From the Lycurgus Cup to Nano Research: The History of Colloids

Long before anyone knew the word “colloid”, Roman glassmakers had already mastered the art of nanoparticles. Their most famous work is the 4th-century Lycurgus Cup, today in the British Museum: lit from outside it shimmers dark green, lit from within it glows ruby red. The secret is the finest gold and silver particles in the glass — a gold share of just 0.01 percent suffices for the intense colour. Only physicists of our own era could fully explain the riddle using X-ray scattering.

The same art made medieval church windows glow ruby red and deep blue: gold particles colour glass — depending on particle size — from soft pink to violet, while silver lends a bluish shimmer.

Gold as the liquid essence of the sun: the idea is older than alchemy itself and reaches back to the pharaohs and high priests of Egypt. In the 16th century Paracelsus raised the transformation of metals to the high art of his craft. In seven steps he isolated a quintessence he called the “Arcanum”; his most famous Arcanum was the legendary Aurum Potabile. The alchemists guarded the exact recipes as their greatest secret, and of the many thousands of pages Paracelsus left behind, only a fraction survives.

The alchemists’ spagyrically prepared Aurum Potabile, by the way, is something different from colloidal gold: there the gold is processed according to alchemical rules, while in a colloid it floats as an elemental particle. Two paths to one metal, separated by four centuries.

Science caught up with the art in the 19th century. In 1857 Michael Faraday produced ruby-red gold sols and was the first to recognize that the colour comes from the tiniest, finely dispersed gold particles — some of his samples float to this day in the archive of the Royal Institution. Four years later the British physicist Thomas Graham coined the term “colloid”, from the Greek kolla, glue, and eidos, form.

Today colloid research is a foundation of nanotechnology: gold colloids work inside pregnancy tests, nanoparticles in coatings, glasses and sensors. What began in the workshops of antiquity now fills research institutes — and the fascination of the floating particles remains unbroken.