This is due to defects. As I understand it, you carry out calcination at elevated temperatures in the air. In this case, part of the oxygen evaporates, creating vacancies. They change the color of TiO2. This can be avoided by calcination in an oxygen atmosphere.

Aslam Ahmad

I wanted to answer this question, but you already defined it as the best. I was surprised. Then I saw the question repeated and realized that you were not sure about it. There is little titanium dioxide in the textile raw material and you had to work hard to remove the textile. You do not write how you did it. It remains and after calcination it turns into coal (graphite). This impurity gives the ashy shade.

Titanium dioxide (TiO₂) typically appears white due to its high refractive index and ability to scatter light across the visible spectrum, but after calcination at high temperatures (400–800°C) to enhance crystallinity or induce phase transitions like amorphous to anatase or rutile, it can develop an ash-colored, grayish appearance due to chemical and physical changes. Oxygen vacancies and lattice defects, formed during calcination in oxygen-deficient or reducing atmospheres (e.g., nitrogen or hydrogen), reduce Ti⁴⁺ to Ti³⁺, creating electronic states within the bandgap that absorb visible light and impart a gray hue, as seen in non-stoichiometric TiO₂-x. Residual carbon from organic precursors, such as titanium alkoxides in sol-gel synthesis, or environmental contaminants can deposit on the surface or integrate into the lattice if not fully combusted, contributing to the ash color, while trace metal impurities like iron or manganese from raw materials or equipment can form colored complexes, further shifting the appearance from white. Phase transitions from anatase to rutile at higher temperatures alter optical properties, and larger particle sizes or agglomerates from prolonged heating reduce light scattering, dulling the whiteness. Extended calcination durations, even in air, can accumulate defects or cause slight carbon deposition, enhancing the grayish tint, whereas a well-oxygenated atmosphere, optimized temperatures (e.g., 400–500°C for anatase), and shorter durations help maintain whiteness by minimizing vacancies and impurities. Using high-purity precursors and pre-treating materials to remove organics also reduces discoloration.