Data will be made available on request.

Acknowledgements
The authors acknowledge the funding of the Leverhulme Trust, grant number RPG-2021–147 and the support of Historic England. We also acknowledge and thank Diamond Light Source for providing access to beamline I12 via experiment number MG35934–1. The authors gratefully extend thanks to the following: Peter Tovey and the entire technical, managerial, maintenance and stage door teams of the Bristol Hippodrome Theatre, Bristol, UK; The Ambassador Theatre Group; Robin Harrison, Russell Hempstead and the whole team at Hayles and Howe Ornamental Plasterwork and Scagliola, Bristol, UK; Gary Buckley, Jon Riley and the team at Locker and Riley Artisans in Plaster, South Woodham Ferrers, Chelmsford, UK; Robert Attwood, Thomas Zillhardt and Genoveva Burca - Beamline Scientists at the Diamond Light Source, Harwell Science and Innovation Campus, UK; William Bazeley, Neil Price, Robert Dyer and Martin Naidu of the Department of Architecture and Civil Engineering, University of Bath, UK; Rod and Eric Stewart, Historic Plaster Conservation Services, Ontario, Canada; Claire Appleby, Theatres Trust, London, UK and Robin Townley, Association of British Theatre Technicians, London, UK. Bristol trchives, Bristol, UK and V&A Museum, London, UK. The data for supporting this manuscript are available from the Dataset for the results of fibrous plaster tests, University of Bath Research Data Archive, https://doi.org/10.15125/BATH-01510.