The Relationship Between Crystal Structure and Colour
Depending on the synthesis method used, iron oxide pigments can be produced in different crystal phases such as α-Fe₂O₃ (hematite), γ-Fe₂O₃ (maghemite) and Fe₃O₄ (magnetite). These crystal phases are the primary parameter that directly determines the pigment's apparent colour. While hematite produces red and brown tones, magnetite is the fundamental building block of black pigments, and goethite that of yellow pigments.
Defects in the crystal structure, partial oxidation or impurities cause the same phase to produce different colour tones across different pigment samples. For this reason, verifying phase purity by X-ray diffraction (XRD) for each pigment batch is the first and most critical step in achieving colour consistency.
Particle Size Distribution
The second critical variable is particle size distribution. Even within the same crystal phase, different particle sizes lead to different colour saturations. As a general rule, smaller particles yield brighter and purer tones, while larger particles tend towards deeper and darker tones. This relationship is particularly pronounced in the red series: the 110 reference offers a relatively light red, while the 130 reference offers a darker one.
Control Criteria
- Reporting the mean particle diameter (D50) analysis for every batch
- Documenting the narrowness of the distribution with D10 and D90 values
- Keeping the surface area (BET) within the range appropriate to the production target
The Effect of the Application Matrix
The matrix in which the pigment is embedded — cement, polymer, resin or ceramic glaze — affects how light reaches the pigment and is reflected back. In highly alkaline environments (e.g. Portland cement), some pigments may exhibit pH-induced tone shifts. The same pigment may appear more saturated in an oil-based paint while remaining paler in water-based emulsions.
Colour stability is the stability not of the pigment alone, but of the pigment–matrix system. A successful formulation optimises both components together.
The Quality Assurance Process
At Alba Pigment, we compare every batch against a standard reference using a spectrophotometer. Batches whose ΔE value exceeds a defined threshold are referred for laboratory examination and, if necessary, withdrawn from the supply chain. In this way, we minimise the risk of colour deviation that our customers might encounter on the mass production line.
Summary Recommendations
- Verify phase purity and particle distribution for every batch.
- Take into account the pH, temperature and binder chemistry of the application matrix.
- Perform a reference spectrophotometer measurement when pigment enters the production line.
- Do not skip the sample validation cycle when formulation changes are made.
The colour stability of iron oxide pigments is an engineering outcome where correct product selection, disciplined quality control and application know-how converge. The Alba Pigment technical team is at your side at every step of this process, from the procurement decision to field application.