Ancient nanotechnology

How the ancient Romans created color-changing glass goblets that shifted colors based on what you put in them.

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  1. Maggie... They changed colours based on the direction of the light source, not the contents of the cup.

  2. The tests they've run on similar material suggest that it would, actually, change color depending on what you put in it. It's there in the story.

  3. I work in this field. The paper cited by Discover Magazine, Gartia et al., is very misleading.

    The glass in the Lycurgus cup contains metal nanoparticles distributed throughout it. These reflect green light and so the cup looks green in reflection. They absorb or scatter blue and green light, so the cup looks red when light is shone through it.

    Gartia et al have made a structure that contains metal nanoparticles, but the fabrication method, the type of nanoparticles and the way the nanoparticles are positioned and structured, are all completely different to the Lycurgus Cup. They claim the association simply because their structures also look green in reflection and red in transmission. Many, many structures share the sample optical properties: it's not called "the Lycurgus Cup effect", it's called dichroism.

    The structures made by Gartia et al are very thin films of gold perforated by tiny cone-shaped holes, which are filled with nanoparticles. The cones and the nanoparticles are exposed to the external environment, and so come into contact with whatever liquids you put on them. The liquids change the optical properties of the metal nanoparticles because they change the local refractive index (and hence the effective wavelength of incident light) which changes the apparent colour. This is a very well known effect, and makes metal nanoparticle extremely useful as optical sensors. However it's not the reason for the colouration of the Lycurgus Cup.

    In the Lycurgus cup the nanoparticles are deeply embedded into the glass. There's no sheet of gold, and no tiny cones, just a whole bunch of nanoparticles randomly distributed throughout the glass. The optical properties of the nanoparticles are only sensitive to the material immediately surrounding them (i.e. the material within about 20-30 nanometres of the particle surface). Liquid poured into the cup does not come into contact with the metal nanoparticles, and so does not affect their optical properties.

    The rest of the Gartia et al's paper actually looks quite interesting, but it has very, very little to do with the Lycurgus cup. I'm sure the authors know this, but "dichroic plasmonic sensing" doesn't sound as sexy as "ANCIENT ROMAN SENSING TECHNOLOGY!!!"

  4. I was going too say that too. Silver and gold... solar metallurgist must have had a nice Guild back then.

  5. Purple of Cassius is a suspension of gold nanoparticles in liquid. The red (or purple) colour arises because the nanoparticles absorb blue and green light, but do not strongly interact with red light. So this is a form of subtractive colouration, just like all pigments and dyes. Adding Purple of Cassius to molten glass results in a red colouration because the nanoparticles become distributed throughout the glass. This is how ruby glass (or cranberry glass) was made from the 17th century onward, and was also used prior to this in stained-glass windows etc.

    However, ruby glass and Purple of Cassius are the same colour if you shine light through them or reflect light off them, it's red in both cases. The Lycurgus Cup is different, and the difference is due to the size of the particles. Particularly tiny gold nanoparticles (with sizes less than a few tens of nanometres) only absorb light, so they act like a pigment. However the particles in the Lycurgus Cup are quite a bit bigger than in Purple of Cassius, around 50-100nm in size, so they also scatter light. The red colour in transmission is due to absorption, just as it is in ruby glass and Purple of Cassius. However the green colour is due to scattering of light, which you only get when the particles are big enough.

    It's this dependence on size that I think justifies the use of the word "nanotechnology" for the Lycurgus Cup and for Purple of Cassius. I use the word when I'm referring to a technology that utilizes feature sizes less than 100 nanometres, and -crucially- that gains unique properties by being this size. The colour of gold nanoparticles is extremely sensitive to their size and shape, and it's only when they're "nano sized" that you get such beautiful effects.

    I feel like I'm writing to an increasingly diminishing audience, but hopefully someone finds it interesting...

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