How hoards are dated

Every hoard has at least two dates. One is when its contents were made, which can span centuries: a Viking-age hoard may hold a Roman coin, a brooch made a hundred years before burial and silver cut up last week. The other is when the whole group went into the ground. That second date is almost never written down. It has to be argued from the objects, the container and the soil.

The starting point is simple logic. A hoard cannot have been buried before the newest object in it was made. If the latest coin can be dated, it gives a terminus post quem, a date after which the burial happened. Coins are ideal because many carry a ruler's name.

The Vale of York hoard, found near Harrogate in 2007, shows how tight this can be. Its 617 coins end with issues of the English king Athelstan (c. 924-939). Many are of his rare 'church' type, probably introduced around 927, the year he took York. Only one is of his Rex Totius Britanniae ('king of all Britain') type, which is common in later finds. The specialists' reading is that the hoard went into the ground just as that type began to be struck, probably around 928 (Current Archaeology).

The Vale of York argument works because a hoard's coins are a sample of the money in circulation at the time, drawn by one person from what was around them. If a type that was minted in huge numbers is absent or rare, the likeliest explanation is that it had not yet reached the owner. Absence is weaker evidence than presence, but in a large group it becomes persuasive.

A big hoard can also show what the circulating pool looked like. Cuerdale, found beside the River Ribble in 1840, held more than 8,600 items, about 7,000 of them coins (Wikipedia). An older account quoted there gives roughly 3,000 struck for a Viking ruler called Cnut, 1,800 memorial pennies of St Edmund, 850 of Alfred, only about 50 of his son Edward the Elder, and just over 1,000 from the Continent. That mix, heavy with new Viking coinage from York and short on Edward's pennies, is one reason most writers put the burial in the first decade of the 10th century, usually somewhere between about 903 and 910.

A coin's date of issue is not the date it was buried. Coins circulated for years or decades, and the amount of wear on the latest coins helps estimate the gap. The Hoxne hoard from Suffolk is the classic case. Its latest coins are silver siliquae of Honorius and the British usurper Constantine III, of types struck before the eastern emperor Arcadius died in 408, so the chest cannot have been closed before 407 or 408. Almost all of its 14,272 silver coins had been clipped, with metal trimmed from the edges (Wikipedia).

Clipping takes time: the coins went on being used, and shaved, in a Britain no longer receiving new coin. The terminus post quem is solid; how long after 408 the hoard was buried is a matter for argument, which is why it is usually described as 'after 407/8' rather than given a single year.

Viking-age hoards often contain silver valued by weight rather than by count: ingots, arm-rings and cut fragments. A cut-up brooch cannot be dated as finely as a coin, but the coins buried with it still set the lower limit. The Spillings hoard from Gotland, found in 1999, holds 14,295 coins, most of them Islamic dirhams, together with hundreds of arm-rings and other silver. The youngest coin identified is a dirham of AD 870, so the silver was buried after that (Swedish History Museum; Wikipedia).

Fragmentation is informative in itself. Silver cut into small pieces belongs to an economy that weighed bullion; silver that is mostly whole suggests a store of ornaments.

When there are no coins, archaeologists fall back on the objects themselves. Brooches, arm-rings and vessels changed shape and decoration over time, and comparison with dated finds places them within a few decades at best. The Galloway hoard, found in Kirkcudbrightshire in 2014, is dated largely this way. National Museums Scotland puts its burial around AD 900 on the strength of its silver bullion and Anglo-Saxon metalwork (NMS).

Style also exposes heirlooms. The lidded silver vessel at the heart of the Galloway hoard turned out, after conservation and analysis, to have been made in western Asia, and it was probably already old when it was buried (NMS; Wikipedia).

Organic material can be dated scientifically. Radiocarbon measures the decay of carbon-14 in anything that was once alive, such as wood, textile, leather or straw packing, and gives a date range for when that organism died. The Galloway hoard shows both the power and the trap. A preliminary radiocarbon date for one of the textiles wrapping the vessel came out at AD 670-780, at least a century earlier than the silver bullion of about AD 900 (NMS). The cloth was old when the hoard was packed. A radiocarbon date tells you when the material was made, not when it was buried, so it only gives a terminus post quem unless the material was fresh at burial.

Dendrochronology, which matches the pattern of growth rings in timber against a master sequence, can give a felling date to the year if enough rings and the outer sapwood survive. It is rarely possible for hoards, whose wooden boxes are usually small and decayed, and it dates the tree, not the burial.

Pots, boxes and bags have their own dates. The Frome hoard of 52,503 Roman coins, found in Somerset in 2010, was buried in a large pot with a small black-burnished ware bowl inverted over its mouth as a lid (Wikipedia).

Careful excavation can reveal how a hoard was assembled. At Frome the coins were lifted in layers. About 85 per cent of the coins of Carausius, the latest in the pot, came from a single layer, which shows they were tipped in as a separate batch (Current Archaeology); one press account notes that the most recent coins lay in the middle of the pot rather than on top (Guardian). The coins themselves run up to Carausius (286-293), and the burial is usually put around AD 290.

A hoard's date is the date it stopped being added to, not necessarily the date its owner lost control of it. Savings hoards could be topped up for years. And the hoards we find are the ones nobody came back for. So the spread of hoards over time reflects reasons for non-recovery as much as reasons for burial.

That is the root of a long debate about Roman Britain. More than 660 hoards are known from Britain containing coins of AD 253-296, a concentration that differs markedly from the rest of the western empire, even though other provinces suffered the same political troubles (CoinsWeekly report on the 'Crisis or continuity?' project). The British Museum and University of Leicester project that studied these hoards argued instead that crises do not in themselves explain hoarding, though they may make recovery less likely. For the late 3rd century it linked non-recovery to inflation and regime change that left the debased coinage of little use, and saw some large deposits, Frome among them, as possible offerings that were never meant to be dug up (BMCR review).