The first time I dug up some vintage denim, I had no idea what it was worth. It just looked like some old rags, so instead of carefully uncovering it, I pulled on it and tore it to pieces. I’d actually been digging for antique whisky bottles, and what I didn’t know then was that those “rags” were likely worth thousands.

Out in the desert in California, Nevada and Arizona, there are abandoned silver mines like buried time capsules, virtually untouched, and you can find vintage bottles down there that are worth a lot to collectors. But as I searched for them, I kept coming across these scraps of denim, because jeans, especially Levi’s, were worn by the silver miners in the late 1800s. When a miner got a new pair of work pants, he’d cut up the old ones and use them for lagging around pipes, so there were a lot of antique jeans buried out here.

I did a bit of research into the history of Levi’s, and realised collectors would pay a lot for them, even for scraps. I started going to the mines regularly with my father-in-law, a geologist, to look especially for denim. But the mines have been covered in rocks and trash, so it can take weeks and months to dig down, lifting big rocks one by one. We crawl around wearing hard hats and head lamps. We don’t tunnel underground, just dig through rocks inside the mine opening, but it can be dangerous: we’re crawling around on probably 100 tonnes of unstable rocks.

The first time, we knew we’d dug down far enough when we found bits of newspaper dating back to the 1800s. We have found scraps of handwritten letters and even some hand-drawn antique pornography. It’s legal for anyone to go in there. One guy did try to copy us, but he used a tractor to dig and it didn’t really work.

I put a few of the denim items I’d dug up on eBay. A Japanese collector contacted me and came all the way out here to look at my collection in person. I sold him a jacket for $1,000. At the time, it seemed a good deal, but he told me not to talk to other people or tell them what I was doing; I realise now that he didn’t want me to find out how much these things were worth. I talked to other dealers and collectors, and found out he was selling the pieces back to Levi’s for its archives – he’d sell them a pair of jeans for upwards of $100,000.

Only a few people are authorised to sell directly to Levi’s, so my father-in-law and I decided to strike out on our own. I started reading everything I could about denim history and it became a sort of obsession.

I’m not focused on the money – I’ve become a sort of archivist and historian. With my father-in-law, I’ve started publishing books about denim history. I work as a commercial painter and a beekeeper, and denim mining is really more of a hobby. But some pieces are worth so much, it’s hard to justify keeping them.

Experience: foraging nearly killed me Read more

A couple of months ago we found just a pocket, which is definitely worth something, but I’ll keep that for my collection. Then last week we found a complete pair of jeans. When you find something like that, it’s an enormous thrill. It’s what we’d been working so hard for: 95% of the time you never find anything complete.

This pair were actually by Neustadter Brothers, who were as big as Levi’s back in the day. They’re from the early 1890s and in good enough condition to wear. We were so excited. We sold those immediately on eBay for $21,000. We could have got more, but my wife and I talked it over and decided to settle for that. The most I’ve sold a pair for is $30,000 to a guy in Osaka, Japan.

A few years ago, my father-in-law dug up the holy grail: the oldest pair of Levi’s from 1873, the first year they were manufactured. They’re in really good condition – they look like a normal modern pair of jeans, really, only back then, they had a crotch rivet and no belt loops. I wish we could keep them for our personal archives, but recently I had an offer of about $100,000. My father-in-law doesn’t want to sell them and neither do I, but I have two daughters to put through college, so they might have to go.

• As told to Antonia Blyth.

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