Guide:Maintaining world purity
This means the page will walk you through a specific task, strategy, or enemy/boss fight.
Upon the activation of Hardmode, Hallow and Corruption/Crimson start spreading, potentially converting the entire world. This guide aims at providing strategies about intervening in this and maintaining world purity.
Since the biomes spread at a relatively slow speed, especially after defeating Plantera and in pre-Hardmode, it takes quite a long time for them to completely convert even a small world. As a result, containing the biomes needn't be considered urgent, especially for shorter playthroughs. However, some worlds are played on for many hours, and if not dealt with, the Hallow and evil biomes will convert most or even all of the world, and add many difficulties for the player.
Spread overview
During pre-Hardmode, Corruption and Crimson spread slowly and only via grass and thorny bushes, but they spread very aggressively and to a multitude of different blocks once the Wall of Flesh has been defeated. Defeating Plantera slows the overall spread speed by 50%.
Although near identical in spreading mechanics there are a few key differences between the three biomes, that being the Corruption's inability to spread by foliage, crimson grass's inability to resist conversion from hallowed grass, and the Hallow's inability to spread through the Jungle, the last of which is particularly notable for preventing spreading in the Jungle, or permanent Jungle destruction in 
Old-gen console and 
versions.
Pre-Hardmode
Sunflowers are a viable option for preventing spread early, as they can limit surface spread of both evil biomes, even in Hardmode. However, because they become less effective when the evil biomes begin to spread underground in Hardmode, they should be used mostly as a temporary barrier for holding back surface grass spread. Because of this, prior to entering Hardmode, it is recommended to physically contain the existing areas of the evil biomes.
To do this, the simplest step, and often the first, is to dig tunnels 3-6 tiles wide completely surrounding the areas. Afterward, another critical action is placing non-corruptible blocks (such as wood, hay, dungeon bricks, etc.) over or replacing any exposed top sides of corrupt/crimson/jungle grass within 6 tiles of the tunnel's edges and along the floor of the tunnel, as well as any exposed bottom sides of any grass type along the ceilings of the tunnels to prevent thorns & vines. If choosing not to cover any top sides of grass that may grow thorns, make sure to at least leave a barrier of 5-7 blocks sticking out from the tunnel's sides to prevent spread from thorns that may grow over the tunnel. It is also a good idea to dig these tunnels around areas of interest, such as the Jungle or the player's base.
(Desktop, Console and Mobile versions) An effective way of digging these tunnels quickly due to the minimum 3-tile width is using the Scarab Bomb(Desktop, Console and Mobile versions). The horizontal and vertical explosions leave tunnels of exactly 3-tile width. Because of this, the player can chain multiple together to produce quick and efficient tunnels. However, because the width of diagonal explosions will leave blocks within the 3-tile reach of Hardmode spread, it is recommended to only use the direct horizontal and vertical explosions.
While attempting to purify the initial world evils with purification powder can be tempting, it is almost always better to create tunnels around the biomes and wait to unlock the Clentaminator, as even in pre-Hardmode total world purification with powder is extremely costly and unlikely to succeed unless the player is very thorough.
Hardmode
Shortly after the Wall of Flesh is defeated, two diagonal strips of Hallow and Corruption/Crimson will generate in a V shape. The location of these strips and distance between them is completely random (but is determined by the world seed, so rolling back to an earlier save cannot be used to generate the strips in different locations), but if the player has done a lot of previous underground exploring, the affected areas will be very clear. These diagonal strips will begin spreading as soon as they appear, so they can wreak havoc if left unattended. However, immediately trying to stop the spread is not recommended. The new enemies can easily kill players at this point, and the tools available are not powerful enough for the task. Therefore, it is wiser to wait until better gear is acquired.
(Old-gen console and 3DS versions) In order to obtain this gear faster, one might consider smashing numerous Altars in order to find the new Hardmode ores more quickly. In Old-gen console and versions however, every single Altar smashed has a chance to turn a random stone block (that can spawn anywhere) into its Hallow/Evil counterpart. This barely speeds up the overall spread, but can be very harmful if the blocks spawn outside quarantined areas. A minimum of 3 altars must be broken in order to gain access to all Hardmode ores, but they can technically be skipped entirely if the player is able to defeat a mechanical boss without their help. Alternatively, the player can obtain hardmode ores by fishing for crates, however, this method is much slower.
In Old-gen console and versions the player's first priority should be preventing the spread of world evils into the Jungle, as in these versions evil biomes do not spread over mud, instead converting mud blocks into dirt which causes permanent damage. Reconstructing the Jungle because of this is extremely tedious and must be done block by block. In the case where tunnels have not already been dug, or if the evil biome strip generated near the Jungle, not many options are available before unlocking the Steampunker for protection. However, if a safe zone underground is set up the player may be able to slow the spread by planting Hallowed Seeds (bought from the Dryad for 20 silver) or pearlstone at the border. This is especially effective against the Crimson, as crimson grass in converted Jungles is quickly overtaken by hallow grass.
Desktop, 
Console, and 
Mobile version players should still consider setting up similar blockades around the Jungle, as overtaking Corruption will still permanently convert nearby mud blocks without Jungle grass to dirt.Verify In any case, note that Chlorophyte Ore (which can convert dirt and normal/converted grass backs back to mud and jungle grass) does offer the Jungle some protection against the evil biomes, though generation is slow.
While biome spread is constant in all block types the amount and concentration of convertible blocks greatly affects the speed of spread, with spread being fastest in Beaches and Deserts, followed by Forest, Underground Snow, and Jungle, with convertible ice blocks being too spread out within most of the surface Snow biome for spread to be possible, outside of World/Hardmode generation and player intervention. Mushroom grass is also immune to spread, although the world evil can still turn nearby mud blocks to dirt. Because blocks are updated 6 times more often on the surface, biome spread is affected accordingly, with the Corruption, Crimson, and Hallow spreading 6 times more quickly there.
Once a Mechanical Boss has been defeated, the Steampunker will arrive, giving the player access to the Clentaminator, which helps massively in cleaning and preserving world biomes. Note that this tool and its Solutions are fairly expensive, so the player should have the necessary funds at their disposal; see Guide:Making money for further reading.
- Green Solution is helpful in purifying vast portions of the world though not always the best option, as purified blocks can and will be re-converted unless the purified area is quarantined or the world evils and Hallow are eradicated first.
- A drawback to using the Green Solution is that all Glowing Mushroom biomes will be converted into Jungle. An easy fix for this is to go back to each pocket of former Mushrooms and spray it with Dark Blue Solution from the Truffle.
- Blue Solution can be useful in converting portions of the world evil near or adjacent to the Jungle to Hallow, but caution is advised as Blue Solution itself can convert jungles into Hallow despite not spreading to Jungles normally.
- This can be worked around by using Dark Blue Solution on nearby Jungle terrain first, since Blue Solution cannot convert mushroom biomes.
- Dark Blue Solution can be used as a cheaper and less infectious replacement for Light Blue Solution in preventing Jungle conversion, as Mushroom Grass is immune to all forms of conversion thereby protecting it, and can be changed back to Jungle later with Green Solution.
- Blue, Purple, or Red Solution can also be useful in conjunction with Hellevators in order to create contained pockets of a particular spreading biome. Hellevators make it easy to clean up the biome after its purpose has been served, on top of allowing you to easily travel to any height level within.
- Already quarantined Corruption/Crimson biomes can be safely converted to the opposite world evil or surface Hallow should the world not have any naturally without risk of further spread outside of quarantined areas.
- Buffs giving chance to not consume ammo affect the Solutions from the Clentaminator/Terraformer. With enough of them, lots of money can be saved as the player can spray with smaller amounts for much longer.
Complete Conversion
If the player is to make purifying an entire world a feasible task, the Clentaminator is an essential tool.
The most reliable way to completely convert or purify a world is to dig multiple Hellevators with a horizontal distance of 120 or 190 tiles (since 60 tiles is the maximum range of the Clentaminator, 95 for Terraformer) between each other, then glide down each one using Wings or a Featherfall Potion, while spraying both sides. Additionally, the Clentaminator/Terraformer produces a decent amount of light that can completely go through blocks, mapping out the entire world. When purifying it may be advisable to equip a Shield which grants knockback immunity, as taking knockback from enemies while falling down a hellevator can be very irritating.
The Biome Sight Potion(Desktop, Console and Mobile versions) allows the player to see Corrupt, Crimson, and Hallowed blocks much easier, making it helpful for targeting and cleansing every single block in a world.
As desirable as completely eliminating the spreading biomes might sound, leaving a part of the world with Corruption/Crimson and Hallow is not a bad idea if contained properly, since there are a lot of items and enemies exclusive to these biomes the player may need later, such as Souls and Biome Mimics, or entities required to complete the Bestiary.
(Desktop, Console and Mobile versions) After defeating the Moon Lord, complete purification of the world is much easier. If the Moon Lord has been defeated, one can upgrade the Clentaminator into the Terraformer(Desktop, Console and Mobile versions) by using Shimmer(Desktop, Console and Mobile versions). The Terraformer is a vast improvement to the Clentaminator as its range is 35 tiles longer and 2 tiles wider, making the conversion process much faster.
Notes
- Thorns can grow from Crimson, Corrupt, or Jungle grass. Corrupt and Crimson variants can grow 6 tiles in all directions, and spread their biome up to three blocks away like any other source, but their spread can be contained with physical neutral barriers, like Clay or Wood. Jungle thorns can spread into range of Corruption, become corrupted, and spread further into the Jungle. If left unchecked, thorns can inadvertently spread the Evil across otherwise safe gaps.
- The Dungeon can completely prevent the spread of Hallow and Corruption/Crimson if it reaches down to the Underworld. If it doesn't reach that far, it can at least slow it down.
- Floating Islands can be turned Hallowed or Corrupt/Crimson during the initial generation from Hardmode, or if converted blocks are placed near them.
- It is much easier to maintain world purity while playing on the Not the bees secret world seeds. The player is able to create artificial mushroom biomes near their evil biome's surface, and chlorophyte ore will protect the cavern layer. The Hallow is almost no threat at all due to almost the entire world consisting of Jungle biome. The only real complication is the Desert, due to the liability of sand to conversion.
