Materials Up Close & Personal: RUBBERS

14 May 2020

Materials Up Close & Personal: RUBBERS

Rubber is a much-lauded material, responsible for the bounce in your ball, the creep in your crepe soles and the waterproofing in your wellies. It’s been the subject of songs (Sesame Street’s glorious Rubber Duckie), featured in poems (AA Milne’s King John’s Christmas), been the muse to various artists (like Eva Hesse and Richard Serra), and has even been the titular character in a film about a tyre that comes to life and kills people with its psychokinetic powers (Rubber, 2011 – I know what I’ll be watching tonight…).

Rubber gets all the attention, the show-off. Rubbers, by comparison, are often thought of as a simple piece of stationery: the little blob we turn to as a last resort when we’ve made a mistake. 

‘But aren’t they the same thing?’, I hear you ask. 

Confusingly, even though rubber-the-material is named after rubber-the-tool-for-erasing-pencil-marks, not all erasers are made from rubber. We have an enormous variety of different erasers for different jobs: sticky putty rubbers for lifting charcoal dust and blotting colour, soft gum rubbers that crumble as soon as you look at them, hard and gritty rubbers that can remove ink but will shred a soft paper surface given half a chance, and those cheap and squeaky rubbers that seem to only smear and make more mess. This profusion of different deleting tools is the result of the variety and versatility of the materials they’re made from. 

The original eraser, developed in 1770 by British engineer Edward Nairne, was made from natural rubber, otherwise known as caoutchouc, latex or gum elastic. Natural rubber is usually derived from the sap of the Brazilian Pará rubber tree (Hevea brasiliensis). In order to make this material, the milky white fluid that fills the tree’s cells (latex) is ‘tapped’ by cutting a gash in the bark of the rubber tree so that it oozes out freely. This flowing vegetable fluid is collected in cups, coagulated by adding an acid (usually formic acid), passed through heavy rollers to squeeze out any excess water, and then dried. The resulting material is waterproof and very stretchy, but in its raw state it also has a tendency to quickly rot and ‘perish’, losing its ability to stretch and becoming hard and brittle as a result of exposure to cold temperatures, light and air. 

The inevitable deterioration of natural rubber can be halted by a process called vulcanization. This process was discovered in 1839 by Charles Goodyear (oddly, no relation to Goodyear tyre manufacturers) and was arguably one of the most important industrial discoveries of the age. Goodyear discovered that by mixing liquid rubber and sulphur and subjecting it to intense heat and pressure during the polymerisation process, he could make the inner structure of the rubber change, making it far stronger and more durable. Named after Vulcan, the Roman god of fire, this hot process allows the normally disparate long chain molecules in uncured rubber to link to each other. Their inner structure goes from being like a bundle of dried spaghetti to a 3D fishing net: geometrically spaced, strong, resilient and elastic in any direction.

That attention-hog natural rubber has taken us off on a tangent again…always craving the  limelight! Now, back to our small friend the eraser. The invention of non-perishable natural rubber allowed it to successfully take over all pencil-rubbing-out duties from its mouldy, short-lived precursor: moist, balled-up bread (which you can still use at a pinch for soft writing and drawing materials). Latex is still used in some rubbers to this day, and can be recognised by its distinctive dusty feel. However, many of the rubbers you’ll find in WHSmith are made from synthetic rubber or plastic. Gradual advances in the manufacture of synthetic elastomers over the course of the 20th century eventually led to much more control over their hardness, elasticity, shape, colour and scent, making possible the novelty rubber that’s shaped like, and smells like, a banana.

Graphite is relatively easy to remove from paper because it slides off the tip of the pencil and sits on the paper’s surface (due to it lamellar structure – for more info go to our Materials Library entry). Ink, on the other hand, soaks into the paper’s fibres, so the only way to mechanically remove it is to scrape away the paper itself. Some rubbers have abrasive powders like glass and pumice (volcanic rock) added to make them aggressive enough to remove stubborn inks. The colour of the iconic rhomboid ‘pink pearl’ rubber and little pink ‘plugs’ of rubber on pencils is said to have originated from the pinkish-red pumice originally used for this purpose.

Some of our most effective erasers are not actually made from rubber, even though they feel rubbery. The classic, white, straight-edged Staelder, for example, is made from a soft vinyl plastic. These plastic rubbers are the firmest of their kind, and leave the least detritus as all the dust clumps together and sticks to the eraser, leaving our pages crumb-free. Brown, semi-transparent art gum rubbers are made from fatice, a vulcanized vegetable oil (often soy, corn or castor oil). Contrary to what we expect from a rubber, these are not very elastic and are designed to crumble easily so that they don’t damage our paper. Soft, kneadable, putty rubbers win the prize for being the stickiest of rubbers. Putty works by absorbing and folding your drawing material into its body, rather than wearing it away. This makes it more suitable for soft materials like graphite and charcoal as it doesn’t leave any small crumbs behind that can smear your work when you sweep them away. Other materials that are popular with art and museum conservators include microporous plastic foams and starch- and silicone-based rubbers.

A piece of putty being used to absorb charcoal markings on paper.

You may be shocked to hear that not everyone is a fan of rubbers. One cognitive scientist from King’s made all the newspapers in 2015 for suggesting they should be banned in classrooms for perpetuating a culture of guilt and shame around errors. To be fair, pretty much every technology has its critics. Umbrellas, bicycles and the printing press all had their share of detractors as well.

In defence of rubbers, erasure is an act as old as writing itself: for example, the Romans wrote on wax tablets or scraped their parchment with knives and washed it in milk so they could start afresh (leaving us with palimpsests). Whilst the ability to erase and rewrite narratives can be used to sinister ends, there are also psychological benefits to being able to forget all our gaffes. As a fan of rubbers, I’d argue that they allow us the freedom to make (and then remove) mistakes as we start to see our written or drawn subject more clearly, helping us to understand the world around us. 

About our blog series 'Materials: Up Close & Personal' 

We are excited to announce a new series of Materials Library profiles. Over the next few months our Materials Librarian Sarah Wilkes will be paying homage to those silent and humble materials that we surround ourselves with daily, but that often we don’t even notice. Every week or so, she will be exploring the expansive inner lives and backstories of the surfaces, substances and stuff around us, and sharing it with our community through our blog and social media #MaterialsLibraryUpCloseAndPersonal.

We would love you to join in too! Send us a picture of one of your favourite household substances with a few words about what you’ve noticed after spending some time getting to know your material cohabitants. A collection of these little gems will go up on our website soon.