Early glasses, known since c. 2500 BC (Henderson 2013), used plants (ash) and minerals (natron) as sources for alkali to flux silica. Mineral and plant alkali samples from the Corning Museum of Glass were combined with quartz from Spruce Pine, North Carolina in a series of experiments to make glass. Oxygen isotope compositions of alkali samples and quartz were measured at various stages of the glass-making process. Initial differences of oxygen isotope compositions gradually converge with higher heating temperatures. Glasses produced with mineral and plant alkali sources could not be distinguished based on oxygen isotope compositions. Therefore, oxygen isotope compositions are not useful in identifying the alkali source used in glass-making.