Fresh Paint Or Patina Of Ages, That’s The Antique Question

July 30, 2021 by Jenny List 46 Comments

The world of antique furniture and the world of hackers rarely coincide, and perhaps the allure of the latest tech is greater for most of us than that of a Chipendale cabinet. But there are times when there are analagous situations in both worlds, so it’s worth taking a moment to consider something.

This late-17th-century dressing box would not be of such value or interest were a restoration to strip it of its patina. Daderot, CC0.

Antique furniture has survived for hundreds of years before being owned by today’s collectors. Along the way it picks up bumps and scrapes, wear, and even the occasional repair. Valuable pieces turn up all the time, having been discovered in dusty attics, cowsheds, basements, and all sorts of places where they may have been misused in ways that might horrify those who later pay big money for them. Thus there is a whole industry of craft workers in the field of furniture restoration whose speciality lies in turning the wreck of a piece of furniture into a valuable antique for the showroom.

The parallel in our community if you hadn’t already guessed, can be found in the world of retrocomputers. They are the antiques we prize, they come to us after being abused by kids and then left to languish in a box of junk somewhere. Their capacitors are leaking, their cases may be cracked or dirty, and they often possess the signature look of old ABS mouldings, their characteristic yellowing. This is caused by the gradual release of small quantities of bromine as the fire retardant contained within the plastic degrades under UV light, and causes considerable consternation among some retrocomputing enthusiasts. Considerable effort goes into mitigating it, with the favourite technique involving so-called Retr0bright recipes that use hydrogen peroxide to bleach away the colour.

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A Deep Dive Into The Chemistry Of Retrobright

January 23, 2021 by Dan Maloney 53 Comments

Considerable effort is often required to rejuvenate the yellowed and grungy plastic cases of retrocomputing gear. One generally does well to know their enemy in order to fight it, though, which is where this guide to the chemistry of plastic yellowing and whitening (PDF) comes in handy.

“The Retrobright Mystery” was written and sent in to us by [Caden Xu], a high school student who also goes by the alias [Saltypretzel]. The paper begins with an excellent description of the chemistry of plastic yellowing. We had always heard that the yellowing in ABS, or acrylonitrile-butadiene-styrene, the plastic most commonly used for cases back in the day, was primarily caused by brominated compounds added to the plastic as flame retardants. It turns out that’s only a minor contributor, with the bulk of yellowing occurring thanks to a complex chain of reactions starting with free radicals liberated from the butadiene copolymer through a reaction requiring oxygen and energy.

Reactive radicals from the decomposing synthetic rubber, added to ABS to increase its flexibility, unroll the benzene ring in styrene copolymers to form a conjugated compound called 2-hydroxymuconic acid. The alternating double and single bonds in this compound tend to absorb light towards the blue end of the spectrum strongly, so the accumulation of 2-HMA in the plastic over time thus makes it reflect more and more yellow and red wavelengths, giving aged ABS its unhealthy bronze glow.

Luckily, just as ketchup smears and grass stains, both rich in conjugated compounds like lycopene and chlorophyll, can be bleached out of existence, so too can yellowed plastics. [Caden] notes that Retrobright, which contains a powerful dose of hydrogen peroxide, does its whitening trick by breaking the UV-absorbing double bonds in 2-HMA. There’s little that can be done about the embrittlement of the ABS caused by the breakdown of butadiene copolymers, but at least it’ll look good.

We found this guide quite comprehensive and instructive, and it should only help retrocomputing fans in their restoration efforts. For those less interested in the chemistry, [Bob Baddeley] published an overview of the yellowing of plastic and manufacturing steps to avoid it, and we covered the more practical considerations of Retrobright treatment too.