Tempered glass is one of two kinds of safety glass regularly used in applications in which standard glass could pose a potential danger. Tempered glass is four to five times stronger than standard glass and does not break into sharp shards when it fails. Tempered glass is manufactured through a process of extreme heating and rapid cooling, making it harder than normal glass.
The brittle nature of tempered glass causes it to shatter into small oval-shaped pebbles when broken. This eliminates the danger of sharp edges. Due to this property, along with its strength, tempered glass is often referred to as safety glass. Notice the thinner TG, 1/8" is more delicate when shattered. Most stained glass is 1/8" and using 1/8" or 3/16" is easier to mosaic with the same heighth.
Automobiles use a different type of safety glass for the windshield and tempered glass for the back and side windows. Windshields are made from laminated glass, which sandwiches a sheet of plastic between two panels of glass. When the windshield breaks, the glass panels stick to the plastic film, rather than falling away to possibly injure the driver or other passengers.
Tempered glass breaks in a unique way. If any part of the glass fails, the entire panel shatters at once. This distinguishes it from normal glass, which might experience a small crack or localized breakage from an isolated impact.
This makes it perfect for mosaics! Tempered glass comes in colors - clear, green, bronze, and gray. It also comes in different thicknesses. 1/8", 3/16", & 1/4". Car windows is 1/4" thickness where as shower doors are usually 3/16". You can also buy textured tempered glass. This picture is comparing the thickness of 1/8" & 1/4" TG.
There are some online mosaic suppliers that sell tempered glass (tg) but I would look in a phone book for glass companies that manufacture tempered glass. They may have scrap glass you can buy. I did that from a local company and he gave me a discount since it was from a special window a customer didn't buy. You can also try Craig's List.
You can paint under TG, add ribbons, pictures, glitter. The sky is the limit. If you do paint your substrate, I would use a sealer before applying the TG and the glue. Mine turned a green color underneath the glass without the sealer.
Here is a box where I used TG. I painted part of the box with gold acrylic paint & then a clear spray before I glued the TG down.