The green tint found in most tempered glass is a result of iron used in the production of glass sheets, where it is added as an ingredient to act as a lubricant. We learn that the greenish tint is strictly caused by the iron in the glass. Also, a green tint is often the source of questions when glass is used in a situation where the edges are exposed. Looking into the glass from the edge displays the green tint most prominently.
If you were using a piece of glass to protect a beautiful piece of furniture, you would notice that the color of the furniture would be tinted green while looking through the glass. Here, the iron becomes an ingredient of the glass and acts as a lubricant.
Most glass is drawn along on top of a molten layer of tin. The liquid quality of the tin is why the glass can be so smooth on both sides, since it is difficult to have bumps in a liquid surface. The iron level in the glass varies from manufacturer to manufacturer depending upon their recipe for their glass. But most glass will contain certain levels of iron.