Good morning, This coin is I believe a Parthian Phraates IV Tetradrachm. These are supposed to be silver, but this coin is 52% silver and copper(according to XRF). Is that the common composition of these coins? Just an interest. 12.9 grams 29 mm
@Parthicus should be able to answer that for you. Phraates IV (38 - 2 B.C.) AR Tetradrachm O: Diademed and draped bust left, wart on forehead, wearing pointed beard and torque with no end visible; all within pelleted border. R: BACILE[WS] BACILEWN ARSAKO U EUERGETOU DIKAIOU EPIFANOUS FILELLHN[OS], Phraates seated right on throne, Tyche standing left before him, presenting a palm and holding cornucopiae; year between throne legs, [month in exergue] Seleukeia on the Tigris mint. 30mm 15g Sellwood 51
My immediate subjective impression is that the Parthians started debasing their tetradrachms pretty early on, and they become billon before too long. Searching my library, I have the classic volume "Chemical Composition of Parthian Coins" by Earle R. Caley (ANS Numismatic Notes and Monographs #129), published in 1955. Most of the text deals with the drachms and with various technical issues involved in determining fineness with 1955 technology (no fancy XRF), But in Table XX it does list a tetradrachm of Phraates IV as having a silver fineness of 522 (52.2% silver). This is basically the same as your reading of 52% silver using XRF, so this seems pretty reasonable. So, technically not billon (<50% silver) yet, but pretty close. Parthian tets do soon turn into billon, and stay that way for the rest of the series, but maintained just enough silver to not look coppery until the end of the empire, unlike the case of Elymais, where both tetradrachms and drachms turn from decent silver to billon to bronze, all within the reign of Kamnaskires V.
Here's a Vologases VI showing some light rust = not too much silver wasted on this tetradrachm. Kings of Parthia, Vologases VI, 208-221/2 AD, BI Tetradrachm (25mm, 12.56 g, 12h), mint Seleucia, date is off flan. Obv: Bust left with long pointed beard, wearing diademed tiara with horn and long curved earflap, earring, and necklace; B behind Rev: EΠI (ΕΠΙΦΑΝΟΥΣ = the Glorious/Illustrious), King seated left on throne, receiving diadem from Tyche holding sceptre; [unknown Seleukid date] between Ref: Sellwood 88
