The pictures in this article come from the BVWS DVD of the 1950s Mullard film.
Mica is a generic name for a group of minerals. All are true phyllosilicates. Micas are composed of sheets of silicate tetrahedrons. The silicate sheets are composed of interconnected six membered rings. The characteristics of micas are that they are insulators, dimensionally stable and very brittle. Like slate, mica splits into thin sheets.
Mica is used as insulators and spacers in valve manufacture. The mica sheets in the 1950s were fed into small stamping machines by hand. The inspection of the cut product was also a visual process, both by the unaided eya and by projecting the mica product onto a ground glass screen. Detailed measurements could be taken from the magnified image to ensure the mica was to the correct tollerance.
An image of a mica component on a ground glass screen