Bronze Age craftspeople tempered steel more than 1,000 years before the Romans did it

Intricate 2,900-year-old engravings on stone monuments from what is now Portugal in the Iberian Peninsula could only have been made using steel instruments, archaeologists have found. The discovery hints at small-scale steel production during the Final Bronze Age, a century before the practice became widespread in ancient Rome.

The 5-foot-tall (1.5 meters) rock pillars, or stelae, are made of silicate quartz sandstone and feature carvings of human and animal figures, weapons, ornaments and chariots. 

"This is an extremely hard rock that cannot be worked with bronze or stone tools," Ralph Araque Gonzalez, an archaeologist at the University of Freiburg in Germany and lead author of a new study describing the findings, said in a statement. "The people of the Final Bronze Age in Iberia were capable of tempering steel. Otherwise they would not have been able to work the pillars." Tempering is the process of heat-treating steel to make it harder and more resistant to fracturing.

The team also analyzed an "astoundingly well preserved" iron chisel that dates to around 900 B.C. and was unearthed in the early 2000s from a site called Rocha do Vigio in Portugal, the researchers wrote in the study, published online Feb. 10 in the Journal of Archaeological Science. Not only did the chisel contain enough carbon to be considered steel (more than 0.30%), but the researchers also found iron mineralization within the settlement site, suggesting that craftspeople may have sourced the material locally.

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"The chisel from Rocha do Vigio and the context where it was found show that iron metallurgy, including the production and tempering of steel, were probably indigenous developments of decentralized small communities in Iberia, and not due to the influence of later colonization processes," Araque Gonzalez said.

The researchers worked with a professional stonemason to imitate the ancient engravings with tools made from different materials, including bronze, stone and a tempered steel replica of the 2,900-year-old chisel. The steel instrument was the only one able to carve the rock, according to the study. A blacksmith had to sharpen it every five minutes, however, which suggests craftspeople from the Final Bronze Age knew how to make carbon-rich, hardened steel. 

The team also noted that the experimental carvings were remarkably similar to the original ones if they accounted for rock weathering.

Up until now, the earliest record of hardened steel in Iberia was from the Early Iron Age (800 to 600 B.C.). Widespread steel production for weapons and tools probably only began during Roman times, around the second century A.D., although the low carbon content of excavated objects points to their mediocre quality. It wasn't until the late medieval period that blacksmiths across Europe learned how to achieve high enough temperatures to make good quality steel.