We’ve already taken time to explore production of both blue and green glass, and the chemical interventions required to create these striking wares; time now to look back considerably earlier than the innovative landscape of 18th century Georgian England which gave us these two decorative staples in search of the processes behind the creation of red glass. This is not, however, a simple tale – so deep breath, gird your loins, sharpen your pencil and prepare to be reminded why you (in all probability) chose not to pursue a career in particle physics, nanotechnology or electromagnetic plane-wave scattering…

It is, I fondly imagine, fairly common knowledge that red glass is produced by the inclusion of gold at a certain point in the production process, but there is a great deal more to the procedure than any arbitrary mixing of constituent parts; Bristol blue glass and its green counterpart were created by the inclusion of quite obviously blue and green ingredients, which seems entirely fair and equitable – but red glass, made by the addition of gold – there’s immediately a disconnect within that statement right from the word go; how on earth does gold colour glass red ?

There is a persistent, long-established and rather vague consensus that the process of manufacturing red glass initially resulted by way of an accident. An unidentified glassmaker carelessly dropping a gold coin in to the melt whilst working alongside his furnace, a nobleman deliberately flipping a sovereign in to a blistering cauldron or some other fortuitous circumstance which first saw something gold being added to molten glass, and immediately imparting the intense carmine hue which went on to become so enduringly popular. Unfortunately, as with most apocryphal stories of this nature, any assertions along these lines are nonsense – they offer far too simplistic an explanation to be anything other than essentially irrelevant.

What cannot be in any doubt is that the albeit initially rather ad hoc preparation of red or ruby-coloured glass has been undertaken since at least the first millennia BC. The process is referenced in Egyptian documents from classical times, and a visit to the British Museum will enable you to see the utterly extraordinary Lycurgus Cup – an artefact from the Mediterranean dated to around 300AD which exhibits not only red glass derived from the addition of gold, but green tints similarly derived from silver; the most amazing facet of the piece, however, is not the use of the colouration itself but the fact that it is what is now termed as “dichroic” meaning that depending on from which direction the cup is illuminated relative to the viewer, it either reflects green light or transmits red – in simple terms, it changes colour.

Successful attempts have been made to replicate the Lycurgus Cup, based on detailed analysis of the original, and what has become strikingly apparent is the infinitesimally small scale of both the actual particles of gold and silver used, and the amounts in which they have to be added to achieve the desired effect. There is no conceivable way that Roman glassmakers had the facility to purposely grind such minuscule metallic particles, or to then accurately measure the proportions required to control the process, so it can only have been an accidental discovery – and an acknowledgement of this is probably the source of the whole fortuitous coin-tossing tales of medieval times. It’s likely that this profoundly fabulous piece was the result of the contamination of glass in a workshop where both gold and silver were filed or ground – an assertion made more credible by the fact that the Lycurgus Cup is unique. Had its creation been anything other than wholly unintentional, then there would surely have been other examples in existence; it’s hard to imagine anyone actually knowing how to conjure up such a spellbinding item, and then making just the one example.

Suffice to say that although other dichroic glass items do exist from Roman times, none have the same balance of colours as the Lycurgus cup; it really does seem to have been a one-off.

It does, however, illustrate the basics behind the production of ruby gold glassware – the use of tiny particles of gold; now, I fully intended for this article to include a complete scientific explanation of how this colouration was achieved, but having barely scratched the surface and been confronted with the phrase “surface plasmon resonance”, and several other similarly impenetrable terms, I have decided that discretion is the better part of valour and to skirt around the more arcane aspects of the curious alchemy involved. It’d be churlish to ignore such a fine phrase entirely, though, and as it is integral to the whole process, we’ll just say that surface plasmon resonance involves molecular particles of gold being excited by the absorption of light – and it’s these excitable particles which create the semblance of colour !

With this inviolable but unfathomable scientific rationale now in play, we are now in a position to return to our original question – how do gold particles make glass appear red ? Well, it transpires, that this is a somewhat rhetorical question as gold particles do not make glass look red at all – they actually make glass look gold, but gold looks red under specific conditions ! This gloriously ambiguous statement has, over time, been addressed on both a philosophical and scientific level, but the former is a little beyond my remit in this particular article (count yourselves lucky), and it’s to the science that we must return if we are to find any semblance of an explanation.

Consider the following: if gold leaf is beaten sufficiently thinly, it becomes too insubstantial to reflect gold/red light and instead looks blue/green (the red/gold light simply passes straight through the extremely thin material rather than being reflected and remaining visible to the observer); it was often thought that many ancient manuscripts, illuminated with gold leaf, had become tarnished, leaving the gold looking “dirty”, but in actual fact, it has been applied so thinly as to exhibit this blue/green effect – it’s still “gold” in every way shape, manner and form, but just not in the more usually-visible state. The existence of “red gold” is another manifestation of this curious propensity of the highly-valued metal to appear to be a different colour depending on its exact formation.

Having established that ruby glass colouration is achieved by gold particles, we now need to take on board that this is possible because gold is a relatively inert element – it can be mixed with other substances and, rather than reacting by dissolving in to solution, it tends to remain intact on a molecular level and is said to remain in suspension as a colloidal solid (basically, no matter how much you fiddle about with the stuff, it always retains the properties of albeit tiny bits of gold). Crucially, when gold is reduced to a state where its particles are of a size which is suitable for this colloidal suspension, they are also at a size where the aforementioned surface plasmon resonance generates molecular excitement to the extent where the particles reflect light in the red part of the spectrum – thereby producing the illusion of red colouration, as dictated by the processes of the splendidly cabalistic mie scattering theory (and that’s now waaaay too much science – that’s it now – promise…). The exact size of the particles determines their precise degree of excitable resonance, and this in turn determines the exact colour – so not all ruby gold is of the exact same hue.

So, having processed your gold to the point where it takes on the form of a colloid, and is happily able to become sufficiently exited to appear red – what now ? The next step in the progression of ruby glass production is to introduce this suspension in to molten glass which, when it cools and sets as a solid, locks the minute gold particles in to the crystalline structure where – although captive - they can still be excited by light to the point where they impart their red lustre. We’re now back to the same physical processes as those used to create blue and green glass – take your clear crystal and add the colouring agent, but instead of actual blue or green particles (oxides of cobalt and chromium respectively), we’re using microscopic gold particulates which have essentially been “treated” to appear red.

Now that we are all absolutely clear on the basic theories behind the production of ruby glass (there will be a test !), and having concluded that its ancient progenitors were superintended far more by luck than judgement, it’s perhaps time to pick up the story in the early part of the 16th century, and look at who was responsible for codifying the process and setting it down for future reference. It’s one of history’s enduring truisms that the rudimentary science of the middle ages was consumed, at least in part, with the much-vaunted pursuit of “producing gold from base metals” which is the proper meaning of (chyrsopoetic) alchemy (as opposed to the lazy authors use of the term to refer to “anything a bit old and mysterious” – guilty as charged !) Inevitably, this involved the exhaustive study of the noble metal itself, and its properties were painstakingly catalogued as they were incrementally unveiled. A German metallurgist, Georgious Agricola, and a Swiss pseudo-scientist by the simply glorious name of Philippus Aureolus Theophrastus Bombastus von Hohenheim (Paracelsus to his mates, for obvious reasons) both made reference in their works to the use of gold compounds to colour glass. An Italian goldsmith, Cellini de Firenze, notes that one of his 16th century contemporaries used gold to make a fine red enamel and Johann Kunckel of Northern Germany also alluded to the process in his extensive writings on glass production. The name Andreas Cassius is also prominent in the field, but his notes, like those of Kunckel (and uniquely involving the use of tin oxides to produce predominantly purple colours), drew heavily on the original works of Antionio Neri whose seminal “Art of Glass” book (L’Arte Vetraria) provided the first and most exhaustive treatise on the subject of red glass.

Neri’s work, however, was largely theoretical, and it was Cassius and the rest who undertook practical demonstrations of the procedures and refined them to the point where they were almost becoming commercially viable. Kunckel’s work was underwritten by Freidrich Wilhelm, Elector of Brandenburg and Duke of Prussia who made significant financial backing available. It seems that Kunckel was close to perfecting the production processes by the mid 1680’s, and in spite of destructive acts of industrial sabotage at the hands of competitors and the death of his benefactor in 1688, he was able to produce bright red glassware using colloidal gold, and ensured that Brandenburg became renowned for its manufacture.

In spite of Kunckel’s best efforts at obfuscation, the secrets of his work spread across Germany, through Saxony and south to Bohemia and the bright red light of ruby gold glass shone brightly for the first half of the 18th century. The product, however, began to lose its appeal from around 1740 onwards – replaced in the affections (and affectations) of European nobility by porcelain and other gilded wares which entailed a far more overt and ostentatious use of gold. This of course, as we have seen time and time again, was the prime directive for wealthy and influential folk; prosperity and affluence had to be advertised with as much conspicuous flamboyance as possible; why “hide” your squanderous ways by procuring obliquely-costly red things, when gold things looked far more lavish !

The use of ruby glass gained in popularity again after an eighty-year hiatus, by which time the world was careering headlong alongside the industrial revolution, and new scientific advances were being made at every turn. This applied to glassmaking as well as seemingly every other endeavour, and the use of newly available, somewhat less expensive, elements was explored. From the latter half of the 19th century, copper, selenium and cadmium were all employed, and this signalled a move away from the alchemical processes of ruby gold production; red glass now involved the introduction of these pigments in much the same way as blue and green glass had once required the addition of blue and green modifiers, although the treatments needed to apply the colouration were entirely new. Selenium, for instance, required carefully controlled heating and cooling of the tinted melt to “strike” the colours in to permanence. This was widely-used in the manufacture of Fenton, Imperial and Carnival glassware in the United States. The process was difficult to control, often producing gradations of colour from yellow to dark red throughout a single piece, but this in itself became an acceptable “colourway” and such glass was considered a sought-after commodity in its own right.

Ultimately, though, properly whole-coloured ruby gold pieces were almost universally held in higher esteem, with the necessary use of elemental gold and the dark arts involved proving to be enduring hallmarks of “real” quality; where high-end pieces were required, the new-fangled heating and cooling techniques were eschewed in favour of the methods which had produced exquisite items in varying shades of crimson, cardinal, carmine and ruby over three millennia – although somewhat tortuous to describe for modern-day scribes (ahem), the old ways, in this instance, were very definitely the best !

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