Short answer
Because the colourants in container glass — iron, chromium and an iron–sulphur complex — are dissolved into the glass and cannot be removed in the furnace, so a little of the wrong colour tints the whole melt. Clear (flint) glass tolerates very little foreign cullet, amber tolerates more, and green tolerates the most, which is why mixed cullet ends up mostly in green glass.
Green container glass gets its colour mainly from chromium oxide, reinforced by iron. Amber is not coloured by a single oxide but by a complex of ferric iron, reduced sulphur and oxygen formed under reducing furnace conditions; remove either the iron or the sulphur and the amber disappears. Clear flint glass still contains some iron from the sand: ferrous iron absorbs in the near infrared and gives a blue tint, ferric iron a weaker yellow-green, and typical container glass carries both. Glassmakers add small amounts of selenium and cobalt to neutralise that tint, and the Glass Packaging Institute notes that the result is technically a pale grey glass that reads as colourless.
Chromium is a strong colourant, and flint contains almost none. A WRAP technical study found that adding just 1% green cullet roughly doubles the chromium in flint glass, and that more than about half a per cent of green cullet is typically where the tint becomes just noticeable. Amber in flint is less damaging because oxidising melting conditions can partly bleach the sulphide amber chromophore. Amber glass, by contrast, can take a substantial share of green cullet without an obvious change, and green glass can take almost anything. The result is an asymmetry: every tonne of mis-sorted cullet can go into green, but only carefully separated clear cullet can go back into clear.
Countries that import a lot of wine and beer in green bottles and make mostly clear containers end up with more green cullet than their furnaces can use. British Glass describes exactly this in the UK: clear spirits bottles are exported, green wine and beer bottles imported, leaving a high quantity of green cullet against a low level of flint. Mixed-colour cullet, which is cheaper to collect, adds to the green pile because green is the only colour that can absorb it. That is the practical reason colour-separated collection, or optical colour sorting of cullet, matters so much for glass.
It is tempting to think a glassmaker can simply decolourise a green-tinted melt. Decolourisers work by adding a complementary absorption — selenium's pink, cobalt's blue — to cancel a faint tint, and they only work for small amounts of iron. They lower total transmission, so heavy decolourising makes glass look grey and dull rather than clear. Selenium is also partly lost from the furnace; the WRAP study notes that only a small fraction of what is added stays in the glass and works as a decolouriser. Chromium from green glass is far too strong to cancel this way, which is why the fix is sorting, not chemistry.
| Target glass | Main colourant | Tolerance of green cullet | Tolerance of amber cullet |
|---|---|---|---|
| Flint (clear) | Decolourised trace iron | Very low (tint noticeable around 0.5%) | Low, partly bleached by oxidising melt |
| Amber | Fe–S–O complex | High (reported up to about 30%) | Not applicable |
| Green | Chromium plus iron | Not applicable | High |
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactModerate evidence
Typical colourant levels reported for UK cullet were about 0.07% Fe2O3 and 0.002% Cr2O3 in flint, 0.36% Fe2O3 and 0.24% Cr2O3 in green, and 0.42% Fe2O3 and 0.02% Cr2O3 in amber.
Caveat: 2004 UK figures; individual glassworks differ.
Source: Feasibility Study for the Reduction of Colour within the Glass Furnace (GLA0023)
FactModerate evidence
More than about 0.5% green cullet in flint glass is typically just noticeable, whereas amber glass can accept up to around 30% green cullet.
Caveat: Indicative thresholds from a technical study; each glassmaker sets its own cullet specification.
Source: Feasibility Study for the Reduction of Colour within the Glass Furnace (GLA0023)
FactStrong evidence
Amber container glass is produced by combining iron, sulphur and carbon, green by chromium oxide, and flint is decolourised with selenium and cobalt to give a faintly grey glass that looks colourless.
Source: Glass Color Chemistry
FactModerate evidence· United Kingdom
In the UK, exports of clear spirits bottles and imports of green wine and beer bottles leave a large quantity of green cullet relative to flint.
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.