Kellastut

Kellastut is a term used in materials science and conservation to describe the process of yellowing or browning of materials caused by chemical changes in light-sensitive chromophores. It commonly affects cellulose-based papers, natural fibers, varnishes, and certain plastics, as well as wood finishes.

The term is derived from Finnish kellastua, meaning "to become yellow," and appears in European conservation literature to describe long-term color changes that are not simply caused by dirt or staining. It is used to distinguish yellowing from other forms of surface discoloration, such as soiling or pigmentation.

The mechanisms behind kellastut are varied but often involve photooxidation. Ultraviolet and visible light can break chemical bonds in lignin, chromophores, and additives, generating new yellow or brown chromophores. Heat, humidity, and pollutants such as sulfur compounds can accelerate the process. Interactions among acids, metal ions, and oxidants may catalyze chromophore formation, especially in older papers, textiles, and varnishes.

Kellastut changes color gradually, frequently beginning at edges or exposed surfaces. It can alter appearance and, in some composites, affect surface properties or mechanical performance through embrittlement or microstructural changes. In conservation practice, Kellastut is often assessed with colorimetric measurements (such as CIE L*a*b*) and spectrophotometric analysis to quantify rate and extent of yellowing.

Prevention focuses on minimizing light exposure, controlling humidity, and using stable materials or protective coatings. In libraries, archives, and museums, strategies include UV-filtered storage, proper temperature and humidity control, and the use of deacidified or inherently stable materials to reduce the rate of kellastut.