by tonytran2015 (Melbourne, Australia).
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(Blog No. 171).
I recently bought some decorative diamond shaped glass display items made in China. Surprisingly some of them have faded after three months being in sunlight on the inside of a glass window. Checking on this strange event reveals that it is a little known fact interested by many people including glass collectors. 
So it is worthwhile to inform people this little-known fact that some types of beautiful color glass do fade under sunlight while some others do not.
#color glass fades under sunlight, #sunlight bleaches, #irradiation, #antique glass, #stained glass, #color glass, #glass art,
Does glass fade ?
1. Some types of color glass fade under sunlight.
Figure: A diamond shaped glass object placed on top of a prism of colorless clear glass. The diamond shaped object is now almost colorless and retains only a faint taint of its former deep orange color.
I placed some glass objects on my window sill and found some surprising results:
Some diamond shaped decorative glass objects of red, yellow, pink colors have not faded while similar objects (bought from the same shop at the same price) of orange, green colors have faded significantly.
The fading is in the bulk of the color glass, not in its surface coating (None of my glass items had any color coating).
To know why the colors fade we have to know why some types of glass have colors.
2. Why do some types of glass have colors ?
Glass makers mix compounds of transitional metals into clear glass to give it colors. The atoms of transitional metals have many unfilled outermost shells of electrons. Electrons can jump between those unfiĺled outermost shells absorbing or emitting light with the exact visible wavelengths corresponding to the differences between discrete energy levels of the shells (Those atoms act like resonating antennas placed in the beam of incoming electromagnetic waves having those wavelengths.).
The energy levels between unfilled shells and the number of electrons in those shells of the transitional elements change with the electronic configuration of their atoms. That is why a single transitional element may have different colors in its different compounds.
3. Practical effects of change of colors in the bulk of glass.
Figure: Colorful object of glass art such as this should be kept away from sunlight when its fade resistance has not been ascertained.
Effectively, some change in the bonding of transition metal atoms with its surrounding atoms is enough to change the color of the glass. Heat, sunlight and irradiation can all cause this change.
Some dishonest antique collectors subjected their antique glass items to irradiation to change their colors. The items are then sold on at higher prices as antique items with rare colors.
” Fakers and forgers are now using a new method to alter the color of antique and collectible glassware. It is called irradiation. … irradiation changes glass internally within the very atoms of the glass .” This precaution also applies to gemstones .
Prized glass items of beautiful colors are usually made by reputable glass makers from non-fading glass. Similarly gemstones are appreciated because theirs color are non-fading.
Stained glass windows should be made from only time-proven non-fading types of glass as sunlight may bleach any untested new type of color glass.
It is little -known but sunlight and irradiation can change (bleach or deepen) the colors of many glass items. Consequently only glass and gemstone items of proven non-fading types be displayed under sunlight.
As sunlight can bleach many types of color glass, stained glass windows should be made from only time-proven non-fading types of glass.
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