Valve lore contains a lot of references to mica but what is a mica? Well, let's start with the raw material. Mica is a silicate material having the chemical formula Na2Al4–6Si8O20(OH,F)4.
The only thing we need to remember about this chemical formual is that it provides a material which splits into very thin sheets of a material that is impervious to gas, rigid but brittle and a useful insulator. Interestingly, the material is named from the Greek word mica which means crumb and also the Greek word micare for glitter.
The mica supports used by Mullard were obtained as cut 'washers' from sources in India and Canada.
In the next blog entry, I'll tell you how these washers were received and treated at Mullard before being incorporated into valves.