Vulcanized fibre is produced in a number of different grades. The final grade depends upon the starting material. Saturating a grade of absorbent paper is the base material for Vulcanized fibre and may be made form cotton rags or wood pulp of high alpha cellulose content. The paper is immersed in a zinc chloride solution, the resulting chemical action causes the individual cellulose fibres to swell and become covered with a layer of “gel.” Plies of “gelled” paper are bonded together under controlled heat and pressure to produce sheets of desired thickness. Vulcanized fibre tubes are made on round mandrels and rods are produced by cross-sectional strips cut from sheets and turned to the required diameter. Vulcanized fibre can be formed by cutting, punching, tapping, milling, shaping, sanding, etc. Methods are described in the (NEMA) National Electrical Manufacturers Association publications.
Vulcanized fibre is often used for insulation in transformers and older style drum cases. Fender style bobbins used a black Vulcanized fibre till the mid 60s than a light gray and in the 70s a darker gray. Since the 80s and 90s Fender has reintroduced the black Vulcanized fibre. They are doing a great job with the Vintage Re-issue instruments. Normally the Vulcanized fibre used for the top of the bobbin is .062″ thick and the bottom uses .093″ thick.
Early Broadcaster, Telecaster, No-Caster and Esquire bobbins used a .062″ and early on changed the .093″. I believe it was changed due to the pressure from the 3 mounting screws on the Broadcaster, Telecaster, No-Caster and Esquire easily warped the thinner flatwork so Leo Fender decided to use thicker Vulcanized for added strength over time. The thicker flatwork helped keep it tighter to the pole pieces and helped avoid the bobbin from falling apart.
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