tin detection, you can purchase a shortwave bulb from them for about $35. That brings the total price up to the $150 range, including shipping.
And here's a link that I was recently given for a very inexpensive UVC light. Purely Products 5004B-P Hand Held Pocket Purifier This one costs only about $15. It is pretty weak though, so you'll need a darkened room and you'll have to check the glass closely to see the glow caused by the tin side.
Notes: UVC light is not good for you! It's not good for your skin, and it is really bad for your eyes. When using any of these lights, do not look directly into the light. The best procedure for use is as follows:
1. Put the light UNDER the glass. Shine the light on the glass at about a 45 degree angle.
2. While wearing #3 welder's glasses to protect your eyes, look at where the light hits the glass. Do not look directly at the light.
3. If the light is shining on the tin side you will see a faint glow. If it is not the tin side, you won't see anything at all.
4. If you have tried this with both sides of the glass and can't detect any glow it is possible that you have a piece of ROLLED window glass, not a piece of FLOAT window glass. If the glass is more than about 50 years old it was made before the tin floating process was used. Also, fusible glass with a COE of 90 or 96 is not made with the tin floating process, so clear fusing glass doesn't need a tin side detector either.
You're welcome, Lou. If you're serious about painting on float glass, I recommend getting one of the stronger lights. The little uvc sanitizer light works, but the glow is really faint. Up to you, of course.
I'm about to give glass painting a first time tryout on some wind chimes I'm working on. Just to see how it looks. Using Color For Earth glass paints. I'll let you know how it turns out.
Lou Pickard wrote
Thanks Gabe, I have tried numerous times to find the answer to the question of how to tell the tin side of glass and what kind of uv light to buy. Really appreciated.