Authenticating coins of the 'Roman emperor' Sponsian

Abstract

The 'Roman emperor' Sponsian is known only from an assemblage of coins allegedly found in Transylvania (Romania) in 1713. They are very unlike regular Roman coins in style and manufacture, with various enigmatic features including bungled legends and historically mixed motifs, and have long been dismissed as poorly made forgeries. Here we present non-destructive imaging and spectroscopic results that show features indicative of authenticity. Deep micro-abrasion patterns suggest extensive circulation-wear. Superficial patches of soil minerals bound by authigenic cement and overlain by oxidation products indicate a history of prolonged burial then exhumation. These observations force a re-evaluation of Sponsian as a historical personage. Combining evidence from the coins with the historical record, we suggest he was most likely an army commander in the isolated Roman Province of Dacia during the military crisis of the 260s CE, and that his crudely manufactured coins supported a functioning monetary economy that persisted locally for an appreciable period.

Fig 1. Coin of the ‘emperor’ Sponsian, currently in The Hunterian, University of Glasgow, UK, catalogue number GLAHM:40333.

Reproduced from Ref. [1].

Fig 2. Coins of the wider assemblage, and their typology.

Note that no image exists of the lost Type 1 of Alexander the Great. A) Type 2 of Plautius, Specimen 2.4 in Table 1, greyscale photograph reproduced from Ref. [3], scale approximate; B) Type 3 of Gordian III, Specimen 3.2 in Table 1, GLAHM:29596, photographed for this study; C) Type 4 of Philip I or Philip II, Specimen 4.4 in Table 1, GLAHM:29820, photographed for this study; D) Type 5 of Sponsian, Specimen 5.3 in Table 1, GLAHM:40333, photographed for this study (same as Fig 1); E) Type 6 of Sponsian, Specimen 6.1 in Table 1, greyscale photograph reproduced from Ref. [28], scale approximate; F) Type 7 of Gordian III, Specimen 7.1 in Table 1, greyscale line drawing reproduced from Ref. [29], scale approximate.

Fig 3. Diagrammatic representation of known and likely provenance trails of the wider assemblage.

Yellow = Type 1, green = Type 2, purple = Type 3, dark blue = Type 4, dark orange = Type 5, white with red outline = Type 6, white with black outline = Type 7. Numbers correspond to individual coins of each type as given in Table 1. Orange boxes = public collections, light blue boxes = private collections. Apart from the Glasgow group discussed herein, confirmed coins are Vienna Münzkabinett RÖ 32.227 and RÖ 32.228; Bologna, Museo Civico Archeologico MCABo Num 26424, Bibliothèque nationale de France K 2339 and number unknown; Münzkabinett der Stiftung Schloss Friedenstein Gotha, 1.Co14611 and 1.Co14612; and Brukenthal National Museum, Sibiu.

Fig 4. Variation of silver and copper content of the six coins analysed in this study by SEM-EDX point analyses.

Gordian III = GLAHM:29596; Philip A = GLAHM:29820; Philip B = GLAHM:29821; Sponsian = GLAHM:40333. Different symbols refer to different zones within each coin; each point is a different surface analysis. For each coin, circles = zone 1, down triangles = zone 2, diamonds = zone 3, left triangles = zone 4, right triangles = zone 5, hexagons = zone 6, stars = zone 7, pentagons = zone 8, squares = zone 9, horizontal half-filled circles = zone 10, vertical half-filled circles = zone 11. Coins of the Rome mint have no detectable copper and either low or undetected silver. Data are presented by coin and zone in S5 File and average compositions are given in Table 3.

Fig 5. Evidence for casting.

A, D, E, GLAHM:29821 (the box on E is the area shown in A); B, C, Coin GLAHM:29820 (same as Fig 2).

Fig 6. Light microscope images of exposed upper areas on the obverse of the six coins.

The two unquestionably genuine coins are at the top. A) GLAHM:29540; B) GLAHM:29697; C) GLAHM:29596; D) GLAHM:29820; E) GLAHM:29821; F) GLAHM:40333.

Fig 7. High magnification electron microscope images of exposed upper areas on the obverse of the six coins.

The two unquestionably genuine coins are at the top: A) GLAHM:29540; B) GLAHM:29697; C) GLAHM:29596; D) GLAHM:29820; E) GLAHM:29821; F) GLAHM:40333.

Fig 8. LM images of the obverse surface and edge of Sponsian coin GLAHM:40333.

A) key to highlighted areas; B) detail of ear and cheek showing subcircular pits interpreted as casting air bubbles, within which the metal has an irregular surface where it did not impress the mould, wear scratches and areas of possible wax residue; C) detail of beading and edge showing wear scratches, possible wax residue and cementation spots; D) detail of lettering showing scratches, chipped edges, earthen deposit between letters and cementation spots; E) perpendicular view of edge; F) area of Sponsian’s mouth showing granular patches in recesses and remnant air bubbles; G) detail of large remnant air bubble.

Fig 9. Coin GLAHM:29821 in visible and UV light.

Areas of white fluorescence at the bottom of the bust on the obverse and in the left field of the reverse correspond to areas of reddish deposit, although not all such areas fluoresce as brightly. Note the small area of bright orange fluorescence near the spear point which is discussed in the text.

Fig 10. Examples of small patches on the surface of coin GLAHM:40333 by SEM.

The spots analysed (crosses) all indicated the presence of Si and O with minor contributions of Au from the underlying metal, suggesting that such patches are cementation spots composed of opaline silica. The three spectra (given in S3 File with data in S4 File) show Si = 46 +/-1.2 wt% and O = 54 +/- 1.2 wt%, consistent with silica.

Fig 11. Appearance of larger earthen deposits on a genuine aureus and questionable medallion.

A, B) Genuine Gordian III aureus GLAHM:29540; C, D) questionable Gordian III medallion GLAHM:29596.

Fig 12. r-FTIR spectrum of light-coloured deposits on the obverse of the genuine Gordian GLAHM:29540 (black) and questionable Philip GLAHM:29821 (grey) coins.

The spectra (see arrows for each spot) show gypsum as the predominant compound.

Fig 13. SEM investigation of the earthen deposit on the questionable Gordian III medallion GLAHM:29596.

A) LM image showing the area of investigation (compare with Fig 11D); B) SEM image showing a dark material with brighter flecks overlain by lighter crystalline layer. Crosses indicate points where EDX spectra were taken. Point 1 has a complex elemental profile dominated by S and O. Point 2 is a flake of gold, presumably spalled off the surface. Point 3 appears to be a Ca/K sulphate mineral such as syngenite. Point 4 may be calcite and / or gypsum.