# How does black CuO impart a green color to glazes and glass?

I read from a source that cupric oxide (CuO) imparts green to blue colour to glazes and glass. But CuO is black in colour. How is this possible?

• Because, bingo, cupric oxide produces different compound in the glazes and glasses. – permeakra Apr 7 '17 at 19:15

This is due to the oxidation of cupric oxide.

The main componds believed to cause this colour comprise a mixture of 3 compounds: $\ce{Cu4SO4(OH)6}$ (green); $\ce{Cu2CO3(OH)2}$ (green); and $\ce{Cu3(CO3)2(OH)2}$(blue).

The following reactions are believed to take place:

Copper (I) oxidised to the black copper (II) sulfide ($\ce{CuS}$) in the presence of sulfur impurities. Under accelerated conditions, the described process occur at a faster rate.

$\ce{CuO}$ and $\ce{CuS}$ reacts with carbon dioxide ($\ce{CO2}$) and hydroxide ions ($\ce{OH-}$) in water (in presence of air) to form $\ce{Cu2CO3(OH)2}$.

The extent of humidity and the level of sulfur have a significant impact on how fast the compounds develop, (under controlled conditions these reactants are varied to produce a favourable hue) as well as the relative ratio of the three components.

$$\ce{2CuO + CO2 + H2O → Cu2CO3(OH)2~~~~~~~(1)}$$

$$\ce{3CuO + 2CO2 + H2O → Cu3(CO3)2(OH)2~~~~~~~~(2)}$$

$$\ce{4CuO + SO3 +3H2O → Cu4SO4(OH)6~~~~~~~~~(3)}$$

References

• Just a couple nitpicks: When you say "over time", I'm not sure what time scale you mean because glazing processes turn CuO green under the right firing conditions. Also, I don't see the relevance of equations 1 and 2 to CuO turning green or blue. I might be missing something, but it seems more like you are talking about long term corrosion type of processes rather than making colored glasses and glazes as the OP asked about. – airhuff Apr 7 '17 at 17:42
• @airhuff Thanks let me fix it. Over time was basically referring to under natural conditions however under accelerated conditions the reactions are much faster. Let me out that into edit – xavier_fakerat Apr 7 '17 at 17:46
• The compounds you mentioned have nothing to do with color of glasses and glazes – permeakra Apr 7 '17 at 19:15
• @xavier_fakerat Ok, let me elaborate. The compounds you mentioned cause coloration of copper-contaning metal pieces where they are indeed formed thanks to oxidation. But, as you brilliantly noted, the question does not involve coloration by copper oxide, not metallic copper. This is different. Since you fail to see the difference, I downvote your answer as wrong and misleading. – permeakra Apr 7 '17 at 20:02
• How is citric oxide oxidized. Copper is known in oxidation states up to +4, but your quoted compounds all have pure Cu (II). – Oscar Lanzi Apr 8 '17 at 1:48

The green-blue color is common for copper compounds, because copper(II) ions in oxide neighborhood usually adsorb light in the red region thanks to relatively low energy of the $d-d$ transitions, that happens to be low because the d-orbitals of copper are small thank to the $d$-shrinking effect (large effective charge affective the $d$-orbitals). The best model for copper glasses/glases would be copper silicates Here are links to description of two copper silicates https://en.wikipedia.org/wiki/Egyptian_blue https://en.wikipedia.org/wiki/Bayldonite Please, note: these two are mere examples, the exact silicate phases formed in glasses and glazes may have very complex composition and structure or be actually amorphous. The point is that the color pallette is typical for isolated $Cu^{2+}$ ions in oxide neigbrohood, which is the neighborhood of copper in glasses in glazes

So, the actual question is where the black color of copper oxide comes from. Black color is a total adsorption in the visible spectrum, that comes from a family of electron transitions. Indeed, the paper on the band structure of copper oxide says that there are two rather wide bands with band gap ~1.4-1.7 eV (the energy of photons of deep-red light is ~1.7) Why it has this particular energy is a much harder question with no simple answer.

Bands are the closest equivalent to orbitals known in quantum theory of solids. Bands are formed from individual overlapping atomic orbitals, but since they are formed from infinite number of atomic orbitals (in theory, but in crystals the size of the crystal makes the theory reasonably close to experiment) they form a continuous spectrum. Individual 'orbitals' in a band are known as states. Bands normally have finite width, thus it is possible to have band gaps - energy ranges with no corresponding electron state for this particular crystal. If the highest band is not completely filled with electrons, the crystal(usually) has metallic conductivity as the electrons may freely move from state to state. If the highest band is completely filled with electrons and there is a gap between it and the next band, the crystal usually is a semiconducor (or an insulator if the gap is large enough). If the gap has wideness near visible spectrum, it may color the crystal. If the gap is larger than energy of violet photons (~3 eV), the crystal is transparent (or white)

• I made it a community wiki because it obviously needs heavy editing. – permeakra Apr 7 '17 at 19:53
• @xavier_fakerat Cu(I) is usually colorless. Cu(III) is hard to obtain. Other than that this IS the correct answer. But it needs editing to straighten wordings and spelling. – permeakra Apr 7 '17 at 19:57
• It doesn't need CW and is not really all that bad. – orthocresol Apr 8 '17 at 9:57
• @orthocresol upvote and upgrade if you thingk it's good. – permeakra Apr 8 '17 at 14:18