The old theatre slides I've seen were a double layer glass 'sandwich', with the film ad layer in the middle.
They did need to be quite robust. Among other considerations, the theatre slide projectors of that era [along with most of the film projectors] used a DC carbon arc light source. So the glass would filter out the UV and IR components of the light, and prevent them affecting the film, as well as physically protecting it.
Cheers,
Gadge
"ODK Mods can explain it to you, but they can't understand it for you..."
"Crazy can be medicated, ignorance can be educated - but there is no cure for stupid..."
| 
