Testing for Tin side of Float Elizabeth L. Braun
The characteristic of float glass having a molecular level of tin left on the “tin side” but not the “air side” is important to distinguish. There are short wave UV light sources to help determine this.
The tin side gives a whiter glow than the air side. Kay Bain Weinner says in her enameling book she can tell which side is the tin side by the "ragged" edge of the cut line when viewed on edge. Another test, which I find more reliable, is to use silver stain, which is Silver nitrate in a liquid suspension and is painted on the glass surface and fired in a kiln. The result is an actual staining of the glass and should be some shade of yellow, from very pale to a rich golden brown. To test which side of the float glass is tin, cut two pieces of glass from the same sheet and turn one piece upside down, Be sure to document which side is which on the test pieces and the sheet they come from!
For mixing silver stain: Glass slab, Glass muller or palette knife, Water jar, Respirator or dust mask, disposable Gloves
1. Place some silver stain on the glass slab.
2. Add water or white vinegar. Do not inhale the paint powder.
3. Use the glass muller or palette knife to mix thoroughly.
Residual tin on the glass will interfere with the uptake of the silver stain. After firing, scrape off excess powder, the darker side of the glass is the tin side, the lighter side is the "air" side.
Careful, these stains are very aggressive on metals and paintbrushes, use separate equipment and clean immediately after use. Silver stain is traditionally applied to the back of the glass and fired at approximately 621-635°C (1150-1175°F) with the silver stain side down. Protect whatever the silver stain comes in contact with during firing. However it may be fired up, (which is how I test for tin side) which prevents staining of the kiln shelf.
Sources for Silver Stain:
Silver stain is quite expensive- but a little goes a long way. You may ask for a smaller quantity from local suppliers- say an ounce, rather than a pound, and save money!
REFERENCES Discussions with Clifford Oster. The British Society of Master Glass Painters, “The Birth of Ancient Walpole Silver Stain”, http://www.bsmgp.org.uk/downloads/Walpole_silver_sta in.pdf. Dorothy L. Maddy, “Silver Staining”, Stained Glass Magazine, Fall 1984. Discussions with Hans van der Valk.