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ThomasGran
New Contributor

Re-dox reactions with ceramic glazes

Hi, I am a retired analytical chemist mentoring a high school teacher in advanced chem.  They are using the Raku process of glazing to show multiple colors due to the reduction cycle reactions.  My question is can you provide me with the re-dox equations for the various reactions of Cu, Co, Cr, Silica, borate to form various colors.

Thank you

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yvonne910rhodes
New Contributor

Re: Re-dox reactions with ceramic glazes


@ThomasGranmilestone card services wrote:

Hi, I am a retired analytical chemist mentoring a high school teacher in advanced chem.  They are using the Raku process of glazing to show multiple colors due to the reduction cycle reactions.  My question is can you provide me with the re-dox equations for the various reactions of Cu, Co, Cr, Silica, borate to form various colors.

Thank you


Hello,

The Raku process of glazing involves a fascinating interplay of reduction and oxidation (redox) reactions that create a spectrum of colors. Here are some simplified redox equations for the elements you mentioned:

Copper (Cu):
Oxidation: ( 2Cu + O_2 \rightarrow 2CuO ) (Copper turns green when oxidized to copper oxide)
Reduction: ( CuO + CO \rightarrow Cu + CO_2 ) (Copper can show reds, blues, and greens under reducing conditions)

Cobalt (Co):
Oxidation: ( 2Co + O_2 \rightarrow 2CoO ) (Cobalt oxide can produce deep blue colors)
Reduction: ( CoO + H_2 \rightarrow Co + H_2O ) (Cobalt can give a range of blue shades)

Chromium (Cr):
Oxidation: ( 4Cr + 3O_2 \rightarrow 2Cr_2O_3 ) (Chromium oxide gives green color)
Reduction: ( 2Cr_2O_3 + 3SiO_2 + 3C \rightarrow 4Cr + 3CO_2 + 3Si ) (Chromium can produce pinks and reds under reducing conditions)

Silica (SiO2):
Silica does not undergo redox reactions in glazes but acts as a glass former.

Borate (Borate compounds like B2O3):
Borate compounds are also not typically involved in redox reactions but act as fluxes to lower the melting point of the glaze.

These reactions are highly dependent on the firing atmosphere, temperature, and the presence of other elements. The actual color development is quite complex and can vary with the specific compounds and the conditions of the Raku firing process.

I hope the information may helps you. 

Best regards,
Yvonne Rhodes

 

 

 

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Armin
New Contributor

Re: Re-dox reactions with ceramic glazes

 

Raku Firing Process - Redox Reactions


Certainly! The Raku firing process involves rapid heating and cooling, often with an intentional reduction atmosphere, to create unique effects and colors in glazes. The colors are a result of various redox reactions involving metal oxides. Here are the redox reactions for common colorants in glazes:

Copper (Cu)

Oxidation State Change:

Oxidation Reaction: 2 Cu + O2 → 2 CuO

Reduction Reaction: CuO + CO → Cu + CO2

Color Results:

  • In an oxidizing atmosphere: CuO (black)
  • In a reducing atmosphere: Cu (metallic red) or Cu2O (red)

Cobalt (Co)

Oxidation State Change:

Oxidation Reaction: 2 Co + O2 → 2 CoO

Reduction Reaction: Co2O3 + 3 CO → 2 CoO + 3 CO2

Color Results:

  • In an oxidizing atmosphere: CoO (blue)
  • In a reducing atmosphere: Metallic cobalt (grey)

Chromium (Cr)

Oxidation State Change:

Oxidation Reaction: 4 Cr + 3 O2 → 2 Cr2O3

Reduction Reaction: Cr2O3 + 3 CO → 2 Cr + 3 CO2

Color Results:

  • In an oxidizing atmosphere: Cr2O3 (green)
  • In a reducing atmosphere: CrO2 (blue-black)
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