This thesis analyzes the mortars used in the construction of Guastavino tile vault assemblies through an in-depth study of St. Paul’s Chapel and observations at five other Guastavino projects. Limited research has been published that addresses the mortars used in Guastavino vaults, but the available literature indicates that only gypsum mortar and Portland cement mortar were used in their construction. Furthermore, literature establishes that vaults were assembled by installing the first layer of tiles with gypsum mortar, while the above tile layers were set in Portland cement mortar. Preservation professionals often encounter discrepancies between this assembly described in literature and the assemblies and mortars observed during restoration projects. To address these discrepancies, this thesis performed a material study at St. Paul’s Chapel, identifying the mortars and their assembly through archival review, physical observation, and cursory petrographic analysis of select samples. It was determined that at least four different types of mortars, including gypsum mortar, gypsum-lime mortar, natural cement mortar, and portland cement mortar were used in the construction of St.

Paul’s Chapel. Furthermore, it was discovered that the used mortar types varied by the form and finish of different vaults at St. Paul’s Chapel. Similarly, in the supporting case studies, mortars other than gypsum mortar and portland cement mortar were identified, again varying based on the particularities of the assembly of each vault. The variety of these materials and assemblies illustrate the oversimplification of the description of Guastavino vault assemblies in published literature. This thesis therefore provides schematic section drawings of the studied vaults that identify the types of mortar used and their locations within the vault assembly. While only a fraction of the projects of the Guastavino Company were analyzed in this thesis, the chosen case studies clearly demonstrate that the company used mortars other than gypsum mortars and Portland cement mortars. Furthermore, this thesis establishes that the type of mortar used in a Guastavino project was intricately linked to the assembly method, and that both mortar and assembly varied based on the specific parameters of particular projects.