They work like any manhole cleaners, except that they are much more thorough.
They don't leave a speck of mud on the hole wall, and even bring home all the cleaning water used.
These men from the Mekong Delta province of Hau Giang were in Ho Chi Minh City for a "pig" hunt, the code word for their job of collecting years of waste from a gold shop's drain, according to a Tuoi Tre newspaper report.
Khuong, who led the group to work at a manhole on Pham The Hien Street on the city's outskirts, said the process of making gold objects emits gold dust that will sticks around the house.
The goldsmith's clean it all up at the end of each day and the gold dust flows into the manhole system designed for the shop. The owners have carefully added filters to keep most of the dust for themselves, but they still hire out a major cleaning job to "pig" hunters every two or three years, he said.
"It looks dirty but it's all money."
Khuong estimated the amount of gold he'd take from the shop by scooping mud from the hole into a bowl, adding some water and shaking the bowl to reveal a little line of glittering gold.
He said he might earn 1.3 ounces of gold, so he paid the owner 0.95 ounces worth of money. Khuong said rookies usually make losses as they cannot estimate the quantity of the gold well, and even experienced hunters still make mistakes sometimes when they fail to recognize that the gold is of low quality.
After cleaning the manhole, Khuong and his group then sailed their boat -- along with all the waste they collected at the site -- back to their gold cooking base in Hau Giang.
Khuong poured each spoon of the dust into a clay pot with soda, borax and graphite powder, and put the pot on a pit oven of red-burned coal.
The heat dissolved the dirt while the metals including gold stuck to the graphite powder to form a small blackish gray mass overnight.
"This is a mixture of gold, silver, copper, lead, zinc, iron and aluminum. I will separate lead first," Khuong said as he turned a torch and lit its liquidified gas to burn the lead.
Then he put the mass into a glass bottle and added a solution of nitric acid (HNO3), a highly corrosive substance which dissolved all other metals.
What remained was reddish-brown gold powder. It was then melted and rolled up into the final product: a piece of shiny gold.
Khuong says his group's method is at the "intermediate level" because they can collect 95 percent of gold from a manhole.
A group run by a man named Long in the nearby Vinh Long Province is considered "primary level" because they use liquified gas fire instead of pit oven for the first stage, which produces much more smoke and stink.
It causes more pollution and only collects 90 percent of the gold, a source said.
The most advanced methods, according to the pig hunters, can collect 99 percent of the gold, but it is also the most dangerous way because it uses cyanide.
"You need utmost care when using this. You'd better do it far from residential area," said Hai, who operates an "advanced" group in the delta hub of Can Tho.
Hai built a cement tank in a deserted area where he mixes everything he collects from a "pig" hole with a cyanide solution.
After three days, most matter is dissolved and some more chemicals are used to separate the gold.
Hai then uses a hydrochloric acid solution (HCl) to treat the cyanide and pours the mixture into the ground, believing that "it will be all gone after sometime."
He said he knows the method is dangerous to his own life, but "you're born with the job, you'd die with it."
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