Numismatic palynology represents a convergence of archaeology, botany, and numismatics, focusing on the identification of microscopic pollen grains trapped within the surface layers of ancient currency. At the Vindolanda archaeological site in Northern England, researchers have utilized this discipline to analyze silver denarii minted during the reign of Emperor Hadrian (117–138 CE). The primary objective of these studies is to detect traces ofVitis vinifera(common grape vine) to determine the extent of viticulture near the northern frontier of the Roman Empire.
The process of numismatic palynology at Vindolanda involves the extraction of palynomorphs from the patina and recessed surfaces of coins that have remained in anaerobic or semi-sealed archaeological strata for nearly two millennia. By identifying specific flora associated with these artifacts, scientists can differentiate between local agricultural products and those imported via military supply chains. This research provides a high-resolution window into the environmental and economic conditions of Roman Britain, particularly regarding the acclimatization of Mediterranean crops to the temperate climate of Northumbria.
In brief
- Location:Vindolanda, a Roman auxiliary fort just south of Hadrian's Wall in Northumberland.
- Artifacts:Silver denarii dating to the 2nd century CE, specifically the Hadrianic period.
- Target Taxa:Vitis vinifera(grapevine) and associated Mediterranean cereal grains.
- Primary Methodology:Ultrasonic cavitation and polycarbonate filter-based acetolysis for pollen recovery.
- Research Goal:Mapping the northern limit of Roman viticulture and validating macrofossil evidence from refuse pits.
- Scientific Impact:Refinement of archaeological dating and reconstruction of ancient trade routes through botanical markers.
Background
Vindolanda is one of the most thoroughly excavated sites in the Roman world, renowned for its anaerobic soil conditions that preserve organic materials, including the famous wooden writing tablets. While macrofossil evidence—such as grape seeds (pips) and wood fragments—has long suggested the presence of wine in the Roman military diet, it was often unclear whether these items were imported as processed goods or grown locally. The study of numismatic palynology offers a unique perspective because coins, as mobile objects, accumulate pollen both at the point of minting and throughout their circulation history.
During the Hadrianic era, the Roman presence in Northern Britain reached its zenith with the construction of Hadrian's Wall. The logistics of maintaining a permanent garrison of thousands of soldiers required a sophisticated agricultural infrastructure. Historians have debated the northern limits of Roman viticulture, with some suggesting that vineyards reached as far north as the Humber or even the Tyne during climatic optimums. Detecting pollen on coinage found in primary archaeological contexts provides a localized record of what was blooming in the immediate vicinity of the fort at the time the coins were lost or cached.
Methodology: Extraction and Analysis
The extraction of pollen from coinage is a delicate procedure necessitated by the minute quantities of material available. Unlike soil samples, which may contain thousands of grains per gram, a single denarius may only yield a few dozen palynomorphs. To prevent the destruction of the coin's historical value while ensuring maximum recovery, researchers employ a multi-stage laboratory protocol.
Ultrasonic Cavitation and Cleaning
To dislodge pollen grains from the complex bas-relief of the coins—such as the hair of the imperial portrait or the letters of the legend—the coins are submerged in high-purity, deionized water. They are then subjected to ultrasonic cavitation. High-frequency sound waves create microscopic bubbles that implode against the surface of the metal, stripping away the granular patina and trapped organic matter without damaging the underlying silver. This method is particularly effective for removing desiccated or fossilized material that has become embedded in the oxidation layers of the coin.
Acetolysis and Microscopic Visualization
Once the washings are collected, the samples undergo differential centrifugation to concentrate the organic fraction. Polycarbonate filter-based acetolysis is then applied; this chemical process uses an acetic anhydride and sulfuric acid mixture to dissolve the non-pollen organic matter, leaving behind the durable exine (outer shell) of the pollen grains. This process enhances the ultrastructural visualization of the grains under a microscope.
Microscopic examination typically utilizes phase-contrast and differential interference contrast (DIC) microscopy. DIC microscopy is essential for discerning the subtle features ofVitis viniferaPollen, which is characterized by its tricolporate aperture morphology and specific exine ornamentation. By calibrating objectives to high magnifications, palynologists can identify whether the pollen is contemporaneous with the coin's circulation or is a modern contaminant.
Vindolanda Findings and Macrofossil Comparison
The analysis of denarii found at Vindolanda has revealed a distinct pollen assemblage that includes not onlyVitis viniferaBut alsoOlea europaea(olive) and various cereal types. To validate these findings, researchers compare the numismatic data with macrofossil evidence recovered from 2nd-century refuse pits (anaerobic middens). While the refuse pits frequently contain grape pips and skins, the presence of pollen on the coins provides evidence of the living plant's proximity to the site.
| Evidence Type | Finding | Interpretation |
|---|---|---|
| Macrofossils | Grape seeds and stalks | Consumption of grapes or wine production remnants. |
| Numismatic Pollen | Vitis vinifera grains | Potential local flowering or handling of coins by vintners. |
| Cereal Pollen | Triticum (wheat) | Local grain processing and storage. |
| Patina Analysis | High mineral concentration | Evidence of long-term burial in specific soil chemistry. |
A critical challenge in this study is determining the source of theVitisPollen. Because denarii were minted in Rome or other major imperial centers before being shipped to the frontiers, some pollen may have adhered to the coins during their initial production or transport through Gaul. However, the concentration of specific pollen types on coins found in the same stratigraphic layer as local agricultural tools suggests that at least some of the botanical material was deposited locally.
Mitigating Modern Contamination
A significant concern in Northumbrian archaeology is the infiltration of modern agricultural runoff into historical strata. Modern fertilizers and current farming practices can introduce recent pollen into deep soil layers through groundwater movement. To ensure the integrity of the 2nd-century samples, the ultrasonic cleaning protocols are paired with rigorous control samples taken from the surrounding soil at the time of excavation.
If the pollen found on the coins matches the modern "pollen rain" of the region (dominated by modern grasses and commercial crops), it is discounted. The identification ofVitis viniferaIs particularly telling because commercial viticulture is not currently practiced in the immediate vicinity of Hadrian's Wall, making the presence of ancient, desiccatedVitisGrains a strong indicator of historical, rather than modern, origin. Furthermore, the degree of fossilization and the state of the exine often allow researchers to distinguish between ancient grains and modern intrusions.
The Northern Limit of Viticulture
The presence of grapevine pollen on Hadrianic denarii at Vindolanda contributes to the ongoing discussion regarding the "northern limit" of the vine. While traditionally thought to be restricted to the southern parts of Britain, the findings suggest that the Roman military may have attempted to establish small-scale vineyards or at least maintained decorative vines within the protected microclimates of the fort’sVici(civilian settlements). This would align with the Roman practice ofRomanitas, where the cultivation of Mediterranean staples was a key component of cultural identity and status among the officer class.
The data suggests that the Roman climate optimum—a period of warmer and more stable weather between 250 BCE and 400 CE—may have allowed for a shift in the agricultural frontier. The reconstruction of ancient trade routes also benefits from this data; if the pollen is found to be non-local, it tracks the movement of the coin from a specific Mediterranean region to the northern frontier, providing a tangible link between the economy of the core and the periphery of the empire.
What sources disagree on
Archaeologists and palynologists remain divided on the interpretation of low-density pollen counts on coinage. Some researchers argue that the presence of a few grains ofVitis viniferaIs insufficient to prove local cultivation, suggesting instead that the pollen may have been airborne and traveled significant distances, or was introduced through the handling of wine-soaked sponges used in coin cleaning or storage. Others contend that the specific association of the pollen with the mineral patina of the coins proves they were embedded during the period of the coin's active use in the 2nd century, supporting the theory of localized, albeit limited, viticulture.
Furthermore, there is a debate regarding the efficiency of ultrasonic cavitation. Some conservationists express concern that even low-power ultrasonic waves might accelerate the micro-fracturing of silver alloys that have undergone significant leaching (depletion gilding). However, the prevailing view in the field of numismatic palynology is that the scientific value of the extracted data outweighs the negligible risk to the artifact’s structural integrity, provided the protocols are strictly monitored.
