Final answer:
The red color in medieval stained glass was produced by iron oxide nanoparticles, which created the deep red hue when the glass was heated and formed. The correct option is 2.
Step-by-step explanation:
The red color in medieval stained glass was often due to the inclusion of iron oxide nanoparticles. These particles were added to the glass and, through the process of heating, they provided the glass with a deep, rich red hue.
During the Gothic period, the construction of churches with flying buttresses allowed for larger windows, which were filled with intricate stained glass that told biblical stories and depicted figures. Stained glass artists used metallic salts to create colored glass, and each color required different metallic oxides.
The creation of red glass specifically required the use of iron oxide, which resulted in the beautiful red tones that we see in such historical pieces as the rose window from the Chartres Cathedral.