Engraved glass plaque made in 1788 in the Royal Factory of Glass of La Granja de San Ildefonso (Segovia, Spain) represents the mythological theme of Hercules with Deianira and Nessus at their feet, as well as the coat of arms of Spain and Portugal. This is a “mastery plaque”, a test to complete an apprenticeship in order to become a glassmaker master.