The material

The melting is heated up to a temperature of between 1,400 and 1,600 °C, depending on its composition. As it melts, the glass releases gases which form bubbles. The higher the temperature, the more these bubbles expand and are able to rise through and be exhausted from the molten glass.
This process can be shortened by adding refining agents to the batch. As the temperature rises, the refining agent produces gas bubbles which remove the smaller bubbles by diffusion. This has the advantage of speeding up the process of larger bubble formation and therefore gas exhaustion When the molten glass cools, the remaining bubbles are reabsorbed.