Dear Dr. Zhu Xinglin,

he first self-cleaning glass was based on a thin film titania coating. The film can be applied by spin coating of organo-titanate chelated precursor, followed by heat treatment at elevated temperatures to burn the organic residues and to form the anatase phase. In that case, sodium might diffuse from the glass into the nascent titanium dioxide, causing a degradation in the hydrophilic/catalytic effect unless preventive measures are taken. The glass cleans itself in two stages. The "photocatalytic" stage of the process breaks down the organic dirt on the glass using ultraviolet light and makes the glass superhydrophilic (normally glass is hydrophobic). During the following "superhydrophilic" stage, rain washes away the dirt, leaving almost no streaks, because water spreads evenly on superhydrophilic surfaces.

For more details, please see Wikipedia: - Self-cleaning glass, or :

- Eco glass cleans itself with Sun

by Jo Twist - BBC News Online science and technology staff

Available at: http://news.bbc.co.uk/2/hi/technology/3770353.stm

Best regards, Pierluigi Traverso.

Annealing of glasses at elevated temperatures, especially treated ones, undergo chemical and mechanical changes. Chemically- oxygen in the over at elevated temperatures can be reactive, and coatings may scavenge or be scavenged by the glass. Mechanically, thermal expansion differences can lead to stresses and micro-fractures. Be careful and best of luck!

Somehow, the temperature/time schedule may perhaps have become harsh enough to induce surface dehydroxylation, through the condensation of silanol group pairs: ≡Si─OH + HO─Si≡ → ≡Si─O─Si≡ + H2O↑ ― rather than just adsorbed moisture loss, as it might have been achieved after a milder drying temperature/time schedule.

This might have demote the areal number density of surface polar groups (e.g. silanol), hence imparting a somewhat lower energy character to the glass surface. That may have then enhanced the glass surface adsorption capacity toward substances of hydrophobic character ― characterized by non-polar (hydrophobic) molecular forces ― which otherwise could generally be rather weak for most common glass types; of hydrophilic character.