On my last visit to London, England I made a point of going to Waterloo station. I wasn’t interested in taking a train, I just wanted to stand at the bus stop in front of the station. I wanted to see with my own eyes the famous spot where one of the most ingenious chemical crimes in history was perpetrated. For it was there, in full daylight, on Thursday, September 7, 1978, that a Bulgarian émigré by the name of Georgi Markov was assassinated by the Bulgarian secret police.

And what an assassination it was! No guns, no grenades, no knives. Just an umbrella! An umbrella specially designed with a spring-loaded device to deliver a pinhead-sized pellet loaded with one of the most potent poisons known to mankind.

Markov, disillusioned with Bulgaria’s communist ideology, defected to Italy in 1968 and then moved to London in 1971 where he joined the Bulgarian service of the BBC. Here, he began to vigorously attack the Bulgarian government over Radio Free Europe. The Bulgarians weren’t pleased and threatened to eliminate him if his attacks did not stop. Markov was undeterred, at least until that fateful September day. While waiting for the bus, Markov felt a jab in his thigh, turned around and saw a man who muttered a quick apology as he got into a taxi. Markov later recounted that the man was clutching an umbrella.

The Bulgarian expatriate soon started to feel ill and the next day had to be admitted to hospital with a high fever and severe abdominal pains. Within a couple of days he was dead. Since he had told his doctors about the jab in the thigh, Markov’s body was carefully examined. A perforated tiny metallic pellet was discovered exactly where he had indicated that he had been stabbed. Inside the pellet was a residue of a poison known as ricin, the same substance found lacing letters to U.S. President Barack Obama and other U.S. officials in April. Ricin is a protein that is found in the seeds of the castor bean plant, the same plant that yields castor oil.

Castor oil sure conjures up some distasteful memories for me. Probably for many of you as well. It’s a classic old remedy for constipation, and I shudder to recall how I was forced to take it when my mother had diagnosed that I was, let us say, in need of relief. What a horrible taste it had. But it did get the job done. So how come I’m here to tell the tale, given that ricin is one of the most toxic substances in existence? Because fortunately, ricin is not soluble in oil.When the oil is extracted from the castor beans it is washed with water which completely removes all traces of the toxin.

While there is no danger in consuming castor oil, eating the whole seeds can be deadly. Such poisonings have occurred. Since the seeds are very pretty they are sometimes used to make ornaments. In fact, in Mexico, castor beans are used to make jewellery for tourists. There can be serious reactions from eating a bean or crushing one in the hand and then putting that hand in your mouth. The symptoms are bloody diarrhea, vomiting and then shock. So obviously it is not a good idea to eat your Mexican bean jewellery. At least not unless you’ve cooked it, since heat destroys the ricin. But if ricin gets into the bloodstream, the prognosis is not good. There is no antidote to ricin poisoning, as Georgi Markov found out.

Joe Schwarcz is the director of McGill University’s Office for Science and Society. Read his blog at chemicallyspeaking.com.