Guide:Maintaining world purity

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 comparably slow speed, more considerably after defeating Plantera, it takes an extraordinarily long time for them to convert even a small world. As a result, containing the biomes might be deemed a minor, optional task, especially for shorter playthroughs. However, some worlds are played on many hours, rendering it an urgent issue.

Spread overview
Corruption, Crimson and Hallow spread actively in Hardmode:

These new properties mean that, eventually, most of the Jungle can be converted. However, in the Underground and Cavern Jungle, Chlorophyte can convert nearby dirt (back) to mud, providing some resistance in these layers.

Spreading of both biomes slows down to 50% of its original speed after defeating Plantera.

Pre-Hardmode
Prior to entering Hardmode, it is recommended to contain the existing areas of the Evil Biome. The simplest step, and often the first, is to dig a tunnel 6 blocks wide completely surrounding the areas. It is also a good idea to build these tunnels around areas of interest, such as the Jungle or the player's base.

Hardmode
As soon as 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 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 create disorder if left unattended. However trying to contain the spread at this point is a bad idea, as the new enemies can easily kill players at this point, and the tools available are not powerful enough for the task. Hence it is recommended to wait until better gear is acquired.

In order to obtain this gear faster, one might consider smashing numerous Altars in order to find the new Hardmode ores more quickly. However, note that every single Altar smashed turns a random block into its Hallow/Evil counterpart. This will barely speed up the overall spread, but can be harmful if the blocks spawn inside contained areas.

If the Hallow did not spawn on the side of the Jungle, it can be helpful to artificially create one around it to prevent the Evil Biome from converting it.

Once a Mechanical Boss has been defeated, the Steampunker will arrive, giving the player access to the Clentaminator, which helps when "cleaning" massive parts that have already been converted. 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 about this issue.

Complete Conversion
If you want to make cleansing your 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 116 blocks (the maximum range of the Clentaminator) between each other, then glide down each one using Wings or a Featherfall Potion, while spraying both sides. Additionally, the Clentaminator produces a decent amount of light that can completely go through blocks, mapping out the entire world.

As desirable as completely eliminating the spread 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 exclusive to these biomes the player may need later.