Why do some U-shaped glass discolor after exposure to sunlight?

Glass, as a very common architectural decorative material, must not only meet the various functions required by architectural design but also possess excellent stability and durability, with no noticeable changes in weather and sunlight year-round.

To obtain an accurate answer, several types of U-shaped glass were tested. Before testing, it's important to clarify the following concepts:

Objects in nature can be divided into two categories based on whether they emit light. Non-luminous objects are divided into transparent objects and invisible objects. The color of a transparent object is determined by the color of light it reflects, while the color of a transparent object is determined by the color of light it transmits.

As mentioned earlier, the color of U-shaped glass depends on the color of the visible light it transmits. So, what determines the color of visible light transmitted by U-shaped glass? This also starts with the composition of U-shaped glass. Glass with different compositions has different colors, as exemplified by the regular glass and ultra-clear glass shown in Figure 7.

In industrial production, the sand, the main raw material for U-shaped glass, contains a certain amount of iron. Ferrous ions (Fe2+) absorb visible light in the blue and red bands, allowing more green light to pass through, giving the U-shaped glass produced in this manner a green hue.

Since building curtain walls mostly use ordinary white float glass, no oxidants are added to the glass raw materials, and thus the valence state of the oxidants remains unchanged under UV light. Ultra-clear float glass is generally produced by large float glass manufacturers, which do not add cheap oxidants or bleaching agents, and therefore rarely changes color after exposure.