Guide:Maintaining world purity

Corruption
Once you enter hardmode, things become a lot more...well, hard. After defeating the WoF for the first time, the rate which Corruption spreads increases dramatically and is no longer stopped by mere sunflowers anymore. It will, if you let it, consume everything. Although there is no sure way to get rid of corruption entirely, especially with the flying Corrupters which spit corruption, there is a way to contain it. The Corruption cannot spread over certain blocks (Dungeon bricks, wood, or any player made bricks for that matter), giving any player rich in resources a way to box up the Corruption.

The Hallow
Despite the childish appearance of the Hallow, it is only slightly less dangerous than the Corruption. But the Hallow can be quite rewarding to anyone with high-tier equipment.(Note: The Underground Hallow is not a place to be caught unprepared, more so than the Hallow Surface. If you insist on going Rambo on the Underground Hallow, guns blazing, at least have a Magic Mirror and acceptable armor & weapons.)

Methods
Upon activating hardmode, two enormous strips erupt from the center of the Underworld diagonally outward toward the surface. One is Hallow, the other is Corruption. Note that existing Corruption chasms are also a threat. The two strips do not stop at the surface. Floating islands in the path of the strips will be converted.

The player has a few tools at their disposal.
 * Corruption and Hallow only spread to grass from infected grass. Placing stone or wooden structures at least five blocks wide will prevent the biome on one side taking over the other side.  Note that corrupted thorns break this rule, so give the structure a little bit of height, too.
 * Stone and Sand can be isolated from the spread of Corruption and Hallow by a 3-block-wide buffer.
 * The Jungle is immune to the Hallow, but not Corruption. A buffer zone of Hallow on either side in the cavern layer is strongly recommended if the Jungle is to be saved.
 * The only advantage of a normal Forest over the Hallow is that it's safer to live in. There are no drops that can be acquired in a normal forest that can't be acquired elsewhere.
 * The Dungeon is entirely immune to everything and provides a natural barrier, which may keep one edge of the map perfectly safe in many cases.
 * Floating Islands have no special immunity, but are relatively safe due to their isolation
 * Corruption and Hallow compete with each other; when they meet, the results are relatively stable.

Trying to save the Cavern layer requires a full quarantine. It is mostly continuous stone, all of which is susceptible to Hallow and Corruption. Given that, you may be best off simply plotting which of the two (hallow or corruption) you want in a given area. Placing Ebonstone and Pearlstone prior to defeating the WoF can make the process run smoothly.

If you are entirely dedicated to saving a section of the Cavern, it's possible to dig shafts from the very surface to the bottom of the underworld that are four squares wide which Corruption and the Hallow can't cross. Note that if the players destroys even a single Demon Altar, they risk infecting this quarantined section anyway.

Trying to save natural deserts is very difficult without a full quarantine. If any sand touches an infected part of the cavern layer, or surface corruption/hallow reaches it, the entire desert will become infected very quickly.

It's possible, to reclaim a section of the surface using Purification Powder and Vile Powder. The most difficult part of this is ensuring you've completely gotten any deposits of Ebonstone and Pearlstone.

Quarantine
To section off a part of the map to be truly safe from the Hallow and Corruption, you'll need to take two steps. Note that this is best done before activating hardmode.


 * 1) Aboveground structure. Build a small structure on the ground to prevent grass from spreading across the surface. Make sure you leave no grass under the finished structure and that it is high and wide enough to prevent Thorny Bushes from simply growing up and over it.
 * 2) Vertical shaft. It must be at least 3 blocks wide, and must extend from your surface structure all the way to the Underworld. It must not have Corruption grass growing on the inside of it. To prevent Thorny Bushes from later sprouting bridging the shaft, the shaft walls should either be lined with bricks or carefully inspected for any corrupt grass.

Once these two steps are complete, Corruption and Hallow on one side will not be able to reach the other.

It is possible to use this method to quarantine the initial locations of Hallow, Corruption, and the surface corruption chasms off from the rest of the world. Note that breaking a Demon Altar creates hallow and corruption somewhere in the world and compromises your quarantine. If you are dedicated to preserving your world from the forces of good and evil, it's best to mine your hardmode ores on another one.

Of course, you must be extremely careful around Corruptors near a quarantine. Extending your aboveground structure vertically may provide some additional security in this regard.