The reflective coating on the back of a glass sheet is what gives it the properties of a mirror. The materials used to create this effect have changed significantly over the years. In the 16th century, mercury was used to create mirrors. It was spread over a thin layer of tin and then the glass was placed on top of the mercury. The reaction between the mercury and tin caused the mirror-like substance to adhere to the glass. These old mercury mirrors tend to age and develop a sparkly, crystalline appearance behind the glass, which is a characteristic of antique mirrors. Nowadays, aluminum and other metals are commonly used to create the reflective coating on the back of glass sheets. These materials are less toxic and more durable than mercury, making them a safer and more practical choice for modern mirrors. The process of applying the reflective coating to the glass has also become more efficient and precise, resulting in mirrors with clearer and more accurate reflections.