Correlation between Surface Area of Goethite Pigments and Their Tinting Strength in Watercolor Paintings

Iron oxide pigments, particularly goethite (α-FeOOH), are highly valued in watercolor paints for their stability and rich color. This study investigates the relationship between the Surface Area (SA) of goethite pigments synthesized from iron-rich lathe waste and their tinting strength in watercolor formulations. Goethite pigments were synthesized through controlled hydrolysis of lathe waste at four temperatures (55, 65, 75, and 85°C). The pigments were characterized for their crystal structure using X-ray diffraction (XRD), and their particle morphology and composition were analyzed using scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX). XRD confirmed the formation of orthorhombic goethite in all samples, with crystallite sizes ranging from 10.8 to 12.6 nm. SEM revealed non-uniform particle shapes with agglomerates and small traces of impurities, such as carbon and sodium. Specific SA was measured using the Brunauer-Emmet-Teller (B.E.T) method, showing a non-linear dependence on synthesis temperature, with a peak at 65°C. Tinting strength was evaluated by incorporating the pigments into standardized watercolor formulations, measuring color intensity against a white reference. The analysis revealed a strong positive correlation between SA and tinting strength, with higher SA resulting in more vibrant colors at lower concentrations. Temperature-controlled synthesis optimized pigment properties, with the 65°C sample showing the best dispersion and color balance. These findings underscore the importance of SA in pigment performance and provide valuable insights for developing high-quality, sustainable goethite pigments for artistic applications.