Guide:Maintaining world purity

Overview
Prior to the activation of Hardmode (by defeating the Wall of Flesh), there exist only a few natural pockets of Corruption and none of Hallow. During this stage The Corruption spreads very slowly and only under very limited conditions. This all changes once hardmode is activated.

After defeating the WoF and triggering hardmode, the rate at which Corruption spreads increases dramatically. When you add the Hallow into the mix it becomes obvious that starting to contain either at this point would be an uphill and most likely futile battle. This means that preparations for containing The Corruption and Hallow are best started before activating hardmode.

Upon activating hardmode, two enormous strips will 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.

Corruption
In hardmode the Corruption spreads far more noticeably and is no longer stopped by mere sunflowers. It will consume everything if allowed to. Although there is no sure way to get rid of corruption entirely, especially with the flying Corruptors which spit corruption, it is possible to contain it. The Corruption cannot spread over certain blocks (Dungeon bricks, wood, glass, or any player-made brick except Pearlstone Brick), giving any determined player a way to box in the Corruption.

The Hallow
Despite the childish appearance of the Hallow, it is only slightly less dangerous than the Corruption. It spreads in a similar fashion except that the Hallow will not spread through the Jungle.

Details
Despite the apparently inexorable infection-like spreading action of both The Corruption and Hallow, they do adhere to several conditions which can be exploited.
 * Corruption and Hallow compete with each other; when they meet, the results are relatively stable.
 * 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 from spreading to the other side. Note that corrupted thorns break this rule, so give the structures some height to prevent them from simply growing over to the other side.
 * Corruption and Hallow will only spread to blocks they can convert that are at most three squares away.
 * 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 snow biome is protected from both Hallow and Corruption.
 * For some odd reason, Hallow is stopped by clay
 * 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.

Tips
As mentioned above, do not wait until triggering hardmode before considering containment; by then it will already be too late due to the speed of the spread of the Corruption and Hallow in hardmode. This should be kept in mind while considering the following tips:
 * 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. If saving the surface is important to you, consider creating safe zones using the methods outlined in the Quarantine section below.
 * 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 dedicated to saving sections of the Cavern layer, it's possible to dig shafts from the very surface to the bottom of the underworld that are over three blocks wide which the 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
Taking advantage of the fact that the Hallow and Corruption cannot cross large gaps, it is possible to create containment barriers and even insulated safe zones by the simple expedient of digging out the necessary shafts. The barriers should extend all the way from the surface down to the underworld, although it does not need to be comprised of one single continuous shaft; turns and junctions are entirely possible as long as all branches of the barrier eventually terminate in a manner which prevents anything on one side to cross to the other.

To create safe zones - primarily to safeguard player cultivated areas such as Underground Mushroom farms - all that is needed is for the barrier to completely encircle it, instead of creating surface-to-underworld barriers on either side.

With this in mind it is possible 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 may compromise your quarantined areas. 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.

To prevent thorny bushes from later sprouting and bridging shafts, the walls should either be lined with bricks or carefully inspected for any grass.

On the surface, structures should be built on the ground to prevent the spread of grass over the barrier. Giving them some height will also prevent the Corruption's thorny bushes from simply growing over them. Tall structures can also act as a barrier against Corruptors. An easy way to make a vertical barrier is by using silt blocks. You can build a silt barrier by using a high altitude platform and then droping silt to the surface. This method prevents corruptors from spawning while you are building the barrier and requires little effort.

Finally, the dungeon does not normally extend downwards completely to the underworld, so if you wish to capitalize on using the dungeon it is necessary to dig out a shaft extending from underneath the dungeon down to the underworld to complete the barrier.