1856

Mirror Silvering

Justus von Liebig (1803–1873), Bernhard Tollens (1841–1918)

This reaction looks like a magic trick, but for nearly a hundred years it was a trick well worth knowing if you were in the mirror business. Mirrors used to be made by coating glass with a layer of tin foil and then exposing the foil to liquid mercury. The resulting tin/mercury amalgam, while reasonably reflective, could corrode over time (and give off drops of mercury, something to note if you own an antique mirror). The alternative was speculum (Latin for mirror) metal, a copper/tin mixture that could be polished to a not-all-that-reflective surface.

German chemist Justus von Liebig found a better way, and refined it enough that it gradually put the mercury method out of business. Liebig’s invention depended on oxidation/reduction chemistry in which a silver/amine complex was formed and mixed with a sugar solution, and this was applied to a glass surface. The silver oxidized the sugar molecules to a soluble acid and was in turn reduced to elemental silver, which was deposited as a thin, highly reflective mirror layer on the glass.

Not only did this procedure provide high-quality mirrors—for personal use as well as for things like reflecting telescopes—but German chemist Bernhard Tollens also modified it to serve as a qualitative chemical test. Before modern analytical instruments, such tests were a major feature of chemistry, with a variety of reagent combinations that would give characteristic colors or precipitates with specific elements or functional groups. Tollens’s test was used when a chemist wasn’t sure whether the carbonyl group (a carbon double bonded to an oxygen) in a molecule had carbons on both sides of it (a ketone) or a hydrogen on one side (an aldehyde). Aldehydes are oxidized by the silver reagent to give a mirror coating on the inside of a test tube, but ketones don’t react.

Tollens’s test, an antique now, had to be performed with freshly made reagent. If silver/amine solutions are allowed to sit around, further reactions gradually form the unpredictable explosive silver nitride, which can detonate for any reason or none at all. The mirror-silvering people also had to learn this lesson, sometimes the hard way.

Suspended in a beaker of warm water, Tollens’s reagent quickly forms a thin layer of silver on the interior of this test tube. With careful technique and a very clean flask, an almost perfect mirror can be produced.