Extraction methods of the time, Mr Isdale explained, meant only half of the gold could be extracted from the hard quartz encasing it underground. But the Government was eager to keep mining going and increase the extraction rate for good economic reasons.
Gold production from Thames mines topped one million pounds sterling at its height, recording a yield of bullion with a present value of $2.5 billion.
When a meeting was held in Thames over the opening of a new mining school in 1885, soon to be one of 30 in New Zealand, the hall was so packed some men were hanging from the rafters.
What became the Thames School of Mines, opened in a Sunday school building built on a sacred urupa, remained a cornerstone of the town for over 70 years.
Three years after the school closed forever to mining students, John Isdale stepped inside for the first time as a 7-year-old boy.
The time capsule he entered - musty desks, creaky chairs, high rafters, beams of sunlight streaming through tall window panes on to long kauri floorboards - has changed little.
Old solution bottles remain lined up along a long wooden shelf and a wide blackboard stretches across the head of a classroom that hundreds of young men once passed through.
Mr Isdale particularly remembers the smell he first recognised inside the darkened old timber building, now one of Thames' most treasured heritage sites and open to the public through the week.
As a curator, he delights in visitors recognising it also. "You can smell history - it's a different smell."
Mr Isdale's father, also a historian, had battled to save the school from demolition, working alongside the local chamber of commerce to keep it going.
When a Canadian mining company offered to use the complex as a base, the school survived - but it wouldn't be the last threat.
In the 1970s, it came within just seven days from being flattened when another mining company, this time from Australia, stepped in.
The New Zealand Historic Places Trust finally acquired it in 1979.
Few schools of mines buildings survive today, and of them, the Thames complex is considered the best preserved and most authentic.
If his father's stories hadn't saturated a young Mr Isdale in Thames' heritage, it was everywhere else around him. Treks through the bush would end stumbling on to old mining roads, and pig hunting was made more exciting by hundred-foot-deep shafts.
Mr Isdale worked a variety of jobs around Thames, but it was almost inevitable he'd take over the job of looking after the school of mines.
"Always, this has kind of been there, while my mother and father were alive, they basically took a life-long interest in the place, and they kept the place up and open."
He believed it "vital" that a slice of history so important to New Zealand's heritage and identity remained for people to see.
"It would be lovely to think about the site and all its layers of our history passing on down the generations, but when you're aware of the threats and times when the very existence of these unique historic buildings have been on a knife edge, you can't guarantee anything," he said.
"It's not going to be here forever, but we want it to be here for as long as humanly possible."
