Re Vin Malone and Ron Clayton’s letters about crozzle topped walls, I have photos somewhere of crozzle taken around the city, including by the river Porter on Mary Street. In my entrepreneurial days I thought of doing a booklet of them!
Newcomers to our strange city may wonder what the origin is of this burnt stone, which is as unique to Sheffield as Henderson’s relish. I’ll try to explain:
It starts with knife grinders’ wheelswarf, a sludge of grindstone and steel dust that built up in his water-filled trough as he worked. Never ones to waste owt, especially if there was a bob or two to be earned from it, the largely self employed grinders sold this sludge to the Cementation steel makers.
The Cementation process, invented in Germany, was adopted in Sheffield about 1700 and held sway, certainly for knife makers, along with Crucible steel (invented in Sheffield ca. 1740) right until about 1920.
Brearley’s stainless steel killed it off, though Daniel Doncaster still made Cementation steel in his furnaces on Hoyle Street until 1951. (One furnace, now listed, is preserved in the HSBC complex).
In the process iron bars, packed in charcoal inside long sandstone chests, called coffins, were sealed with a thick ‘piecrust’ of wheelswarf sludge, and heated to 1000 degrees for 7-10 days. The sludge kept heat in and oxygen out. After 7-10 days the furnace was allowed to cool, and men entered to break off the burnt crust, to reveal the bars, now converted, thanks to the charcoal, into the celebrated Sheffield ‘Shear Steel.’
Like the grinders, never ones to waste owt, the steelmen sold the crozzle, cut to size, as topping for whoever built the stone walls along rivers, etc. Its rough, sharp surface deterred youngsters from running along it and falling in the river.
Knab Road,S7 2ET. Listed Cementation furnace near Hoyle Street.