Authenticating coins of the ‘Roman emperor’ Sponsian

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.


Introduction
Four coins of the Hunterian collection were analysed non-destructively by µ-FTIR in reflection mode. Each coin, laying on a polystyrene support, was placed under the microscope of a benchtop FTIR instrument, a Nicolet iN10 FTIR AutoImage microscope. A cooled mercury-cadmium-telluride (MCT) detector was selected. The parameters chosen for the analysis at high spectral resolution were: 22 sec, 64 scans, 4 cm -1 resolution, 100x100 µm aperture, 4000-675 cm -1 spectral range, Beer-Norton strong apodization. No further spectral correction was applied after the collection of each spectrum. Point analysis was carried out on both sides of the coins to characterize the main organic and inorganic compounds present at surface. Points of interest were chosen based on the results of the microscopic investigation by visible and UV light.

Position and intensity of the main IR bands at Point and their intensity
Discussion: the analysis indicates the presence of gypsum and an organic substance. The presence of an organic substance, not better characterized, is also hypothesized due to the bands at 2919, 2850, 1463, 1415 cm -1 . These bands might be attributed to calcium distearate [11], possibly coming from a cleaning activity. Calcium stearate is a carboxylate salt of calcium, classified as a calcium soap. The salt is a component of some lubricants, surfactants, as well as many foodstuffs. It is a white waxy powder.

S.6.5.2 Reverse side
Two areas were selected for study on the reverse of Coin GLAHM:29821 as shown in Figure  S  The first of the two spectra was obtained from an area of bright orange fluorescence in UV light which had tentatively been identified as shellac resin (Supplementary information 2 Section S.2.5). This is highlighted in Figure S.6.18. The spectrum obtained is compared with a reference for shellac in Figure S.6.19 with an interpretation in Table S.6.6 below.  Discussion: The analysis confirmed the presence of shellac resin, possibly an adventitious spot on the coin because the substance is common in museum environments and was not detected elsewhere on any of the studied coins. Also present are minor carbonates and possibly an organic substance such as calcium distearate.
A spectrum was also obtained from the area of a mineral grain seen in Figure S Figure S. 6.20 Discussion: Sulphates are present, possibly different from gypsum like jarosite (KFe3(SO4)2(OH)6) [13] as indicated by the combination modes in the region 2200-2000 cm -1 and the small, sharp bands at 1193 and 1080 cm -1 . An organic component, not better characterized, is also present, as suggested by the sharp bands at 2955, 2918, 2850 cm -1 , the doublet at 1576 + 1540, and the band at 1470 cm -1 , which might be attributed to calcium distearate. This composition is very similar to that found for the 2 spectra in area B (not shown).

S.4.6 Coin GLAHM:40333 (Questionable Sponsian medallion), obverse
Three areas were examined as shown in Figure    The spectrum from point 2 on a light coloured patch below the emperor's nose is shown in Figure S.6.24 beside a reference spectrum was wax. The main absorption bands are given in Table S

Position and intensity of the main IR bands at poin2 and their interpretation
Discussion: The above spectrum shows bands assigned to wax. Other organic bands are present, specifically the doublet at 1576 + 1540, and the band at 1470 cm -1 , which might be attributed to calcium distearate. Minor carbonates and oxalates. This composition is highly compatible with the species detected from spectra 1-2 in the 3 rd area analysed, near Sponsian's ear. Silicates are also present, as magnesium silicate hydroxide, talc, which might have come from gloves used to handle the coins.  The main absorption bands from point 1 in this area are given in Table S