TOUGHENED GLASS INSULATORS

Toughening

Today, all glass insulators for high voltage applications are toughened. A toughened glass cannot be chipped, as it shatters in many smaller parts. (Some cap and pin discs have been manufactured in North America for high voltage many years ago in standard non-toughened glass).
Toughening makes the shell stronger, but even the breaking into "thousand" pieces is a fantastic propriety as it clearly indicates that the remaining insulator stub must be replaced.
Localizing a failed porcelain or composite polymer insulator is a very difficult, slow and costly operation, but it is easy to localize the toughened glass insulators to be replaced by a visual inspection of the insulating strings, either from the floor, or by helicopter.

Residual strength

An insulator with a damaged glass shell keeps a residual mechanical strength, typically 80% or more of its rating and the line won't drop.
International electrical standards applicable to residual strength: IEC797, IEC1325

Color

The glass color used to be transparent green. The green shade came from the raw materials used in the manufacturing process.
Because the glass plants tend to manufacture more than insulators (glass block for construction for example), a totally transparent glass is today preferred by most suppliers.
The Saint-Yorre Sediver plant in France was probably the last plant to use green. The owner has however recently decided to stop manufacturing insulators in this plant. Future "Sediver" insulators will be transparent and manufactured in other group facilities in Italy (Dielve), Brazil (Electrovidro) and China (in Shanghai), all without the green color.
With a green shade or not, the transparency of the glass shell enables an easy visual check during manufacturing.
Seel also additional comments around insulator materials