Amazing rare postcards reveal a vanished world

In 1971, browsing in a French flea market, the art dealer John Kasmin found the postcard that turned him from a fan into a serious collector. It was made in 1906 by a Normandy printer from a photograph of two figures in a bare room. One is a man in an ermine-trimmed suit, the other is a bear, wearing a frock. They both wear rollerskates and appear to have just completed a slalom around some cider bottles. Both the man and the bear – whom the card names "Lolotte" – look exhausted and glum. "It is," the veteran gallery owner says, "just the most wonderful thing."

Postcards don't figure much in today's Instagram and Snapchat-obsessed world. In the 1970s, 27% of Britons sent them when on holiday; now only 2% do. The number of cards sent through the US postal service has dropped by more than a third in the last five years.

But they are the honourable ancestor of all digital photography apps: the first technology that gave everyone access to cheap images – and the ability to write about them. Like the three-minute pop song, the postcard was an artefact shaped by the technology available when it was invented, but that now seems perfect for what it does.

Kasmin has many thousands of cards – he doesn't know the exact number. Most are from the first quarter of the 20th century; a time, he says, when millions of cards were posted in the US and Europe every week. In Britain, posting them cost half as much as did a letter. So printers churned out cards of every conceivable eye-catching subject, from circus acts to car crashes and studies of poverty. Many cards were home-made: Kodak used to sell amateur developer's photographic paper pre-printed with a postcard form.

Some of the most intriguing treasures in Kasmin's collection date from the First World War, when German servicemen used the Zeiss cameras they were equipped with to send mementoes home. There's a tragic group of Senegalese prisoners of war, African faces solemn or crazed amid the mud and death. Another is a trench-side sculpture made of stacked shell cases and horses' skulls.

As long as there have been postcards there have been collectors, and some are very rich: the international market is dominated by Leonard Lauder, heir to the Estée Lauder cosmetics fortune and owner of 125,000 cards or more. How did Kasmin come to join them? "I had been cheerfully spending £5 here on a postcard, or occasionally £10. It was another cheap way of enjoying yourself," he says. "I collected views of places I'd been, and project them on the wall at home. Then, one summer, I was in the flea market in Rouen and I saw this – the roller-skating bear and his friend – in a stamp dealer's shop. I asked the price and it was €300. So I didn't buy it. Then, for a whole year, I was describing this card to people and talking of the crazy price, until I realised it had lodged a tremendous impression in my brain. So the next year, back in Rouen, I went in and bought it. I was hooked. I just thought, if that's where postcards can be, I don't care what I have to pay. I'll just sell some things."

Some of the "things" Kasmin mentions are almost priceless, as you'd expect from a man who was friend and gallerist to David Hockney and Anthony Caro and who used to tour the darker parts of the world fossicking for antiquities with the writer Bruce Chatwin. His west London flat is dotted with simple, beautiful objects acquired over six decades of exploring and dealing.

But the postcard collection is something else. Now 80 years old, Kasmin has amassed thousands of cards, each one carefully catalogued in a series of huge volumes. The catalogue titles are a tour of his mind and its fascinations: "Wheels", "Catastrophes", "Size," "Beggars", "Enigma", "Steps, staircases and ladders", "Chairs". "Chairs" is largely pictures of empty armchairs. Why those? "I like the hunger and self-importance of empty chairs." Hunger? "Hungry for bums."

Leafing through the albums with him takes half a day, a time punctuated by anecdotes and cries of joy as he finds old friends. "Look at that! The sort of thing you could have looked at for hours, when we used to smoke spliff." Or: "This is my best catastrophe..." It's a sepia shot of two men contemplating the tangled wreck of an early motorcar, which appears to have hit a telegraph pole. Behind them billboards advertise chewing gum and "Gold Dust" washing powder. "Everyone loves advertising," muses Kasmin.

Is his greatest joy the hunting down of a card? Or the sharing? "It's very exciting when you find it, but I get a particular pleasure out of showing it. I hardly ever take the albums out and look at them alone." David Hockney is among many friends who, over the years, have come to inspect the newest acquisitions. "Occasionally he would look at them when he was short of a subject, but he only ever made one painting inspired by a postcard – a picture of the Japanese inland sea."

Kasmin is, he says, what professional card dealers dismiss as an "aesthetic collector" – interested in the "look of the thing", the half-told narratives and unintentional beauties in an image. Pristine items – unwritten-upon, unposted – actually fetch more in the fastidious world of postcard collection. But Kasmin prizes the human detail. He's fascinated by the banality of messages on remarkable cards. One from 1907 shows a leper being wheeled in a box by his wife through the Brittany town of Paimpol. On the back a daughter has written to her parents a mere "Love to both of you".

Many serious collectors seem a little autistic. "Real fusspots," says Kasmin. Less interested in stories, they want perfect, unmarked cards and complete sets. There are odd fans around too: "Some only want erotica, others only want coastal shipping. The most expensive cards at American fairs are those featuring murderers or lynchings ... I met rather a strange-looking young fellow at a fair, very well dressed, come all the way from Texas. He collected male figures in wet bathing suits, those old-fashioned woolly, itchy things."

Talk of the death of the postcard may be premature. After all, the demise of radio was universally predicted when TV invaded middle-class homes half a century ago. Yet, it holds on. Like the postcard, its form of communication is unique enough to keep a niche. And the 21st century is catching up – there are apps now that let you take a photo with a smartphone, and then send it as a postcard by snail mail.

Artists continue to use postcard formats to work and many are intrigued by the postcard's possibilities. A current exhibition at London's Barbican shows postcards from the collections of artists including Peter Blake, Andy Warhol, Damien Hirst and the photographer Martin Parr. Parr's own interest is the dramas of everyday life, and over several decades he has amassed 20,000 postcards that celebrate the extraordinary. They include a lot of catastrophes – mining disasters, railway accidents, epidemics – a category the Victorians appear to have particularly enjoyed. Among them is a photograph of a lace-up boot with a ragged hole near the little toe. The hand-written caption reads: "The actual boot the lad was wearing on Thursday 24 June at Ringmore when struck by lightning. A horse being felled at the same moment not more than a yard away."

As so often, the information you really want – did the boy survive? – isn't there. But postcards of the aftermaths of tragedies, starving children and even corpses tell something of a cultural shift: our great-grandparents were less coy about inquisitiveness. What now might seem bad taste was then a matter of proper public interest.

But it is the card-buyers' tolerance for the banal that makes some of the most famous commercial collections of cards. Parr's best-selling book is of his colourised cards from the middle of the last century, featuring hotel swimming pools, farm equipment and newly-opened motorways: it is published under the frank title Boring Postcards.

Kasmin's postcards aren't dull: they're moving, sometimes tragic, and replete with stories. He has already published one volume of them, with the Royal Academy, featuring tramps and beggars – a favourite subject for early 20th-century photographers with the first portable cameras and he is in talks with other publishers about more books.

But the best way to see Kasmin's postcards is in an armchair with the man himself.