Reddish Color Variation of Clay bricks,Why?

when firing clay bricks, even if the same content of Fe₂O₃ in two types of clay bricks fired under the same system, but the fired clay bricks colors look different, and in some case, the more higher content of Fe₂O₃ gives an orange look rather than reddish, why does this happen?

Iron Oxide Mineral Forms and Transformations:  

• During firing, Fe₂O₃ doesn’t exist in isolation—it interacts with the clay’s mineral matrix. At high temperatures (typically 600–1000°C for brick firing), iron oxides can transform. If the starting clay contains hydrated iron oxides like Goethite (FeOOH, yellowish), it dehydrates into Fe₂O₃. However, the final color depends on how the iron integrates into the fired structure.

• In some cases, higher Fe₂O₃ content might not fully crystallize into hematite (red) but instead form amorphous or fine-grained phases that lean orange or yellow.

Interaction with Other Elements:  

• Clays often contain impurities like calcium, magnesium, or alkalis (e.g., CaO, Na₂O, K₂O). These can react with Fe₂O₃ during firing to form complex compounds (e.g., calcium ferrites), which dilute the red color and shift it toward yellow-orange. For instance, a higher calcium content might lighten the brick, giving it a more yellowish cast even with elevated Fe₂O₃.

So mostly the possible reason is of the Goethite in the clay leading to the yellowish or orange red variation,

The Role of Goethite in the Clay

Under typical oxidizing firing conditions up to nearly 1000°C, hydrated iron oxides like goethite (FeOOH) should lose their water (H₂O) and convert to Fe₂O₃ (hematite), which is reddish. The reaction is:

2FeOOH→Fe2O3+H2O;

But the bricks are still looks orange, which might suggests that the Fe₂O₃ derived from Goethite in the clay doesn’t fully develop into the deep red, well-crystallized hematite due to:

• Form fine-grained or amorphous Fe₂O₃, which scatters light in a way that leans orange rather than red.

• Retain some yellowish optical properties from its goethite origins, blending with the redder Fe₂O₃ from the base clay to produce an intermediate orange hue.

Reddish Adjustment:

if we have a sure understanding of role of Goethite, then we may take some actions in favor of the Reddish Adjustment:

• Stong Oxidation Atomsphere Firing

• 10-20% longer of Soaking for conversion from Goethite to Hematite of richer red,directly addressing crystallization;