Any ancient painting like that in the UK would be protected by a sheet of perspex.
From the guardian.co.uk
Stephen Wiltshire creates an impromptu self-portrait with sharpie and perspex.
From the couriermail.com.au
Perspex is then vacuum formed over this bust creating an immobilization shell.
From the en.wikipedia.org
Perspex jars were used for LAG and ILAG, and metallic jars for neat grinding.
From the nature.com
The car's exhaust is connected by a hosepipe to a perspex box on its roof.
From the morningstaronline.co.uk
Made of black perspex, it may well scuttle the giver into your good books.
From the guardian.co.uk
Swede's design includes covering the cargo shed area with a weatherproof perspex dome.
From the nzherald.co.nz
Perspex, bull's heart, silver, assorted needles, scalpels, and formaldehyde solution.
From the en.wikipedia.org
Is there any indication of the thickness of perspex needed to be implanted in the window?
From the newscientist.com
More examples
Lucite: a transparent thermoplastic acrylic resin
Poly(methyl methacrylate) (PMMA) is a transparent thermoplastic, often used as a light or shatter-resistant alternative to glass. Chemically, it is the synthetic polymer of methyl methacrylate. ...
Any tough, transparent plastic that is used instead of glass for windows
An alternative name for Plexiglas
Clear plastic material, trade name for polymethylmethacrylate. Used in some applications where a cheap, clear vessel or window is required. Easily scratched, cannot be welded. See also PET-G
A clear plastic sometimes used for window glazing, especially of sheds.
Plastic material glass finish effect that comes in 2,3,4,5,6 mm. maximum size 8 x 4 feet.
N. 1. Lucite, plexiglas, clear plastic. The term is a trade name in the U.K.
Acrylic; Polymethyl methacrylate; clear, translucent or opaque plastic; ICI registered trade name