Re: Why was bronze used before iron

The archaeology of ancient metalworking and smelting has a long way to go 
before findings can be explained and understood.  In the distant past, 
metal was was a rare and valuable commodity, so that worn out metal 
objects were recycled rather than discarded in rubbish tips - which is 
where archaeologists get their most useful information.  Although we have 
many metal objects in museums, they are mostly from burials, and may not 
therefore be "everyday" objects.  Also, museums generally do not take 
kindly to requests from archaeologists to cut up ancient metal objects, 
limiting opportunities for investigation by microscopy of the smithing and 
casting techniques.

It is not easy to determine exactly where ancient peoples obtained their 
metal ore, because primitive mine workings are usually destroyed by more 
advanced techniques developed later and used at the same mines.  It might 
have been possible to trace metal objects back to their source ores 
through analysis of unique isotopic signatures, but recycling has mixed 
the metals from various sources and disguised the signal.

Even the smelting processes are unclear because kilns were usually 
temporary structures made of clay, leaving very little trace.  Often, the 
only evidence left for the archaeologist to analyse is slag, the waste 
product composed of various minerals which melted and separated from the 
metal during smelting.  Slag is a notoriously variable substance, 
containing a complex mixture of elements and giving surprisingly few clues 
as to the temperatures reached or the fluxes and other ingredients of the 
smelting.

Nevertheless, archaeologists can show conclusively that bronze was used 
for millennia before the discovery of iron, and that ancient peoples must 
have been skilled in the smelting of copper, tin and iron, as well as 
other metals.  I think that two factors contributed to the earlier 
discovery of bronze.  Most importantly, copper can be found "native" - in 
other words as lumps of pure metal needing no smelting, so that its 
existence must have been known about long before the development of true 
metallurgy.  The other factor is the ease with which tin can be smelted.  
It has a relatively low melting point and separates easily from the ore.

Pure copper is of limited usefulness, being quite soft, and the discovery 
that adding about 10% tin (or other metals such as antimony and arsenic) 
increased its hardness significantly was a major step forward.  Advanced 
smithing such as hammering, heating and quenching of bronze, as with 
steel, increases its hardness.  Unfortunately, tin ore and native copper 
are both rare, so that the production of bronze in the past was a 
remarkable feat requiring many advanced techniques and co-operation 
between people in widely separate areas.  The finished product must have 
been a source of wonder - this was a "new" substance, created from 
unpromising-looking raw materials by the art - or perhaps the magical 
powers - of the metalworkers.   Some archaeologists believe that the 
colour of the bronze may have been important, perhaps identifying the home 
area of the owner.  Proportions of tin and antimony found in ancient 
bronze vary widely, high proportions giving a more silvery colour.

Despite the hard work and co-operation required to make bronze, it was 
still a more obvious route to metal tools than ironworking.  The smelting 
of iron ore requires very high temperatures and the skilled use of various 
mineral additives.  The iron initially obtained from the kiln is virtually 
useless, requiring hammering at high temperature to purify it, and then 
the addition of small amounts of carbon to turn it into steel and regulate 
its hardness according to the proposed use.  
{Admin note: Crude cast iron actually contains much more carbon than you 
would want in a good quality steel. The real problem is how to remove 
undesirable impurities, including most but not all of the carbon. 
See Chemistry of steelmaking. In the overall process, 
there is a net loss of carbon, not an addition.}
Some peoples, notably the Saxons of the 8th and 9th centuries, were 
extremely skilled in the production of various grades of steel, often mixing 
two or more types in one blade to give both flexibility and a hard edge.  
Much evidence of these techniques has been discovered recently in 
excavations in York and Southhampton.

Although steel is difficult to produce, once it had been discovered its 
vastly superior qualities in most applications was a great incentive to 
learn the techniques required.  Not only that, but the virtually ubiqitous 
distribution of iron ores meant that steel production was under the 
control of local populations and did not require long-distance trade in 
mysterious raw materials.  It was a great leap forward, but also, as with 
all technological advances, could be used to advance the technology of 
warfare.  The Roman army consumed iron (and therefore also the fuel 
required for smelting - charcoal) in huge quantities.  There are immense 
deposits of iron slag from their operations covering many acres near 
Worcester and thousands of tonnes of this were "mined" as raw material for 
the early blast furnaces of the industrial revolution.  The 
word "metalling", meaning creating a hard road surface, comes from the use 
of such slag for road surfaces.

Bronze was probably used before iron because the raw materials and their 
extraction methods are more "obvious".  Iron (in the form of steel) 
rapidly replaced bronze as soon as it was discovered because of its 
superior qualities and because the inhabitants of even a small village 
could control the whole process of production from local raw materials.

For a useful summary of ancient Egyptian metallurgy, see http://nefertiti.iwebland.com/trades/metals.htm