How Glass is Made

The Difference Between Primary and Secondary Glass Manufacturing

This entry will give a short description of both primary (how flat glass is made) and secondary glass manufacturing processes (how flat glass is processed) and a description of what the tin side of float glass is and how it affects certain processes.

Primary glass manufacturing is the mixing and melting of finely ground glass ingredients to make float glass as shown in the picture. It is called float glass because once the glass mix has melted, the molten glass is floated on top of a pool of molten tin (called the float or tin bath). This is done to ensure that the glass is perfectly flat. Molten glass will naturally pool to a thickness of approximately 6mm or ¼” thick. By controlling the speed at which the molten glass travels over the tin, the thickness can be changed. Increase the speed and the glass becomes thin, slow the speed down and the glass becomes thicker. Once the glass is cooled through the annealing process, it is cut into large sheets and sold to secondary glass manufacturers.

Secondary glass manufacturing is the cutting of glass to required sizes, drilling holes, polishing the edges, applying ceramic frit to the glass surface, tempered and/or made into sealed units, whatever is required by the customer.

Knowing which side of float glass was in contact with the tin bath is important when cutting the glass to size. Glass is always scored on the non tin side (air side) because it breaks more cleanly. Ceramic frit will change colour (white will look burnt) if applied to the tin side rather that the air side. Casting or slumping float glass over a mold tin side down, will result in cloudy glass. Lifting glass using suction cups on the tin side can cause ghost like images of the suction cups to appear when water (rain or condensation) is present on the tin surface.