Identifying the coated side of glass

Dichoric and iridised glass can present difficulties in identifying the side that has the coating. For some applications it is important to know which is the coated side to place it up or down or even to make sure the pieces are all the same way up.

Several methods are possible:

On coated clear glass you can use a pencil or other pointed object. Hold the glass so you're looking a glancing angle then bring a pencil down onto the surface. The coated side will show a clear reflection of the pencil tip and the backside will show a gap, or multiple image of the pencil tip.

If the glass is dark or black this method will not work conclusively. Instead you can use grozing pliers to nibble at the edge of the glass. The surface that shows damage to the coating is the coated side. If there is no damage visible to the coating, the other side is the coated one. You could mask the glass and sandblast a small corner. If the coating blasts away, that is the coated side. If not the coating is on the other side.

Having gone to all this trouble, mark up the glass side with a permanent marker to identify the coated side. Also mark each piece cut from it so you do not have to repeat the test on each cut piece.