Approximately 19.5cm x12cm. When making fused glass projects it is vital to know whether any stress has occurred during the annealing process. This is not always obvious to the naked eye and a piece that looks fine may actually be stressed causing it to break anything up to a year after it was made. Polarizing film is a cheap and easy way to check for stress. To use: Cut the piece of film in half and place one piece on a light box, place your fused glass piece on this then cover with the other piece of film. Rotate the top piece of film so that it is at right angles to the bottom piece with as little light shining through as possible. Now check the edges where different parts of glass meet to see if there is a white glow. Depending on how bright the glow is will determine how severely the glass is stressed. If the glass is compatible then there should be no glow or only a very slight halo. The brighter the light, the more incompatible the glass is, it may still be useable but will be more likely to crack.