Tindicator

Just like art glass, float glass (the stuff used in windows) is fire polished on its top side; the molten glass flows to solidify into a smooth flat shiny surface. But unlike art glass, the underside is smooth, flat and shiny too. How is this achieved? The process was invented by Pilkington, and relies on the sheets of float glass being extruded onto the top of a very big trough of molten tin. As the glass cools and solidifies, it does so against the smooth liquid surface of the tin, which is still liquid even when the glass has become solid. This gives us our perfectly undistorting windows and mirrors.
        The tin has a subtle effect on the glass. A tiny amount of it is absorbed into the glass structure on the underside. That side of the glass is more liable to devitrification, may not take enamels as well, and may be harder to fuse. So, if you want to use float glass as a base for fusing, then the tin side should be down against the kiln shelf, away from the fusing interface and partly protected from devitrification.
        But how do you know which side is which? The supplier won't tell you. You need a Tindicator or tin indicator. Here is a picture of a commercial battery operated Tindicator, retailing for around $A90 ($US50). It is a simple source of ultraviolet (UV) light.
        To determine which side of the glass is which, take your (cleaned) float glass and the Tindicator into a dark place. Hold the glass at about 45° level with your waist and shine the UV from under on it. See the sketch. If the tin side is down, you should see a slight bluish-white fluorescence. Turn the glass over and compare. Since the glass itself absorbs the UV light, the fluorescence only occurs on the side near the UV source. Some float glass fluoresces more than other varieties, depending on manufacturer, batch, and many other variables. Float glass manufacturers don't much care about it. Some people shine the UV on the top of the glass. Whatever you do, don't look directly at the tube.

Making your own Tindicator
It is not overly difficult to make your own Tindicator, if you want to save money. Here's what you do:

Hunt through your hardware stores and camping shops until you find a torch (flashlight) with a 4W 6 inch fluorescent lamp. Sometimes these are made for campsites, so the bulb bit is useful or a beam and the tube for lighting up a small area such as inside a tent. See the photo of one that I found. You might find something different, even a mains-operated desk lamp, as long as it takes a 4W 6" tube.

Then ask an electrical supply store to order you in a 4W 6 inch 'black light, blue' tube. It may have the part number GT45 (or it may not). It should cost you say $A7 or less.

Replace the tube in the torch by the UV tube, and you are nearly in business. But wait, read the safety instructions.
It is dangerous to look directly at the UV tube. Extended exposure to UV light may cause cataracts and vision problems. In the dark your pupils will be wide open, and your torch is designed to spray light everywhere. Depending on the design of your torch, stick opaque tape (such as packaging tape) to block the light everywhere except in a wider slit in the direction you want it to go. It would be a good idea to use a permanent marker pen to write 'Do not look at the tube' prominently on the torch.
With luck, you now have a Tindicator. It should make most plain white copy paper and most white cotton shirts fluoresce in mauve, as a simple test. Try it on several pieces of float glass from different sources.

Disclaimer
Use of this information is strictly at your own risk. I disclaim all responsibility for injury to yourself or others, loss of funds, or damage to any glass product you may make.

Float Glass?
Why would you want to use float glass? In a word, cheap. It also comes in many thicknesses. There are many options for creating a sparse colour palette with a float glass base, but the commonest is to use it in transparent products. I also try out 3D glass ideas in float before committing to more expensive glass.