Ask athletes what goes into Olympic gold medals, and they will likely say sweat and years of training. For Brazil’s National Mint the answer is simpler: recycled silver.

What most people don’t know is that the gold medals in Rio de Janeiro are nearly 99 percent silver. They contain just 1.2 percent gold, mostly used as plating.

The three types of Rio 2016 Olympic medals are made at the Casa da Moeda do Brasil, the Brazilian mint, in Rio de Janeiro, Brazil

The Rio 2016 Olympic medals are pictured at the Casa da Moeda do Brasil (Brazilian Mint) in Rio de Janeiro
REUTERS/Sergio Moraes

Each Olympic gold medal weighs around 17.6-ounce, or about 500-gram. They contain 1.2% gold.

A worker from the Casa da Moeda do Brasil (Brazilian Mint) checks a Rio 2016 Paralympic medal in Rio de Janeiro
REUTERS/Sergio Moraes

“It’s a great honor and a great responsibility,” said Victor Hugo Berbert, head of medal-making, as he showed Reuters around the mint in Rio de Janeiro.

Nelson Carneiro, craftsman from the Casa da Moeda do Brasil (Brazilian Mint) works on the Rio 2016 Olympic medal mold in Rio de Janeiro
REUTERS/Sergio Moraes

Nike, the winged goddess of victory in Ancient Greece, is minted on one side below the five Olympic rings, while the discipline for which the medal has been won is engraved along its edge. The other side bears the Rio 2016 logo.

A sculptress from the Casa da Moeda do Brasil (Brazilian Mint) works on the Rio 2016 Olympic medal at her computer in Rio de Janeiro
REUTERS/Sergio Moraes

Each of the 5,130 Olympic and Paralympic medals takes about 48 hours to make, said Berbert, who has an 80-strong team working shifts around the clock.

Workers from the Casa da Moeda do Brasil (Brazilian Mint) prepare the Rio 2016 Olympic and Paralympic medals in Rio de Janeiro
REUTERS/Sergio Moraes

The medals are the most sustainable in Olympic history.

A worker from the Casa da Moeda do Brasil (Brazilian Mint) pours molten metal into a mold to prepare the Rio 2016 Olympic and Paralympic medals in Rio de Janeiro
REUTERS/Sergio Moraes

The gold is free of mercury, which is often used to separate gold from ore and can poison local ecosystems if not carefully disposed of.

A worker from the Casa da Moeda do Brasil (Brazilian Mint) shows a Rio 2016 Olympic medal in Rio de Janeiro
REUTERS/Sergio Moraes

Much of the silver is recycled from old mirrors and X-ray plates.

A worker from the Casa da Moeda do Brasil (Brazilian Mint) prepares a Rio 2016 Paralympic medal in Rio de Janeiro
REUTERS/Sergio Moraes

The attention to detail is incredible.

A machine works on a Rio 2016 Olympic medal at the Casa da Moeda do Brasil (Brazilian Mint) in Rio de Janeiro
REUTERS/Sergio Moraes

Each medal is carefully inspected for flaws

A worker from the Casa da Moeda do Brasil (Brazilian Mint) varnishes a Rio 2016 Olympic medal in Rio de Janeiro
REUTERS/Sergio Moraes

The exhaustive effort shows in the final product: perfectly made Olympic gold medals

A worker from the Casa da Moeda do Brasil (Brazilian Mint) takes out gold-plated Rio 2016 Olympic and Paralympic medals in Rio de Janeiro
REUTERS/Sergio Moraes