Preserving the look, feel and sound of degrading plastic artefacts presents particular problems

We live with man-made materials all around us, to the point that most of us have long since stopped noticing all of them. It’s true that there’s far more awareness now of waste disposal problems that these can generate, and of methods to recycle and repurpose such objects. The attention paid to how to get rid of these items is fitting, and in many cases long overdue, but there is a little-known problem at the other end of the scale: what if you want these plastics and polymers to stay around? Just like they were when they were made?

Stevie Wonder plays a Hohner Clavinet C

Source: © Ron Howard/Popperfoto/Getty Images

The unique sound of the Clavinet’s rubberised hammers on metal strings was used extensively by musicians such as Stevie Wonder. Some even came to prefer the slightly ‘sticky’ sound as the hammers began to degrade over time

If you are a collector, a museum curator, an archivist or historian, or a musician who admires the sound of mid-20th-century electronic instruments then you may well have already encountered this problem. The earliest synthetic materials date from the 19th century, and ever since then we’ve had an explosion of new polymers and composites with an incredible range of properties. But that leaves you with a similarly incredible number of ways that these items can fall apart. Sometimes you can slow the deterioration by protecting things from light and heat, but there are some polymers that are just going to decompose no matter what you do, and some of them will take other nearby objects down with them.

An extreme case is nitrated cellulose, a key ingredient in the production of celluloid, which was the base of almost all photographic film in the world before the 1950s. As you might well guess, it was dangerously flammable even in the best of times. But on storage over the decades, celluloid turned out to decompose to a sludge that was still flammable but also released toxic gases that would spread the ‘celluloid rot’ to any other items within range. This is not what conservators want to hear. Other common polymers like polyvinyl chloride (non-rigid kinds) and polyurethane can get the ‘rots’ over time as well with similar release of monomer gases and other reactive compounds. Older forms of rubber are known to outgas sulfur compounds that end up as a film of sulfuric acid all over everything else in the museum case, while they themselves crack and crumble. And that’s no good, either.

Celluloid turned out to decompose to a sludge that was still flammable, but also released toxic gases that would spread the ‘celluloid rot’ to any other items within range

Complicating all this is the way that many earlier plastics are simply not labelled with any clues as to their composition or even their broad chemical class. Examination of the physical properties is often not the way to solve that identity problem, either. Famously, you can make many polymers take on properties ranging from elastic bands to motorcycle helmets depending on the ratios of plasticisers, and co-monomers in the mix, as well as the casting conditions. And determining these additives after the fact (and the chain lengths and types of crosslinking) can be a non-trivial analytical chemistry problem in itself. Collectors and curators find themselves trading advice about ancient Bakelite knobs, celluloid buttons, vinyl boots, and polyurethane sculptures, and some of them have taken to re-creating old plastic recipes and trying to artificially age their new samples with heat and light to uncover their hidden problems.

The electromechanical era of device construction has led to some unusual situations. Compared to modern equipment, these were the awkward years, with levers, mechanical relays, and glowing wires co-existing with transistors and early integrated circuits. This almost always meant the use of many different kinds of polymer materials along the way. Take the Clavinet, an electronic keyboard produced in the 1960s and 70s. Pressing a key caused a rubber hammer to strike its own tensioned metal string, and electromagnetic pickups (as on a guitar) modulated the resulting vibrations into sound. Over time, though, the rubber deteriorated and the hammer tips became somewhat sticky, leading to a slightly plucked-sounding delay on the string.

But wouldn’t you know it? Some musicians liked the sound of that effect once it began creeping in. There are Clavinet sounds built into modern synthesisers, as you’d expect, although there are some musicians (as you’d also expect!) who insist that only playing a real one will produce the right sound. But the synthesisers often also have a ‘sticky Clavinet’ setting as well, for people who are seeking to bring back the sound of the perfect level of polymeric deterioration. That’s a challenge that not even museum curators have to deal with: achieving the right amount of rot!