Design rationale and field testing of a gypsum-based grout for wall painting stabilization in the Chapel of Niketas the Stylite, Cappadocia, Turkey

Abstract

The rock-cut chapel of Niketas the Stylite (Cappadocia, Turkey) is decorated with 7-8th C CE painted gypsum plasters on a tuff support. A 2015 survey found that detached areas of plaster required stabilization through injection grouting. This paper presents the rationale behind the preliminary development of calcium sulfate hemihydrate (HH, CaSO4·0.5H20)-based injection grouts for this purpose and their testing in the field. Research into gypsum-based conservation materials is limited compared to lime, and there are very few examples of the development of proprietary gypsum grouts for the stabilization of wall paintings on gypsum plaster. This study therefore reviews the existing literature, and explains the rationale behind the selection of materials for the conservation of the paintings, including a discussion of the potential risks and benefits associated with the use of lime and gypsum binders. Field testing focused on the development of HH-based grouts formulated with a range of aggregates and fluidizers. The grout mixes were tested against clearly-defined working properties and some of the performance characteristics for the intervention. Existing field-testing protocols were adapted as necessary to the working conditions, context and materials of the site and paintings.