Glassing, or purification, is a protoss war technique used to destroy all life on planetary surfaces, usually to eradicate the presence of zerg.
A purification attack is carried out by ships firing massive energy beams to sterilize planetary surfaces.Carriers are equipped for this purpose, and can project energy from their bows.Motherships, and possibly other large ships may also be used to purify planets.
Glassing may disperse most of a planet's atmosphere, and release magma onto the surface by burning through the crust.Minerals on glassed worlds are often altered by the attack.
"Mr. Liberty, let me make myself perfectly clear. We have made first contact with another alien civilization. This contact consisted of them vaporizing the colony of Chau Sara. They burned it to the ground, and then burned the ground beneath it."
When the protoss discovered the extent of the zerg infestation of Chau Sara, Executor Tassadar reluctantly ordered his expedition's ships to power up their weapons, destroying the planet, firing at both terran settlements and other points. At least two ships, firing massive energy beams, obliterated most or all life. A small number of marines, including Andre Madrid, survived the attack by hiding underground.
Tassadar avoided attacking the next infected world, Mar Sara, until after the terrans had evacuated. Several hours after the escape, Tassadar's forces again sterilized a planet.
In 2504, Dr. Ariel Hanson intended to relocate several hundred colonists to a world called Haven, a world ideal in all aspects aside from that it was very close to protoss space. After supporting Jim Raynor's campaign for a few weeks, she returned to Haven to find that the zerg had infested the world and several protoss warships had entered orbit to purify the planet. The protoss, recognizing the Hyperion, asked for Raynor to help in the purification, while Hanson asked for time to complete a cure for the infestation.
Plot branch : Raynor's Raiders defend Haven from the protoss. (A canon)