Written by Agatha Christie fan and research chemist Kathryn Harkup, A is for Arsenic considers the chemical and physiological characteristics of each poison used by the crime writer, which provided vital clues to the discovery of each book’s murderer.


Here, writing for History Extra, Harkup explores the history of arsenic…

Arsenic is a word almost synonymous with poison, and with good reason. Its toxic properties known since ancient times, arsenic has a long history of being used to dispatch enemies, rivals and wealthy relatives. It is said that Cleopatra, when she decided to end her life, was keen to find a painless means that would leave behind an attractive corpse. She tested several poisons on her slaves, including arsenic, and watched the effects before deciding on the bite of an asp, a poisonous Egyptian serpent.

The Borgias

In 15th-century Italy arsenic was favoured by the Borgias, who produced their own unique poison recipe called 'La Cantarella': the guts of a freshly slaughtered pig were liberally sprinkled with arsenic and the resulting mess allowed to liquefy, then dried to leave a pale powder that could be sprinkled on the chosen victim's food. From the Borgias' point of view arsenic had the advantage of having no taste that might alert a victim, and the symptoms produced were similar to those caused by cholera or food poisoning. Any unfortunate deaths after dinner with the infamous Italian family could therefore be blamed on natural causes.

By the 17th century, arsenic's poisonous popularity had spread to France. During the reign of Louis XIV, several members of the French aristocracy were accused of poisoning and of witchcraft. Bumping off rivals and wealthy relatives was said to be particularly prevalent at the time: indeed, arsenic became known as 'poudre de succession' or ‘inheritance powder’.

Rat poison

After such a long, distinguished history, arsenic poisoning was brought to the masses by the industrial revolution. Arsenic is a common contaminant in metal ores such as iron, copper and tin. The 18th and 19th centuries saw a huge increase in demand for these metals. To get rid of the arsenic and purify the metal the ores were roasted, producing fumes of white arsenic trioxide that condenses in the chimney as a waste product.

Industrialists wanting to maximise their profits decided to sell the white arsenic, and its known toxicity meant the most obvious use was in rat poison. When people refer to 'arsenic' they are usually referring not to the pure element but to arsenic trioxide, or ‘white arsenic’. Arsenic is considerably less toxic than arsenic trioxide, but it’s nevertheless a very bad idea to eat it.


Bottles of arsenic and tincture of capsicum, 1921. (Photo by SSPL/Getty Images)

Death by wallpaper

Arsenic also proved to be a very versatile ingredient in a number of other products: it can be combined with sulphur or copper to produce brilliant red, green and yellow dyes. The Georgians and Victorians were extremely fond of the colours red and green, and arsenic compounds were used to decorate almost every conceivable household object. Consequently, people could be poisoned by their clothes, candles, playing cards and even children’s toys.

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Arsenic wallpaper was particularly hazardous to health. The use of green arsenic compounds to colour patterns on wallpaper was of immediate danger to those involved in its manufacture, but also posed a risk in the home.

The problem was not flakes of arsenic dislodging from the paper, but how the paper was attached to the walls: a flour and water paste was normally used, and in a damp climate this provided the perfect growing conditions for mould.

Arsenic compounds are toxic to mould, but some moulds were able to process the arsenic into a gas, trimethylarsine, which released the arsenic from the paper. This resulted in it being inhaled by the human occupants of the room.

It has been suggested that Napoleon Bonaparte may have succumbed to the effects of the green wallpaper in his home on the damp island of St Helena. Analysis of Napoleon's hair and a sample of wallpaper both show high levels of arsenic, but not enough to kill him. However, his decor could well have contributed to his poor health.

Murder weapon

When Victorians weren't accidentally poisoning themselves, they were deliberately poisoning others. Arsenic trioxide was freely available to buy from chemists in rat poisons, tonics or cosmetics. It was cheap and was a perfectly normal addition to any shopping list. Because the vomiting induced by arsenic is likely to remove much of the poison before it can be absorbed into the body, the poisoner often needed to re-dose to kill their victim.

Poisoners in the early 19th century could act with impunity because arsenic was undetectable in a corpse. All this changed in 1832 when James Marsh, incensed after a poisoner walked free from his trial for murder, decided to produce a robust method for detecting arsenic in a body. Eventually he devised the ‘Marsh Test’, which was first used in a criminal trial in France in 1840.

Marsh Test Apparatus, Steel engraving, 1867. (Photo by: Universal History Archive/UIG via Getty Images)

Marsh Test apparatus, steel engraving, 1867. (Photo by Universal History Archive/UIG via Getty Images)
However, given the number of everyday items that contained arsenic, finding traces of the element in a dead body was of no surprise. Indeed, several women accused of poisoning their husbands or lovers explained the presence of arsenic using the ‘Styrian defence’ – that is, the claim that their partner ate arsenic by choice – when they appeared in court.

The background to this was that a number of peasants in Styria – a state now in Austria – were known to spread small amounts of arsenic on their toast two or three times a week, because in small doses arsenic could give women a more curvaceous figure and men more bulk. This is because arsenic causes oedema (fluid retention), and improves the complexion by killing bacteria that cause spots. Indeed, arsenic became a popular tonic or cosmetic to make an individual more attractive.

Advertisement for Fould's arsenic complexion wafers by H. B. Fould in New York, 1901.

Advertisement for Fould's arsenic complexion wafers by H B Fould in New York, 1901. (Photo by Jay Paull/Getty Images)

However, regular small doses of arsenic would accumulate in the body and would prevent a corpse from rotting, because the arsenic had killed the bacteria that would normally cause decomposition. This may have been the source of inspiration for vampire stories as we know them today.

Fast-forward to the 21st century and arsenic murders, thankfully, appear to be a thing of the past. Getting hold of arsenic compounds is considerably more difficult today than it was a century ago, and the tests used to detect arsenic in a corpse have greatly improved over the years.

The ‘Styrian defence’ had mixed success even in the Victorian era. Today, it would be almost impossible to get away with.


Kathryn Harkup is the author of A is for Arsenic (Bloomsbury, 2015). To find out more, click here.