The mixture of ingredients needed for making glass are called glass batch. Once mixed (at room temperature) they are "cooked" in a furnace. The result is molten glass.
Typical soda-lime glass batch, for example, includes silica (sand), soda ash, and lime. It can also include other ingredients, including those for color (frequently metal oxides).
Glass artists have the choice of purchasing pre-mixed glass batch, mixing their own batch or a combination of the two -- purchasing mixed batch and adding their own additional ingredients.