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I recently went on a tour of a glass manufacturing facility who produce bottles for the prestige alcohol market. I was informed on the tour that red glass cannot be directly produced - any bottles that I have seen had made by applying a spray to a plain finished bottle as opposed to adding the colour into the molten glass.

Why is this?

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It seems likely that costs associated with the material prevents economical production of red glass. Rather than a matter of possibility, it may be a matter of viability. Without knowing more about how the glass is produced or the specific materials used for their bottles, it is difficult to say the exact reason. The best answer I can think of is that the only glass I am aware of which selectively transmits red light requires gold nanoparticles suspended in the silica glass network. Such glass is referred to as Cranberry Glass or Gold Ruby Glass (Wikipedia).

It isn't clear how much gold is required, though the abstract of this paper, which uses an alternate production process, suggests about 0.2% by weight. The price of gold is about 40,000 USD/kg per the current spot price from apmex.com as of 08 March 2016 at ~2:45 PM CST. Thus a glass bottle weighing 1 kg would have an added cost of about 80 USD from the gold alone. Expanding the process to mass-produce such bottles would require storing and shipping significant quantities of gold, which may add logistical and overhead costs for security. There would also be equipment requirements for producing colloidal aqua regia in the proper concentration and drying, handling, and mixing the nanoparticles into the liquid glass.

As a side note, flint glass (Wikipedia) contains zirconium and/or titanium. I can't think of any reason that either element would interfere with the cohesion of the gold particles, but I also can't strictly rule it out. If such a thing did happen, then Flint glass specifically might be impossible to make red by the use of gold nanoparticles.

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  • $\begingroup$ Maybe this helps you to expand: its flint glass used for alcoholic drinks, furnace which feeds in to blow mould machines. Great answer nonetheless! $\endgroup$
    – Phizzy
    Mar 8 '16 at 21:05
  • $\begingroup$ @Phizzy I added a comment about flint glass. I don't know enough about the chemistry to say for certain, but I can't think of any reason why flint glass specifically would make the process impossible. $\endgroup$
    – wwarriner
    Mar 8 '16 at 21:08
  • $\begingroup$ I would guess that it is to do with the fact that traditionally flint glass traditionally contained significant quantities of lead oxides which contribute to its high refractive index and that the modern lead-free versions which would be required for storing drinks don't interact with the gold in the same way. Bearing in mind that the colouring of glass is a rather more complex physical phenomena than simply adding pigment of the desired colour a it would be with resins etc. $\endgroup$ Mar 8 '16 at 21:45
  • $\begingroup$ @ChrisJohns That's true, however Cranberry Glass contains Au as a colloid, not a solution, whereas the Zr/Ti (or Pb) is in solution. So unless the latter atoms dramatically increase the solubility of gold in molten glass, it isn't clear what happens. I can't find any published results on gold solubility, and there may not be any. Artisans probably use the traditional recipe with Pb and wouldn't publish, and mass-producers don't bother to try when they can just paint the glass far more economically. This may be one of those questions that may never have a clear-cut answer. $\endgroup$
    – wwarriner
    Mar 8 '16 at 21:55

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