Design By Nature: The Origin of the Rubber Sole

Design By Nature is an ongoing GC series in which Garden Collage explores aspects of modern clothing design that were inspired by plants. This week, GC investigates the origin of the rubber sole– a modern development in shoe-making attributable to Amazonian tribes in the 1500s, which came from the milky sap of Hevea brasiliensis— also known as the rubber tree.

In addition to claiming to “discover” many lands that were already inhabited by natives, European explorers also stumbled upon a myriad of sartorial innovations when they arrived.

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Once such example was the discovery of rubber soles– more specifically, of rubber shoes, which were later tailored and modified by Europeans after they discovered them in the Americas in the late 1500s. The people of the Amazon rainforest wore rubber shoes made from the milky sap of a special tropical tree that’s known today as Hevea brasiliensis, or the sharinga tree (or, even more commonly, “the rubber tree”).

To make their footwear, indigenous people tapped the gummy white sap of the rubber tree and dipped their bare feet into it before drying them by the fire. When the sap hardened, it formed a rubbery protective “sneaker” that made rough treks through the dense rainforest less damaging to their feet.

Today, we know much more about the properties of rubber. The milky latex extracted from the Hevea brasiliensis tree, which occurs in latex vessels in the bark, mostly outside the phloem, is the world’s primary source of natural rubber. These latex vessels spiral up the tree in a right-handed helix that forms a 30 degree angle with the ground. Rubber trees can grow up to 100 feet tall, and gummy white latex can be extracted at heights up to 45 feet.

The rubber tree requires a tropical or subtropical climate, and grows primarily in a 700-mile area flanking either side of the Equator. This region of the South American rainforest is known as the “rubber belt”– an area that receives a minimum of about 1200 mm of rainfall a year.

When tapping a tree for latex, harvesters make incisions across the latex vessel at a depth shallow enough to tap the vessels without harming the tree or impeding its growth (tapping starts when a tree is about 7 years old). Ideally, the incisions are made at a 25-30 degree angle from the horizontal, from high on the left side to low on the right. This cut exposes the maximum number of latex fibers per length of incision, which in turn maximizes the rubber yield.

Much like the process of maple syrup production, the tree is tapped and a bucket is hung below the spigot to collect the gummy white resin.

Tapping needs to be timely, as it cannot be done in the rain– one frost can also cause the rubber from an entire plantation to become brittle and prone to breakage once it has been refined, which is why attempts to reproduce rubber trees outside of the Amazon often prove unsuccessful. In 1873, european explorers attempted to grow H. brasilensis outside of Brazil, and after some effort, 12 seedlings were germinated at Kew Gardens in England– though they died as soon as they were sent to India for cultivation.

Today, increasing development and deforestation in the Amazon threatens the rubber tree population, which is already undergoing genetic selection to meet our rapidly-expanding global demand for rubber. Latex production in individual rubber trees declines with age, so most rubber trees are felled when they reach the age of 25 to 30 years. Since the modern world’s reliance on rubber already outstrips the capacity of our rainforests to supply it, rubber farmers have developed new standards for the sustainable harvest of this vital natural resource, which continues to line the soles of our sneakers today.

Watch below to find out how the rubber tree is tapped:

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