An x-ray machine helped determine the accurate age of the gold bar.
CNN  — 

Almost four decades ago, a gold bar was found on land that used to be Aztec ruins in Mexico. Now, scientists confirm it was part of a plunder by Spanish conquistador Hernán Cortés.

Scientists have been waiting on technological advancements in order to confirm the date of the gold bar.

“The mexica pieces may have come from the treasure of Moctezuma’s ancestors, which the Spaniards found in Teucalco or maybe among the spoils of war stored in royal warehouses in Petlaclaco, gun factories in Tlacochcalco or the crafts workshops in Totocalli,” said Leonardo López Luján, an investigator with Mexico’s National Institute of Anthropology and History.

He explained that researchers used fluorescent X-ray equipment to analyze the gold bar’s chemical composition and match it with other historical pieces discovered in the same area.

The gold bar is  4.35 pounds and was found in March of 1981 in Mexico City.

The 4.35 pounds gold bar was found in May of 1981 in a construction site of what is now Mexico’s Tax Revenue Service offices near Mexico City’s historic downtown. A worker found the bar buried about 15.7 feet deep and gave it to a team of archaeologists.

Luján said the gold bar is at the National Museum of Anthropology and serves as “key material witness of the Spanish conquest and archaeological testimony of the ‘Noche Triste’.”

It is said that Cortés got the gold when he sought out Aztec emperor Moctezuma for his treasure. A year after being welcomed into the capitol city of Tenochtitlán, now Mexico City, he arrested the emperor and demanded all of his gold. Cortés and his men then melted the treasure down into bars.

The “Noche Triste” or “Sad Night” is a battle where Cortés tried to flee Tenochtitlan quietly on June 30, 1520, the night after Spaniards massacred Aztec leaders. The Aztecs learned of their plan and attacked. Some of the gold from Moctezuma’s treasure was lost in the battle.

The area where the gold bar was found is believed to be the location of an ancient canal that Cortés and his troops used to try to escape.

The bar will remain at the museum for visitors to view.