Isinglass
Thin transparent sheets of mica called "isinglass" were used for peepholes in boilers, lanterns, stoves, and kerosene heaters because they were less likely to shatter compared to glass when exposed to extreme temperature gradients. Such peepholes were also used in "isinglass curtains" in horse-drawn carriages and early 20th century cars. A book about a journey in a Model T Ford car describes isinglass curtains as follows: "Oiled canvas side curtains were put up over the windows for wind, rain, and cold (there were no heaters) and were held in place with rods that fit into the doors and twisting button snaps around the perimeter... 'Isinglass' peepholes in the curtains allowed limited visibility. Isinglass was made of thin sheets of cracked mica."
