The Arsenic Works job was to process all the delicious arsenic from the mines roundabout. It was was built in 1875 closing very soon after due to financial difficulties. (This sort of thing seems to be quite a theme. Mining was clearly a precarious industry, prone to bad decisions.) Fortunately, the Gunnislake Clitters mine just down the hill came to the rescue and bought the place, lest a handy 212-foot chimney designed to gout acid and mildly arsenical smoke into the atmosphere should go to waste. It operated probably until the early 1930s, gaining its current chimney in 1893, and the woods around were stunted and blackened. I'm guessing that the people were probably moderately stunted and blackened too, poor things.
This is not the only bit of the works that survives. It was a big place. There are ruined buildings a bit further down the hill. I walk past them a lot, but have only once visited, because even by the standards of the rest of the valley they feel terribly ruined, with lots of slag and burned stuff under the heather, piles of unstable stone and scary chunks of twisted rusty metal.
The HER feels that the place should get credit for other jobs, not just the preparation of arsenic: "including the smelting of tin, preparation of complex arsenic compounds and even the separation of gold and silver which occurred in some of the ores of the district."
Despite appearances, Greenhill works did not explode and blow the top twelve feet off the chimney. It was hit by lightning in 1989.