The island owes its name to the homonymous founder Siphnos, son of Sounios. An Ionian colony "of Athenians" (Herodotus VIII 48, 4-5), Siphnos was renowned in Antiquity for its wealth, which evidently came from exploiting the mines of gold and silver ores (Herodotus III 57, 4-6; Pausanias X 11, 2). This prosperity, mainly in Archaic times, is reflected in the excellent quality of the Siphnian silver coins of the period, such as the staters with the head of Apollo on the obverse and an eagle on the reverse. Impressive too are the Archaic marble walls of the acropolis at Kastro and, above all, the marble treasury with the superb sculptured decoration, which the Siphnians dedicated in the sanctuary of Apollo at Delphi in 525 BC (Pausanias X 11,2).
In the late sixth century BC the Siphnians were obliged to pay 100 talents to Samian refugees who plundered the island. Later (during the Delian League), the Athenians exploited the resources of the island. Gradually, the metal ores became rare and the mines were abandoned, leading the island into decline. Indeed, tradition has it - although this is not confirmed by archaeological finds to date - that the sea flooded the galleries in the mines, on account of Apollo’s rage because the Siphnians no longer paid the due tithe of their income to the god (Pausanias X 11.2). However, the reasons for the islands decline must have been more complex, and even today scholars disagree over what happened and when. An attack by Cretans pirates, most probably in 153 BC (Diodorus Siculus XXXI 45), led to further decline, though a number of sarcophagi and the grave goods of imperial times from the ancient cemetery at Kastro, suggest that the Siphnians still enjoyed relative prosperity during the Roman Age.
Several ancient mines have been located on the island, but none can be associated securely with the extraction of gold (the mine at Aghios Sostis was for silver). Some of the numerous round towers seem to be linked with mining and metallurgical activities. Some ca. 70 towers, dating mainly from the Classical period, have been identified in all, a considerable number for such a small island. The use of these constructions remains uncertain: some must have served as phryktories (for bonfire signals), others for protecting the rural population and the harvest. However, it is not known whether the towers were included in a state-organized defence network.
Stephanos of Byzantium refers to two further cities in addition to Siphnos -which is usually identified with Kastro -, Apollonia and Minoa, but so far these have not been identified with certainty. It has been suggested that the citadel of Aghios Andreas could be identified with Minoa, but this is no more than a hypothesis. The site lies in the middle of the southern part of the island. It was an important fortified citadel of the Mycenaean period, founded in the second half of the thirteenth century BC (Late Helladic IIIB period) and was inhabited until the end of the Bronze Age (12th century BC). The site was therafter deserted for several centuries and was settled anew towards the end of the Geometric period (second half of 8th century BC). Life continued there uninterrupted into Hellenistic times.
Prehistoric Siphnos is little explored. An Early Cycladic cemetery (3rd millennium BC) has been investigated at Akrotiraki, a hillock south of Platys Yalos, and a significant Late Mycenaean installation (12th-11th century BC) has been located on the steep rocky height at Froudi tis Baronas.
The precipitous and naturally fortified site of Kastro, on the east coast of the island, has been identified convincingly with the ancient city of Siphnos, the asty of Herodotus, which was inhabited already in prehistoric times, as surface sherds of Middle Cycladic pottery (first half of 2nd millennium BC) indicate. At several points on the acropolis imposing marble walls of the Late Archaic period (late 6th century BC) are preserved, in some places to a height of 4.5 m. The schist fortification wall enclosing the city is dated most probably to Hellenistic times. Very few building remains have come to light, mainly of the Geometric-Early Archaic and Hellenistic periods, since the city of historical times lies beneath the modern village. The ancient cemetery was in continuous use from the seventh century BC into Roman times (most of the excavated graves are of this last phase) and extended over the south slope, opposite the city, in the Seralia Valley. At the northwest end of the acropolis a deposit of a sanctuary, of Geometric and Early Archaic times (700-550 BC), was found. The divinity wordhipped may have been Artemis Ekbateria, mentioned by Hesychius. Hesychius also records the cults of Apollo Enagros and Pythios (the second epithet is attested in Hellenistic inscriptions, and of Zeus Epibemios, who is represented on Siphnian coins. Another sanctuary has been found in excavation, in the south corner of the acropolis of Siphnos, and is dated to the late sixth century BC. The agora and a prytaneion would hevebeen built of Parian marble (Herodotus III 57,16-17). Inscriptions record that there was also a theatre, where drama contests were held in Classical and Hellenistic time, and a sanctuary dedicated to Dionysos.
Important work has been conducted on the island of Siphnos by the Ephorate of the Cyclades, by Zozi Papadopoulou and Christina Televantou, as well as by other researchers (see recently Πρακτικά Γ' Διεθνούς Σιφνιακού Συμποσίου, Σίφνος 29.6-2.7.2006, Athens 2009). The ongoing surface survey of Siphnos has already identified a host of new sites (many new towers among them), which show that the island was flourishing throughout Antiquity and had relations with the wider Aegean region and the Greek Mainland. Z. Papadopoulou has conducted a new study of the Kastro, especially its fortification walls (Z. Papadopoulou, Σιφνίων Άστυ, Athens 2002?; id., Νέα στοιχεία για την ακρόπολη του αρχαίου άστεως της Σίφνου, in Πρακτικά Γ' Διεθνούς Σιφνιακού Συμποσίου, Athens 2009, 41-56; see also N. Kourou, Πότνια και Εκβατηρία. Παραλλαγές της Λατρείας της Aρτέμιδος στη Σίφνο, in Πρακτικά B' Διεθνούς Σιφνιακού Συμποσίου, Σίφνος 27-30.6.2002, Athens 2004, 227-242), while Athena Tsingarida is currently studying afresh the old finds of the British excavations at the site, including the material from the votive deposit of the acropolis, usually associated with Artemis. Ch. Telavantou completed an important excavation and restoration project at the inland fortified acropolis of Ay. Andreas. The new excavations there led to the discovery of an "urban" sanctuary, presenting several building phases from the Geometric to the Classical period. The excavator suggests that here too Artemis Εκβατήρια was also worshipped (C. Televantou, Ακρόπολη Αγίου Ανδρέα Σίφνου. Οι πρόσφατες έρευνες, in Πρακτικά Γ' Διεθνούς Σιφνιακού Συμποσίου, Athens 2009,23-40).
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