The leads are inserted through a glob of hot glass, along with a glass tube. The bulb's open base is heated to make it soft, the blob containing the leads with attached filament are inserted, then the base is welded to the lead blob. Last the tube is used to evacuate the bulb and/or inject inert gasses. While under this vacuum the tube is melted and smashed closed, permanently sealing the bulb. Attachment of the base comes later.