The first self-cleaning glass was based on thin film titanium dioxide coating. The glass cleans itself in two stages. The "photocatalytic" stage of the process breaks down the organic dirt on the glass using ultraviolet light in sunlight (even on overcast days) and makes the glass hydrophilic (normally glass is hydrophobic). During the following "hydrophilic" stage rain washes away the dirt, leaving almost no streaks, because hydrophilic glass spreads the water evenly over its surface..
Titanium dioxide is a material of choice because it is characterized by high photocalytic properties, chemical stability and low price. Its anatase phase is most photocalytic. What is more, under the UV irradiation its surface structure changes to generate OH groups, making it superhydrophilic.
Titanium dioxide–based glass cannot decompose thick non-transparent deposits, such as paint or silicone, waterstop fingerprints or bleeding after weathering, or stucco dust produced during construction.
Another approach to self-cleaning glass is suggested basing on the hydrophobic "lotus effect".
Since 2001 the TC24 "Coatings on Glass" committee International Commission on Glass has been trying to set up test methods for evaluation of photocatalytic self-cleaning coatings on glass.