Core forming was one of the earliest methods used by glass makers to make glass objects & items. The underlining concept behind technique was that a shaped “core,” dipped in molten glass, could be used to give a basic form to the cooling glass & thus create a void for storage purposes. These cores were most likely made of a material like clay or animal manure that could be baked & chipped away from the cooled glass leaving a useful vessel.
Another interesting technique used by early glass makers was Casting, in which powdered glass particles were poured into a mold, as with core forming this mold was most likely 2 pieces of clay. As the powder melted more would be continuously added until the mold was full. Upon cooling the cast would be opened revealing a solid, formed glass object. If this object were to become a usable vessel it would then need to be drilled out.