This resulted in blooms and iron products with a range of carbon contents, making it difficult to determine the period in which iron may have been purposely strengthened by carburizing, or reheating the metal in contact with excess charcoal.Carbon-containing iron had the further great advantage that, unlike bronze and carbon-free iron, it could be made still harder by quenching—i.e., rapid cooling by immersion in water.Later the art apparently spread to the Philistines, for crude furnaces dating from 1200 charcoal demanded a high temperature, and, since the melting temperature of iron at 1,540 °C (2,800 °F) was not attainable then, the product was merely a spongy mass of pasty globules of metal intermingled with a semiliquid slag.
Production may have been based on minerals that were not widely available and became scarce, but the relative scarcity of tin minerals did not prevent a substantial trade in that metal over considerable distances.
Arsenic contents varied from 1 to 7 percent, with up to 3 percent tin.
Essentially arsenic-free copper alloys with higher tin content—in other words, true bronze—seem to have appeared between 30 , beginning in the Tigris-Euphrates delta.
The discovery of the value of tin may have occurred through the use of stannite, a mixed sulfide of copper, iron, and tin, although this mineral is not as widely available as the principal tin mineral, cassiterite, which must have been the eventual source of the metal.
Cassiterite is strikingly dense and occurs as pebbles in alluvial deposits together with arsenopyrite and gold; it also occurs to a degree in the iron oxide gossans mentioned above.