The extra clear glass is a type of silica glass, but it has low iron content, it removes the greenish-blue tint and hence glass becomes extra clear.
The extra clear glass is a type of float glass, which has very low iron content and hence is also called low iron glass. The iron content in float glass is usually 830 – 850 ppm (parts per million), while iron content in an extra clear glass is less than 150 ppm. It allows almost 92% of visible light to be transmitted.