Fibrous Plaster: An overview of research and an example of audience impact upon ceiling environments in historic theatres

Fibrous plaster was used extensively to provide ceilings and other decorative features in period buildings such as theatres, premium private houses and municipal buildings with a particular proliferation occurring during the late 19th century and early 20th century. It was patented in the UK in 1856 and has a matrix of gypsum plaster (also known as ‘Plaster of Paris’) with fibre reinforcement. Historically, fibres were woven hessian scrim which derived from the Indian Jute plant. A timber ‘lath’ framework spaced typically at 0.5m spacings for panel elements provided extra reinforcement. Fibrous plaster ceilings were often suspended from structural elements in roof spaces or floors using fibrous plaster wads. In addition to gypsum and hessian, wads can contain further reinforcing steel wire; modern wads include wires, but historically it has been found that most wads did not. Prior to fibrous plaster, lath and lime plaster was a popular choice for ceilings, with plaster covered timber laths affixed directly to structural members. There is a risk of fibrous plaster being subject to degradation over extended timescales, and failures of fibrous plaster ceilings in the 21st Century, most notably the 2013 partial collapse of the ceiling at London’s Apollo Theatre, london, place importance upon the conservation of historical fibrous plaster elements. There is a wide range of knowledge and experience in a small specialist fibrous plaster industry but to date, there hasn’t been a major amount of scientific research concerning material properties. This paper provides an overview of research being conducted to contribute to existing knowledge. Results of material behaviour will deepen understanding of long-term degradation and environmental interactions. Continuing experimentation includes mechanical testing, moisture and fungi-induced deterioration and in-situ monitoring of ambient conditions in theatres. Data collection of temperature and relative humidity levels in a theatre environment with a historic ceiling is presented and the impact of audience attendance is evaluated, with further monitoring and analysis in progress. It is demonstrated that attendance levels during performance times in a theatre affects temperature and relative humidity levels within the venue, demonstrating that historic ceilings are located in environments which vary in relation to human activity. Project results will contribute towards the enhanced conservation of fibrous plaster elements, particularly ceilings, in heritage buildings, keeping them in both in prime visual condition and safe for the public to enjoy in future years.