This one was rough. It is difficult for me to rescind my 'there's no way they could have gone to Cornwall' statement, but reading I have done since supports the idea that there may indeed have been Roman Empire-period trade into Cornwall, at the very least, and it is not impossible for Phoenician trade to have occurred there. Why is tin important?
Tin is instrumental in making stronger kinds of bronze. Bronze can also be made as a mixture of arsenic and copper, but the bronze we usually think of is copper and tin. Tin is fairly useless on its own because it is fragile, hard to work, and hard to acquire in useful quantities. Tin is also combined with lead to make pewter. Tin is found on its own as small decorations (bits on saddles, nails and thin sheets on pottery) Once it is found lining a pottery bottle, and a bracelet containing over 90% tin was found on Lesbos, dating to the 3rd millennium BCE. Where does one get tin?
Tin is rarely, if ever, found in metallic pure form in nature. It is only found near certain types of granites, which are not present in most areas of the world. It is found as a part of certain minerals, sometimes in assocsiation with copper or iron, and more commonly as SnO2, or as an oxide called 'casserite'. Casserite is the most common form of 'found tin', the important 'stream tin' which was most likely used the most in antiquity. Stannite, another mineral, is a natural combination of iron, lead, tin and copper. This would be, if worked, a natural bronze, but it is somewhat rarer than other forms of tin-bearing minerals and is not commonly found in the Mediterranean and Near East. Casserite is found in pebbles or small stones, and can sometimes be panned from rivers in association with gold. The pebbles would have a metallic density and might have been panned with gold deposits as a curiosity at first. There is no mention in Mesopotamian texts of association with tin and mining, which lends credence to this panning theory. In Egyptian wadis on the more mountainous eastern side of the Nile, occasional heavy rains bring up casserite pebbles which can be collected after the water drains away again. The only source where tin was possibly mined before industrial times is the western mountains of Spain; by Pliny's time, tin was said to come from the mountains in Iberia. Other possible sources of casserite and other tin-bearing minerals include areas of Turkey, South-Eastern Europe near the former Yugoslavia and Armenia, the area around Lake Van, Byblos, and the island of Sardinia. There are no remaining deposits of tin of note on mainland or the islands of Greece or in the Levant. However, with the way that casserite can be panned, it is quite possible that small local deposits were exhausted. Ancient authors variously place the source of tin as from the north or west, or more commonly from the east, but authors never mention local tin deposits. Tin is always an import. In Egypt, tin comes through Cyprus, and the tomb of Rekh-mi-re shows copper and probably tin ingots being imported by Aegean people. This tin possibly came from Turkey, which is the closest source. Various Near-Eastern sources associate tin with the import of lapis lazuli, which came from Afghanistan, and classical sources associate tin with amber, which may point to a more northern source. The Trade of Tin: Evidence
Evidence of the trade for tin can be found occasionally in written descriptions. The painting in Rekh-mi-re's tomb is disputed as to whether the ingots which aren't copper are of tin at all, instead of lead. Lead and tin, as well as arsenic and antimony are all soft grey metals, possibly confused in antiquity. (There is debate as to what 'annaku' means in Akkadian; the modern theory is that it means 'tin'.) Mesopotamian texts occasionally mention the trade of tin; by the late third millennium bce tin was recorded as passing through Dilmun. (Ebla texts) Other Mesopotamian texts refer to other far-off lands as sources for tin, tin captured as booty from raids into areas now in Eastern Iran, and other eastern, mountainous sources. In Mesopotamia at Mari, the tin trade was highly organized. Tin travelled in the form of approximately 5 kilogramme ingots by land-travel, moving westward as both a trade good and a form of royal gife-exchange. There was massive trade of tin between Assur and Kanesh, resulting in 100 tonnes exchanged over a period of 40-50 years. Texts from Tell-Rimah indicate the metal's source as from the area of Nairi, a region now around the Upper Euphrates and Lake Van, extending somewhat eastwards. The site of Nuraghe in Sardinia has produced a find of 10 kilos of 1-3 cm pebbles of casserite nuggets from the Late Bronze age. This may possibly be some metalworker's 'stash' of casserite to add to copper. There is also evidence of casserite being melted in crucibles at the same site. Two shipwrecks, the Cape Gelidonya and the Ulu Burun, have evidence of tin on board, as well as copper. The 1150 (BCE) Cape Gelidonya wreck, as reported initially by Bass, included evidence of tin probably in ingots stored with copper ingots; time and reaction to the copper have corroded the tin down to mostly oxide again, although it may have been shipped as casserite pellets. Bass advances the idea that between the tools and weights on board, the crew of the Gelidonya wreck may have been 'tinkers', who would carry copper and tin to make repairs to bronze. This ship was smallish, 8-9 metres long, and carried approximately half a tonne of cargo. The copper on board was from Cyprus, and the ship was probably sailing from east to west along the Turkish coast. The Ulu Burun wreck was earlier, around 1300 BCE. The tin ingots found in this wreck bear Cyprian marks, which possibly suggest that Cyprus was a trade-center for metals at this time. The ship holds 10 tonnes of copper ingots and 1 tonne of tin, which is the same as the preferred 10:1 ratio of copper to tin to make bronze. (Most often, bronze has less tin in the Mediterranean, probably because of its rarity.)
Bass, G.F. (1961) The Cape Gelidonya Wreck: Preliminary Report. American Journal of Archeology, 65, 267-276 Forbes, J. R. (1950). Metallurgy in Antiquity. Leiden: E.J. Brill. 231-271 Moorey, P. R. S. (1994) Ancient mesopotamian materials and industries : the archaeological evidence. Oxford, New York : Clarendon Press 297-301 Muhly, James David. (1973) Copper and tin : the distribution of mineral resources and the nature of the metals trade in the Bronze Age. New Haven : Connecticut Academy of Arts and Sciences / Hamden, Conn.: Archon Books Olin, Jacqueline S., Franklin, Alan D., Wertime, Theodore A. (1979) The Search for Ancient Tin. Washington : Smithsonian Institution, Smithsonian Institution Press Penhallurick, R. D. (1986) Tin in antiquity : its mining and trade throughout the ancient world with particular reference to Cornwall. London : Institute of Metals