Carborundum — Silicon Carbide

This process runs at electric-furnace temperatures and it liberates carbon monoxide. Work outdoors or under forced extraction, never in a closed shop. Full face shield and dry gloves for anything near the electrodes.

Acheson called it "Production of Artificial Crystalline Carbonaceous Materials". He believed he was making a carbon compound — the title records a genuine misunderstanding, preserved in law.

The charge is quartz sand and carbon. Roughly equal parts by weight is a working start; the reaction wants carbon in excess of the strict stoichiometry.

Sawdust burns out and leaves porosity so gas can escape; a little salt helps volatilise metallic impurities. Both are in the original practice and both matter.

Blend until uniform. Damp charge turns to steam at temperature and can throw material out of the furnace violently.

Line a trough with firebrick and set a carbon electrode entering from each end. This is a resistance furnace, not an arc furnace — the charge itself is the heating element.

Run a spine of coarse graphite or coke from one electrode to the other. The core carries the current and becomes the hot zone — everything grows outward from it.

Heap the sand-and-coke mixture over and around the core, burying it completely. The outer charge insulates the reaction as well as feeding it.

Raise power slowly. The core resistance falls as it heats, so a fast ramp overshoots and can blow the charge apart.

The reaction needs roughly 1700-2500 °C. Blue flames playing over the heap are burning carbon monoxide — expected, and the reason for the ventilation rule.

Cool for many hours. Opening hot exposes incandescent material to air and destroys the crystal faces you are trying to grow.

You will find zones: unreacted charge outside, then a shell of crystals, then graphite at the core. The best silicon carbide sits in a band at a specific distance from the core.

Iridescent black or green plates and needles. The colour is impurity, not chemistry — pure silicon carbide is nearly colourless, and the green and black grades differ by trace nitrogen and aluminium.

Try to scratch a corundum abrasive with your crystals and then the reverse. Silicon carbide wins — Mohs 9.5 against corundum's 9, and that half point was worth an industry.

The patent. Edward Goodrich Acheson was granted US 492,767, "Production of Artificial Crystalline Carbonaceous Materials", on 28 February 1893. Acheson had worked for Edison and was trying to make artificial diamonds by heating clay and coke. He got hard iridescent crystals instead, assumed they were a compound of carbon and corundum, and coined carborundum from those two words. The material is in fact silicon carbide, SiC — no aluminium in it at all. The name survived because the Carborundum Company made it a trademark, and a chemically wrong word became the standard trade name for a material now made by the megatonne.

The chemistry, correctly. SiO₂ + 3 C → SiC + 2 CO, at roughly 1700-2500 °C. The carbon monoxide is why ventilation is not optional, and why blue flames dance over an operating furnace. The Acheson process is a resistance furnace: current passes through a granular carbon core buried in the charge, and the core is the heating element. Because heat radiates outward from that core, the ingot forms in concentric zones — graphite nearest the core where SiC has decomposed again, the best crystals in a band further out, unreacted charge at the edge. Industrial practice recycles the outer material into the next run.

Why half a point on the Mohs scale mattered. Silicon carbide is about 9.5 where corundum is 9 and diamond is 10, but Mohs is a ranking, not a scale — the absolute gaps are enormous. Practically, SiC cuts materials corundum merely polishes, it can be made in quantity from sand and coke rather than mined, and its grain fractures to keep exposing fresh edges. Grinding wheels, cutting discs, wet-and-dry paper and lapping compounds all changed within a decade, which meant everything made by grinding got cheaper — including the precision machine parts that the next generation of industry depended on.

Where it went afterwards. Acheson's furnace also gave him artificial graphite, patented separately, when he noticed SiC decomposing near the core. Silicon carbide later turned out to be a wide-bandgap semiconductor: it holds up at voltages and temperatures that destroy silicon, and SiC power devices are now standard in electric-vehicle inverters and solar converters. A material invented as a substitute for diamond dust, named after a mineral it does not contain, ended up in the power electronics of the twenty-first century.