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[* black] The front glass panel of the iPhone is constructed of Corning [link||Gorilla Glass], a chemically strengthened alkali-aluminosilicate thin sheet glass that is reported to be 20 times stiffer and 30 times harder than plastic.
[* black] [link||Gorilla Glass] holds many advantages as the iPhone 4's front panel including its high resistance to wear and increased strength from a strengthening process.
[* black] What about that [link||strengthening process]? Corning uses an ion-exchange process that creates a deep compressive layer on the surface of the glass. Due to these high compressive stresses and the large effective depth of the treatment, pits, cracks, and scratches are not likely to form in the first place and even less likely to propagate to the surrounding glass.
[* black] A high amount of compressive stress is advantageous in glass under stress because glass will only crack under tensile (pulling) forces. If a sheet of pre-stressed glass is put in bending, the pre-existing compressive stresses will first have to be overcome before the glass is in a zero-stress state, and during this time the glass can deform greatly without breaking.
[* black] Corning accomplishes the chemical strengthening process by immersing sheets of its Gorilla Glass in a molten salt bath. As the glass undergoes this process, larger ions migrate toward the surface of the glass exposed to the salt bath, replacing smaller ions. It is the larger ions squeezed into their smaller predecessors' places that induce the advantageous compressive stresses that make Gorilla Glass so desirable for mobile products.