Poisonous pullover

Many years ago I read a short story, only one aspect of which stuck in my mind - everything else I've forgotten.

It was a murder story where the victim was poisoned with cyanide. Nothing unusual about that of course, but the poisoner made the cyanide from raw sheep's wool and caustic soda. I'm sometimes reminded of it when out walking and see bits of sheep's wool caught on barbed wire or a hawthorn bush.

As I recall it, the poisoner heated the wool with the caustic soda in a cast-iron crucible, but I don't remember if there were any further details. Maybe that was as much chemistry as the writer had imbibed or cared to pass on. You probably could make cyanide this way. Sheep's wool, like human hair, is a form of protein with lots of nitrogen and carbon, the two elements comprising cyanide (CN) ions. But extracting sodium cyanide from the horrible mess left behind after heating raw wool with caustic soda - well I can't see that being a simple problem outside a laboratory.

I always thought the writer would have been better off inventing a poisoner who, if he really had to make his own cyanide, would at least make it from clean wool. An old pullover for example, or a cardigan. Real wool of course. He could even have had his poisoner nick the victim's pullover, turn it into cyanide and poison him with it. Poisoned with your own pullover - now that's what I call a short story plot.
The Fair Isle Poisoner.