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possible machaniem for the format can ba «nvlsag®d us ih® formatlon ot surface apele® wh®n th® arsenie wouid dlffu®® tuto th® copper to balance •cUvltf_r

Advent of tln-bronge

Afcer th® copper-arsenic phase we enter th® truo Bronce Ag®. Th® ua® of copper-arsenic alloys and th® new tin-bronzos goes on band ln hand for a period. Th® tin-bronzes produced rareJy contaln elgnlflcant arsenie and the copper-arsenic alloys ar® usually free of tin. It seems elear, th«rafora, chat the sort of purposeful alloylng that I write of above had become estab-lished, adding selected materials to copper for thelr effecta. In some regions the arsenical phase does not appear to have exlsted (e.g, IreJand) and th® period of alloylng commences directly wi th tln-bronze. Ho w did the effect of tln addltlons become recognlced?

In some aulphldlc copper deposits stannite may occur, but it is falrly rare. It usually contains arsenie and where a copper-arsenic tradltloo was already established could have been selected because of the characterlstlc odour of the arsenie. It al60 has the semi-metallic appearance they were used to in the copper sulpharsenides and arsenopyrlte, both presuraably established as arsenie sources.

However, as stated, stannite ls relatively rare and we are left witb the problea of how they madę the connection to the only other tln minerał avail-abl®, cassiterite, available partlcularly ln placer deposits assoclated with granit es and pegmatites at high concentrations.

Cassiterite al60 occurs in the gossans of some copper deposits. Tln oxide is lnsoluble and is left behind by the weathering and leaching process often in the wood-tin form. Of a generally brown appearance with no flamę colour, it may first have been identlfied with a maglcal flux in association with the brown iron oxide. This wouid initially introduce a Iow level of tin, but theo obserwation and deduction wouid lead to the separate component which madę all the difference.

Identifylng cassiterite is not difficult if you are prepared to handle the materiał: it is extremely dense and therefore heavy. Together witb its lnertness, this results in it being concentrated from the breakdown of weathered rock in existlng or ancient but now dried stream beds. Once knowlng what to look for they wouid have already known where it was from the earller mining for gold. Furthermore, in the same placer deposits is arsenopyrlte, already suggested as a possible source of arsenie ln the earller phase,

There remalns, of course, the direct approach. Perhaps an ancient soith. already using arsenopyrlte collected from a placer deposit or working the gold, just declded to try these very dense brown Stones as a surface addltion to his cruclble surrounded by charcoal just to see what happened - for the smlth mlght be regarded as the earliest sclentist experimenting on the basi6 of that lnnate curiosity which ls part of the human character, partlcularly developed ln skilled craftsmen and the modern-day sclentist.

Tin oxide can be reduced relatlvely easlly directly to tin, although with substantlal losses to the slag phase unless the furnace is very strongly reduclng and the temperaturę ls high, not an easy combination to achieve ln a prlmltlwe furnace. Some few tln artlcles have survived from the Bronce Age. but these are not as numerous as one wouid expect if tln were always added to copper as tln metal ln the alloylng procedurę. Some tin lngots have been

found la the Moditerranean, but tbay ar#    uk<mm. There are v«y

l#w records of #ny Lin motał being found 1« ar.y Broara A«a foyi.ders hoarde, although there la aJways bulfc copper and tcrap brooia artlfacta, Kayba Che urna J J er amount of metallJc tln, the vltal lrigredient, vm always kapt on blf per eon, •• his most valuable poaaaaaloa. But parbapa tba a r-chaeologlsts dli not rccognlM tba brown atonca that wlghr hav« ba ar. naar tba hoard - tba cae-ilttrlt* ■ for tba aslth could perfectly wali bave added cha tln cbla wy, cruablng lt and puttlng It lato tba copper aurfoca undar charcoal, kaductlaa ls, ln fact, morę aaally achieued la tba preser.ca of coppar, sińce tba copper ln dlaaolvlng the tln away from the reactlon (decreaslng lta Chemical actlvlty> increases tba thermodynamlc drlulng forca for tba reductlon reactlon and lt la achieyed at lowar temperaturę.

Ecllpse of copper-arsenic alloys

Eventually, after about 700 years at most, copper arsenie alloys wara no longer employed, and tin-bronze alone was madę. Why? Technlcal reasons may be advanced. Once the arsenates were exheusted in deposits the additioo of arsenie from sucb materiale aa enargite, tannantlta and arsenopyrite wouid involve substantial losses of arsenie ln the rcasting atage of heatlng, and the volatllity of arsenious ozide wouid make control of arsenie content diffi-cult. Once the arsenates were no longer available and once the lead to tin-bronze had become establlehed, perbaps the somewhat better propertlea of tba tin-bronze and certainly its consistency from measured additions of cassite-rite wouid have eventually won the day.

Personally 1 believe that lt was almcst an evolutionary changę, based not only on the inereasing difflculty of finding arsenical alnerals and inconsistent results, but also on that very garlic odour ueed to deteralne the mineralogy. Of the Greek gods ln the Komeric record only Hephaestua tbe the smith was imperfect: he suffered with a llap, and was a bit of a Joke, presumably the mark of smiths generally - and terminal neuritis is a sympton of arsenie poisoning.

Eventually, the smiths concentratlng on tin-bronze wouid be clearly seen to be in better health and live to greater ages tkań those aaklng the terrible smell! Eventually understandIng wouid work ltself lnto the smiths* codę, Just as we now recognize the dangers of working with carclnogenic materiale. la later history alchemists in particular were fasclnated by arsenie la potlona and clearly a tradltion of its strange effects on huaan beings had been handad on.

The change from bronze to Iron

We have establlshed that smali ąuantities of iron were produced in the Bronze Age and that, eventually, selective smelting for Iron blooms wouid have been posslble with high enough temperatures, as in the primltive Afrlcan iron furnaces of today. Levels of metallurgical skill had been achieved ln the Bronze Age which make tbe6e presenc Iron Age aaiths, who ln some lnstances have no social status in their tribes, look posltively neolit hic. For eitample, the Mlnoans invented 'Sheffield Platę', diffusioa bonding silver to copper at each end of the rivets holding the handle to the tang of a dagger. The Nycenaeans bonded both silver and gold to bronze as inlay oa che faaous sbaft grave daggers, presumably also by diffusion bonding. The addition of lead to bronze for the marked effect it haa on caatability was identified as a metallurgical technikum of particular value in the production of thin wallad castings.