Material Matters: Vulcanised Rubber Sole Construction
A world without good rubber would be a world without sneakers. People have used natural rubber for many thousands of years, but on its own the tree juice doesn’t cut it for the daily abuse of the shoe sole. There were plenty of attempts to mix additives into or process natural latex and create a superior compound, but it wasn’t until the mid-nineteenth century that a couple of clever minds came upon our modern day method – after that it didn’t take long before the vulcanised sneaker established its place in the annals of footwear history.
The ancient Olmec people of South America were the first to discover how to make use of the natural latex from the Hevea tree – they would make a small incision in the bark of the tree and collect the milky substance that dripped out. The milky white goo would be boiled to produce rubber balls, which were used to play the traditional Mesoamerican Ballgame – examples of these have been discovered dating as far back as 1600 BCE. The techniques were passed around the ancient world, eventually earning the Olmec people’s name, which in the Aztec language means ‘rubber people’. Although the earliest civilisations to make use of rubber, rubber plants can be found in temperate climates all around the world and have been used for all kinds of traditional purposes. These days the majority of the world’s natural rubber is cultivated in Asia, which accounts for about one third of the world’s production.
In the Western world rubber remained an exotic curiosity until relatively recently. The Aztecs had long since used rubber to waterproof fabrics, but it wasn’t until 1819 that an English coach-builder named Thomas Hancock had the same idea – it’s said he was thinking about materials that could protect the passengers on his coaches from the elements. Hancock continued experimenting with rubber, while at the same time an American named Charles Goodyear was cooking up some rubber of his own. Neither one knew it at the time, but they were both about to stumble upon something revolutionary. Hancock submitted a patent for vulcanised rubber to a UK patent office in 1844, eight weeks before Goodyear submitted his in the US. Although Goodyear tends to be the person credited with the discovery, nowadays the common view is that Hancock had managed to refine his technique earlier and rightfully claimed his patent first.
The original method of vulcanisation, and still the most common, is to mix sulphur with natural rubber and then heat it. The sulphur changes the chemical structure of the rubber, creating crosslinks between polymers to make them stronger and more durable. The name of the process is derived from Vulcan, the Roman god of fire, in reference to the heating process that pushes the whole reaction along. The resulting rubber has increased elasticity, resistance and is less perishable than its natural predecessor.
Vulcanised rubber was an obvious choice for shoe soles. It provided more grip than leather and wouldn’t weaken in the wet. The earliest use of rubber soled shoes goes all the way back to 1876, when a company in the UK began making plimsolls for croquet. In 1892 there were companies in the US making rubber-soled shoes with canvas uppers, which quickly grew in popularity among athletes and holiday-makers.
The perfected formula for vulcanised sole construction is simple, the rubber outsole is placed on the base of the shoe, then a strip called the foxing is wrapped around the midsole to hold it all in place – the same arrangement is still used these days. The assembly is done before rubber has completely cured, then once everything is attached, the entire shoe is placed in a vulcanising oven to finish off the process.
One of the main consumers of vulcanised rubber from the 1890s was the automobile industry. As the vehicles became more and more common the need for rubber seals, belts, hoses and – most of all – tires grew exponentially. In 1909 the first breakthrough on synthetic rubber was made by a group of scientists in Germany, as they discovered polymers that could be synthesised from petroleum by-products. By 1910 Russian scientists had synthesised synthetic rubbers suitable for small scale commercial production – which were utilised when the World War I kicked off and a global rubber shortage took hold. Once the war was over synthetic rubber faded into the background, vulcanised natural rubber was still the superior product. Brands like Converse and Keds sprung up after the war and the public embraced the vulcanised rubber soled sports shoes for their new-found spare time. In the 1920s the demand for rubber was so high that Henry Ford built an entire town in Brazil, named Fordlandia, with the intention of farming rubber plantations for the raw materials needed to furnish his cars – though this turned out to be a spectacular failure.
As the athletic footwear industry flourished, more and more brands joined the ranks. While the 20s ticked on, Charles ‘Chuck’ Taylor was promoting basketball around America and with it he spread his choice of shoes, the Converse All Star and Non-Skid. The Dassler brothers, who would go on to be the founders of adidas and PUMA, rose to fame in Germany during the 30s making leather track shoes – most famously the 1936 track spike of Jesse Owens – but it wasn’t long before they were creating indoor sports shoes with vulcanised soles.
The second world war saw the introduction of rubber soles to military uniforms for the first time. After the war ended, the rise of youth culture and a broader desire to forget the recent horrors lead to a boom in the popularity of spectator sports, which went hand-in-hand with demand for the footwear that the pros used. The surfing revolution that hit the West Coast during the 50s and 60s meant the world was primed for a brand like Vans – still one of the most recognisable producers of vulcanised soles – who set up shop in 1966. Vans shoes were adopted by early skateboarders and are still used by many, thanks to their grip and the feeling they offer of the board underfoot.
Due to the construction process which requires the whole shoe to be heated up, synthetic materials are no good for the uppers of a vulcanised shoe – they tend to melt. As athletic shoes began making use of lightweight nylon and EVA foam during the 70s, the vulcanising process lost its place at the pointy end of athletic footwear. Luckily, by that point the style had become a casual mainstay and these days the canvas or leather uppers of a vulcanised shoe offer welcome respite from the teched out craziness of the modern runner. It’s impressive that one of the original materials which lead to the rise of the sneaker is so relevant today – you just can’t beat a winning formula.
Material Matters is our weekly tech section, where we take a peek behind the mesh curtain and break down the building blocks of the industry. Recently, we’ve looked at Nike Flyknit, adidas Boost Technology and 2016 Olympic tech.