Ebonite (Hard Rubber)

Vulcanised rubber is rubber with a few sulphur bridges between its chains. Keep adding sulphur and the bridges multiply until the chains cannot move at all: the soft, stretchy material turns into a hard black solid you can turn on a lathe, polish like ebony and use as an electrical insulator. **US 8,075**, "Improvement in the Manufacture of India Rubber", Nelson Goodyear (brother of Charles), granted 6. May 1851, claims "the combining of india-rubber and sulphur… for making a hard and inflexible substance hitherto unknown": about equal parts by weight, heated to 260–275 °F for three to six hours. It was one of the first rigid materials made by chemically changing a natural polymer, over half a century before Bakelite. **You turn a part from commercial ebonite rod and measure what the sulphur did** — its density, and its charge when rubbed. Curing ebonite needs a heated press and releases hydrogen sulphide, so the blueprint starts from finished rod; the notebook does the chemistry.