Arising from questions concerning the long-term stability of the mortar, this research establishes the composition of the epoxy resin mortar used for the construction of the dalle de verre lantern of Liverpool Metropolitan Cathedral. Water ingress had manifested itself early in the life of the lantern as a result of flaws in the mortar. These defects are described and illustrated herein. The paper also describes the process of characterising the epoxy resin, quartz flour, and carbon black in the lantern mortar in order to prepare mock-ups for weathering assessments to gauge the mortar’s future behaviour. The mortar specification was accomplished by archival research and personal interviews, complemented by laboratory analyses of lantern mortar samples using Fourier-transform infrared spectroscopy, Raman spectroscopy, scanning electron microscopy, and thermogravimetric analysis. The difficulties in sourcing sufficiently detailed documentary information on the original mortar components are discussed.