Updated: Aug 18, 2021
While researching the manufacture and use of fish sauce in the ancient world, we came across an article published in 2008 that claimed: 'Remains of rotten fish entrails have helped establish the precise dating of Pompeii's destruction' .
Nearly 2,000 years ago, Mount Vesuvius erupted burying Pompeii and Herculaneum, two ancient Roman towns on the Bay of Naples, in volcanic rock and ash. Covered in between 3 m and 6 m of debris, the existence of these towns was completely forgotten within a few generations. In the 18th-century, however, Pompeii was discovered having lain undisturbed and protected from the elements since AD 79. Extensive excavations soon began, and Pompeii and Herculaneum have given us some of the most precise knowledge of how a Roman city looked and how its inhabitants lived.
In 2008 it was reported that seven jars, unearthed some years before in the house of Aulus Umbricius Scaurus, had had their content analysed. Given that Scarus was Pompeii's most famous fish sauce producer, it should come as no shock that the jars contained Roman garum. Moreover, the desiccated remains found at the bottom of the jars revealed that the last Pompeian garum was made entirely with bogues, a Mediterranean fish species that abounded in the area in the summer months of July and early August .
According to Annamaria Ciarallo, director of Pompeii's Applied Research Laboratory: 'Analysis of their contents basically confirmed that Mount Vesuvius most likely erupted on August 24th, AD 79, as reported by the Roman historian Pliny the Younger in his account on the eruption.'
For the last five hundred years, articles about the eruption of Vesuvius have typically agreed that it began on August 24th. Moreover, this date is seemingly confirmed by a 1508 printed version of a letter between Pliny the Younger, an eyewitness to events, and the Roman historian Tacitus, which was written some 25 years after the eruption. The problem, however, is that hand-copying of the original letter before 1508 may have led to the date being corrupted. Manuscript experts believe that the date originally given by Pliny was either August 24th, October 30th, November 1st, or November 23rd ; an imprecise set of dates caused by how the Roman calendar was calculated. Regardless, given that the August date had been reproduced in the majority of extant medieval manuscript copies  it was accepted by most scholars and incorporated into nearly all books written about Pompeii and Herculaneum for the general public.
Since at least the late 18th-century, a minority among archaeologists and other scientists have suggested that the eruption began after August 24th, during the autumn, perhaps in October or November. Some of the evidence for a later date include:
In 1797, for example, Carlo Rosini reported that excavations at Pompeii and Herculaneum had uncovered traces of fruits and braziers indicative of an autumn date, not a summer one.
The remnants of more autumnal fruits (such as pomegranates) were discovered by archaeologists in 1990 and 2001. About the same time, it was reported that the remains of victims of the eruption were noted wearing heavier clothing more appropriate for later in the year. Furthermore, large earthenware storage vessels laden with wine at the time of their burial by Vesuvius implies the eruption occurred after the year's grape harvest and wine making.
In 2007 a study of prevailing winds in Campania  showed that the south-easterly debris pattern of the 1st-century eruption is quite consistent with an autumn event, but inconsistent with an August date. During June, July, and August, the prevailing high-altitude winds flow to the west meaning the volcanic debris would have been carried in the wrong direction to bury Pompeii.
Two coins, from early in Emperor Titus' reign (June 24th, AD 79 to September 13th, AD 81), were found in a hoard recovered at Pompeii's House of the Golden Bracelet. Although the coins' minting dates are somewhat in dispute, a numismatic expert at the British Museum, Richard Abdy, concluded that the latest coin in the hoard was minted on or after June 24th (the first date of Titus' reign) and before September 1st, AD 79. Abdy states it is 'remarkable that both coins will have taken just two months after minting to enter circulation and reach Pompeii before the disaster' .
In October 2018, Italian archaeologists uncovered a charcoal inscription in new excavations. The scrawled inscription, likely made by a worker renovating a home, is dated 16 days before the 'calends' of November in the old Julian calendar. By our modern dating method this becomes October 17th. Given that the inscription is unlikely to be have been a year old in AD 79, it sets the earliest possible date for the eruption .
The signs for an October eruption are stacking up but how to explain the evidence from the garum jars? If the bogues were caught in August, the fermentation process by which fish sauce is made requires several weeks of macerating in the sun. Andrew Dalby states this process would take about two months , which again fits well with an eruption in October. So, despite Annamaria Ciarallo’s 2008 discovery, the August 24th date is most likely incorrect meaning Pompeii and Herculaneum were probably devastated sometime after October 17th, AD 79.
1. Lorenzi, R. (2008), 'Fish Sauce Used to Date Pompeii Eruption', Discovery News, (accessed July 24th, 2020).
2. The eruption froze the sauce right at the moment when the fish was left to macerate. No batches of finished garum were found, since the liquid evaporated in the heat from the eruption.
3. Berry, J. (2013), 'The Complete Pompeii', London: Thames & Hudson, p. 20.
4. There are no surviving Roman copies of Pliny’s letter.
5. Rolandi, G.; Paone, A.; De Lascio, M.; Stefani, G. (2008), 'The 79 AD eruption of Somma: the relationship between the date of the eruption and the southeast tephra dispersion', Journal of Volcanology and Geothermal Research, 169 (1), pp. 87-98.
6. Abdy, R. (2013), 'The Last Coin in Pompeii: A Re-evaluation of the Coin Hoard from the House of the Golden Bracelet', The Numismatic Chronicle 173, pp. 79-83.
7. BBC News, (2018), 'Pompeii's destruction date could be wrong', (accessed July 24th, 2020).
8. Dalby, A. (2003), 'Food in the Ancient World from A to Z', London: Routledge, pp. 156-157.