Cleansing Corruption / Crimson and Hallow (With lots of pictures)

Lots of pictures! Are you tired of your world being taken over by Corruption or Hallow? Do you want to know how to protect your biomes from the sudden influx of hardmode spread? Want to know the most efficient way to take care of this? Ever wondered what the big deal is? -If you answered yes to any of the above questions then you might want to read this.

Some things that are good to know about the spread

As the title might reveal, there are 3 Biomes (areas of different types) that can spread throughout your world. If left unchecked they might go out of control.

When your world is generated there will be either Corruption or Crimson (you can see which one by looking at the tree icon on the world selection screen). They are nearly identical and serve the same purposes but both will not spawn. I will sometimes only mention corruption and hallow. In this case "corruption" refers to both the Corruption and Crimson biome. (This is because when I made this guide there was no Crimson biome, it was added later).

For actions to take for preventing spread check the section called "Preemptive Actions". If you want to know what to do when the corruption is already there go to "Damage Control"

Spread in a Pre-hardmode world
There are few dangers with spread pre-hardmode when you start the game. In a pre hardmode world Ebonstone/crimsonstone won't spread to other materials. Neither will red/purple ice or ebonsand/crimsonsand.
So your only issue is the corrupted/crimson grass growing on dirt. It only spreads to Dirt and only to adjacent tiles. More specifically, the grass does. So this is the time if you want to take preemptive meassures.

A world usually starts with two corrupted areas. One in the east and one in the west, this can be good to know so you don't miss them. The size of the world also increases the number of biomes of course.


Gaze upon this awesome map depicting a typical world with crimson

How do the biomes spread?
There are different materials that become converted and in turn spread the biome further. Dirt and Stone are the most common. All types of grass and moss can spread to adjacent blocks of the correct type. So dirt above the underground layer will grow grass that spreads to adjacent dirt blocks.
In the Snow biome ice blocks are converted while snow is not.
In the desert everything can be converted, sand block, sandstone block, hardened sandstone blocks.
And even the mud blocks of mushroom and jungle biomes are slowly turned into dirt as it is converted.
Ores do not become corrupted, but trees are when the grass below them is converted. Ash, Clay, Granite and Marble cannot be converted and all playermade bricks are immune as well; grey bricks, glass, smooth granite etc.
Even demonite bricks and crimstone bricks are immune to corruption and won't spread it either despite their names. In fact, very few things can spread and be converted when you see the bigger picture.

When it comes to the actual conversion in Hardmode though the general rule is that a block can convert blocks up to 3 tiles away on each side.

• This means that a block can convert a block 3 tiles away even if there are like 2 platinum ore inbetween.
  
• It can also spread even if the two blocks aren't connected. So your everyday 2-tile-wide hellivator can spread from one side to the other.

A picture of an experiment done early, I wanted to see what could spread to what.


However, in hardmode dirt, or rather grass, will have foilage. These grow on grass and can increase the "range" vertically with thorns growing up and vines growing below. They can sometimes grow thorns slightly to the left and right as well. So if you dig a trench on the surface you might want to make it 4 or wider or line the edges with something that can't grow grass.
The rest of the blocks will spread to blocks within the 3 tile radius. Once spread begins this creates a snowball effect that quickly gets out of hand.

The following GIF shows what a single Crimsonstone Block does to the Desert Biome if left for a day in hardmode. It was placed at 2 pm and the last picture is taken 12 pm the next in game day.

2:50PM > 10:24PM > 4:23 AM > 4:38 AM > 12:21 PM. Note that each in game minute is a real life second so 15 seconds have passed between picture 3 and 4.

Corruptions vs hallow
Something to remember is that dirt with hallow grass cannot be converted into corrupted grass. Neither can Ebonstone blocks be converted into pearlstone. And the other way around, so they cancel eachother out. (this is also true for crimson and corruption, they can't convert each other.)

Walls don't spread to blocks:
Although walls are converted by blocks and can spread corruption to nearby walls the walls themselves do not spread corruption to nearby blocks like this picture shows.

Differences in Hardmode

The coruptions spread isn't a problem in a prehardmode world, the conversion rate is pretty slow and only affects grass. However, as hardmode is activated Hallow will spawn in the world and the conversion rate is increased a lot and more blocks can spread.
Besides the faster spread, a V-shaped line of corruption and hallow is drawn from the underworld up to the surface. (Flying islands can become converted this way)
This will also be the start of the underground corruption/crimson and hallow biomes and they can spread rapidly thanks to the access to blocks that can be converted (Stone mostly).
Since Ash blocks can't be converted you can'tt tell where these "lines" started and what direction they went. But as you move up you can usually tell.
(It's also possible to notice if you made your hellbridge out of stone.)


The amazing map is back, now also animated just to show how varied the lines can be, note that it will never strike the center.


You can see the red area that has been converted to Crimsonstone seconds after the WoF has been defeated.

Breaking altars
A spot of corruption and hallow can spawn randomly (66%) underground whenever you break a demon/crimson altar with your hammer. This only happens in the Cavern Layer. It's worth noting that it doesn't just randomly spawns, Stone is converted into ebonstone/crimsonstone. This can easily escalate.
With The Labor of Love update this supposedly isn't a thing anymore.


This shows another anomaly. When the line is drawn that creates hallow after WoF it sometimes crosses the corrupted biome converting all the crimsonstone into pearlstone. This is however the only time that can happen.


Slowing the spread in hardmode
To make your life easier you need to defeat the boss Plantera. Defeating plantera does 2 relevant things.
1 it spawns more chlorophyte (which prevents jungle spread)
2 it slows the spread down to a comfortable level. So the time you take to defeat plantera in hardmode is usually what decides how bad the spread becomes before you start cleaning it up.


Checking the spread:
At any point in the game you can ask the Dryad about your world's "Status" this will give you a percentage telling you how much of your world is corrupted and hallowed.
Sometimes you need to leave and reenter the world for the numbers to be correct.
With The Labor of Love update there is now an Achievement for achieving 0% crimson, corruption and hallow.

Useful FaQ

Q: What can spread corruption or be converted?
A:
In pre-hardmode mode:
Corrupted/Hallowed Grass, aka dirt on the surface
Corrupted/Hallowed Plants such as thorns, vines, mushrooms.

In hardmode:
Corrupted Grass, aka Dirt blocks now also below surface.
Hallowed Grass, aka Dirt blocks now also below surface.
Corrupted/Hallowed Plants such as thorns, vines, mushrooms.
Corrupted/Hallowed Stone blocks
Corrupted/Hallowed Sand blocks
Corrupted/Hallowed Sandstone blocks
Corrupted/Hallowed Hardened Sand blocks
Corrupted/Hallowed Ice blocks

Note that Dirt without grass isn't converted unless it's by another piece of dirt with corrupted grass.


Q: How far can it spread?
A: In normal mode only adjacent tiles.
In hard mode anything up to 3 tiles away. However, Dirt with grass can reach even further on the surface thanks to the growth of thorns/vines.

Q:Does Crimtane Ore and Demonite Ore spread corruption?
A: No.

Q:Does corrupted icicles forming below ice and snow blocks spread corruption?
A:Based on my tests, no.

Q: Can Background Walls convert Blocks?
A: No, but it can convert other background walls and the background is in most cases what decides enemy spawns.
Courtesy of Alucard:
https://steamcommunity.com/app/105600/discussions/0/2791621151153302818/

Q: What Background Walls can spread corruption?
A: They don't really spread corruption, they are affacted by the nearby blocks.

Q: What useful blocks can stop spread?
A: Only Chlorophyte Ore and Chlorophyte Bricks (crafted from Ore) actually has a positive effect on this. According to datamining having 5 or more C-blocks within 10 block range will stop that tile from being converted. Note however that this only protects Mud.
In any other case, just any type of object or block that can't be converted.

Q: How fast does it spread
A: It reportedly spreads above ground every 140 seconds and below every 830 seconds. This is taken directly from the Wiki. I probably don't have a way to test this.

Q: How does it help to defeat Plantera?
A: It reportedly halves the spread rate (how fast it takes to convert) so if the above numbers are true then every 280 seconds above and every 1660 below.

Q What about these other things that spread / come back?
A:Jungle Grass and Mushroom grass will spread on Mud.
Chlorophyte Ore can convert nearby Dirt to Mud and Mud to Chlorophyte Ore.
The 8 different kinds of Moss spread across Stone and Grey Bricks to adjacent tiles.
Cobwebs don't spread, instead the spider mini biome has backgrounds that makes it come back.

Q: How is the V actually generated?
A: It depends on your world size and which side the dungeon is in regards to starting poitn. But the middle of the surface can't be targeted. In a small world the width is approximately 200, medium 320, large 450 blocks..

Q: What was it like in previous patches?
A: A lot has changed. When I started playing in 1.1 the Corruptor enemies would spread corruption with their ranged attacks. Spread slowed down when all Mecha trio bosses were defeated because that was the end game back then.
There was no Ice that could spread the corruption in the snow biome (because there was no icea, just snow) there wasn't any corrupted versions of sand either.
Because of the lack of mini biomes in the underground (granite, marble, spider, desert, tundra etc) pretty much the entire world quickly became corrupted when you let it get out of hand (because just about everything was Stone or Dirt down there)
Crimson was added in 1.2 including a lot of new things.
And finally, before 1.2 there was no Clentaminator. That thing is a life saver.
Another thing that is helpful now is the generated minecart tracks underground which creates some natural division between areas.
The introduction of more underground biomes and mini biomes (such as Marble and Granite) also makes natural barriers.
And finally with the labour of love update the game no longer randomly spawn corruption when breaking Altars, and Mud is no longer converted into Dirt which removed the fear of the Jungle disappearing.

Preemptive Actions

As the name suggests, this section lists things you can do before the corruption actually reaches you. If you need advice on what to do when things are already corrupted - go to the next secion "Damage control".

Sunflowers
Close to your spawn you will most likely find sunflowers, in this game it gives you a buff called "happy" that decrease monster spawn rate and increase your movement speed. But what they also do is slow down the spread of corruption on the grass blocks around them and counter the Graveyard Biome.
However, according to the terraria wiki this corruption prevention is basically lost once you enter hardmode. If that wasn't the case - these would be the perfect way to stop corruption. Note that they will prevent grass under it from getting converted, but you need plenty of them. And the corruption can still spread underground.

But that is enough to protect against the corruption > grass spread in pre hardmode.

Digging a trench
If you read the previous section about how corruption and hollow spreads then you probably know that they spread to adjacent blocks (and blocks up to 3 tiles away in hardmode). This means that one way to prevent the spread is to dig a hole 3 tiles in width, that way the corruption can't reach across or spread to the other side.
Note that corrupted grass on dirt sometimes grow vines so it migth be worth it to make it wider, like 4-5 tiles wide, line the outsides with a non dirt block (to avoid thorns growing) or placing a Rope type object running down the side (or platforms) which occupy the spot.
You can either dig your trench near your base to shield it, or you can dig it around the corrupted area to prevent it from spreading from there.
It is adviced that you do this for your jungle if it is located close to a corrupted area because it's easy to lose parts of it in hardmode.

Oh no corruption is getting close to the middle.

That settles it.

Explosives
If you don't feel like digging you can always use bombs to take care of the problem. Bombs will make small explosions and dynamites will make larger ones. Just be careful not to get caught in the explosion or you might just end up dead. The hole will not be as straigth but it will be wider, especially if you used dynamites. You can read more about explosives in the "Useful items" section.
Remember that desert? Maybe we need to cut it off before it continue spreading.

This is how deep you get with 80 bombs.

The Floating Island solution
Even if you dig/bomb a trench you need to dig it all the way down to the underworld or it will eventually reach the bottom and spread up the other side. Another solution is to go around and cut off your base area from the cavern instead. This is called the floating island strategy. An extreme to this strategy is that you build everything 6 tiles above the ground from the beginning.

Either way, in 1.3 they added underground railways. These are always high enough to stop corruption from spreading vertically. So if you find one under your base this speeds up the process a lot. But the basic idea is that you dig/bomb down to the desired depth and then starts digging under your base horizontally until you reach your other trench. You have now successfully separated your base from the rest of the world and it's safe.

If you want to be extra sure you can dig out even more like this:



Building something in the way
This one is the least effective way to prevent corruption. In the picture below you will see a house made of wood blocking the crimson spread. In pre-hardmode this is fine since a single block can interrupt the spread. In this case since it is wider than 3 tiles the corruption can't reach the other side of the house in hardmode.

The good thing about this is that corrupted plants can't grow. Some people fill their trenches' sides with glass blocks and such, but it's more worth it to just replace the corrupted side with some blocks along the edge on the surface layer. Or add ropes and platforms, if there is an object there already the game can't spawn a new one.

Also note that the NPC still lives in the house. There is too little corruption nearby to upset her currently.

However, it's hard to know what lies below. The crimson could easily find some Stone Blocks below and travel around. As you can in this picture from the snow biome. Snow can't spread corruption, but there is ice below that became corrupted and then spread upwards again to the other side.


Planting Hallow to block the corruption
This is by far the easiest way to do it. But it is also only availabe in hardmode. Or if you grab hallowed seeds from a hardmode world and bring it to a normal world.
Since the corruption cannot spread on hallow and the hallow cannot spread on corruption that means that planting hallow between you and the corruption will prevent it from getting closer. However, since the hallow will also spread that means that the purity between where you live and where the hallow was planted will turn hallow not before long.
So if you simply want an easy way to keep parts of your world safe, make the area around it Hallowed to act as a natural wall against corruption.

NPCs can handle living in a hallow biome unlike a corrupted one. But you will have to deal with daily pixies and nightly gastropods.
In 1.4 Hallow is a biome that NPCs live in and can give you a Pylon. But it also includes a certain new critter that could cause problems.



Chlorophyte:
Limited to Jungle use really. The Chlorophyte Ore found in the underground jungle is rather unique since it spreads to nearby Mud blocks. Not only that but it will also slow down the conversion rate of nearby mud blocks. It is not adviced to rely on chlorophyte however. It's much more efficient to separate the jungle from the corruption.



Preparing for hardmode
Before defeating WoF and starting hardmode I tend to dig 3 hellivators. One in the middle, one to the east and one to the west. That way the entire world is separated into 4 different (vertical) pieces. If you want to cut it up even more you can do that with more hellivators or making a tunnel in the middle of the world right across. Even if one of the pieces are corrupted or hallowed it won't spread to the other ones. So if you have cleaned up an entire zone, you can leave it.


I usually do something like this. Effectively reducing the chance of anything going out of control. Digging down is always easier than digging sideways so it's usually all you can muster in normalmode.

Note that with 1.4 and the additions of pylons you will no longer just keep a single base in the middle of the world with all NPCs. So you may need more trenches to protect all of those bases.

Damage Control

You snoozed off and your base is now covered in red slimy crimson. You don't know what to do to make your best friend the Arms dealer to move back in with you. This is the section that explains what you can do to cleanse the spread when it's already there.

Clentaminator:
The Clentaminator is by far the best tool to remove unwanted corruption or Hallow. Using Green Solution you turn all of it into ordinary grass, stone, desert and whatnot. You need to reach Hardmode however and buy ammo and that's why ammo reservation can help a lot.The range of the clentaminator is also great making the lesser alternatives look like a joke.
Just note that Green Solution turns Mushroom Biomes into Jungle biomes.

Read more about the clentaminator in the useful items section.

Remember the picture of the desert being corrupted? That's the result of a single block during 24 minutes. So make sure that you cleanse all of the corruption when you spray.

Lesser alternatives:
Purification Powder and Vile Powder.
Purification powder turns corruption into purity and vile/vicious powder changes Hallow into purity. The range is puny however and you easily miss blocks, but it is the only way to cleance corruptions in a pre hardmode world.


Pretty lame huh?

Unholy Water, Holy water and Blood water:
These spread their respective biome. It's not overly useful if you want to cleanse your biomes but good if you want an isolated spot of biome somewhere or want to stop corruption by planting hallow.

Trenches/Hellevators and Bombs 2:
Although they work great as preemptive meassures you might want to dig more trenches and do more bombing once hardmode starts because of the new biomes that are created with the V. Do this to separate your world into smaller zones rather than one big.

Look at the Useful Items section for different suggestions on how to do this.

Just before hardmode begins it's good to open your map and loot at what you might want to keep intact. Usually the jungle and an underground mushroom biome, your NPC housings etc. Do some more trenches, or divide it up even further.

Why is it important to stop?

Several people ask why they should remove corruption or prevent its spread. Here are some reasons.

• The Invading biomes will replace most of the wildlife in the biome it converted removing the "Purity" as it is called on the official wiki.
  
• For starters, having too many corrupted blocks near a housing will prevent NPCs from spawning there. The house will be deemed "invalid" until it has been cleansed. However, the NPCs don't react this way to the Hallow.
  
  
• Underground Mushroom and Junlge biomes will be destroyed and the materials that grow there could become unavailable for you. Such as jungle spores and glowing mushrooms, and plants etc..
  
• Several Fishing quests from the Angler requires pure biomes. For example a pure jungle, snow, underground snow, surface, underground and so on. Fish of different kinds can also be used for potions as well.
  
• Added in 1.4 is a Town and Pylon system (teleports), if the biome they are in get corrupted you can no longer teleport there since each pylon only works in the biome they were made for.
  
  
• Pre 1.3 all plants needed to grow on specific materials. If everything became corrupted only Vile Mushroom and Deathweed grew. In 1.3 however they added Planter Boxes where you can plant any type of seed - if you have any.
  
• The Jungle and Underground jungle can be destroyed by corruption. Mud blocks are turned into Dirt as they are converted. Using Green solution on them will turn them into pure grass rather than mud again, which means it can't be easily fixed. More on that in the section "Save the Jungle"
  
• A corrupted or hallowed ocean will no longer spawn Ocean monsters. 1.3 made the converted oceans still spawn the ocean monsters. However, Duke Fishron will now enrage when outside an Ocean biome.

Should you remove all of it?

Unless you want the achievement or a creative world then no, not really. A world completely without Corruption or Hallow will have a hard time to progress on its own.

Here are some reasons why you might want to leave some of these invading biomes:

• Underground Corruptions drops Souls of Night, an important crafting component. Underground Hallow drops Souls of Light, an equally important crafting component. They also each spawn a rare Mimic that can be forcefully summoned using Key of light/ key of night placed in a chest, But those keys are crafted using the souls as well.
  
• Corrupted undergrounds drop Cursed Flames, Crimson underground drop Ilchor, Hallow Underground grow Crystal Shards on Pearlstone. These are all used to craft early hardmode gear and ammo/flasks.
  
• Each of these Biomes have a chance to drop a Biome Key used to open their respective chests in the dungeon.
  
• A converted underground snow biome will spawn some unique enemies such as pigrons and fishing there has a small chance of rewarding the player with a mount.
  
• Converted biomes drop materials that are used in come recipes such as potions, boss summons etc.
  
• Hallowed Biomes are the only biomes that can spawn monsters to acquire Pixie Dust, Unicorn Horn and Rainbow Blocks.
  
• Each biome has their own fish and Fishing crates, also for fishing quests
  
• Each Biome has its own type of tree that can be used to craft furniture and most enemies grant you a Monster Banner once you kill 50 of a type of monster.
  
• You can even turn Crimstone blocks into Flesh Blocks using a Grinder (dropped by enemies in the Crimson) to access even more furniture (with the Clone Vat from the Stempunker).
  
• You can even turn Ebonstone blocks into Lesion Blocks using a Grinder (dropped by enemies in the Corruption) to access even more furniture (with the Decay Chamber from the Steampunker)
  
• Added in 1.4. There is a Hallow Pylon so it makes sense to build a town in the hallow, several NPCs also prefer to live there.
  
• Added in 1.4 are two new bosses. One in the Hallow overworld, one in the Hallow underground.
  
• Also in 1.4 is the Bestiary, to complete it you need hallow and corrupted enemies from each biome and hybrid biome.

Also even though your biomes gets destroyed you can always restore them.

This is what happens to your mushroom biome when the corruption gets it.

Saving the Jungle

The most important thing to do in your world is to save the Jungle Biome.

This is what happens to the jungle if the corruption gets to it in hardmode: (pre labour of love)*


The Jungle is important because: You need to fight Plantera in the Jungle underground, find Life Fruits and mine Chlorophyte and eventually enter the Jungle Temple to progress

In pre-hardmode you're safe. The Corruptions can't convert the Mud blocks the Jungle rests upon or the scattered Stone Blocks hidden beneath.
But as WoF is defeated and hardmode is activated, the race against time begins.

Preemptive action
First of all, you want to ensure the jungle is isolated before you start Hardmode (kill WoF). Do this by giving some berth and then digging or blowing up the things on the side 3-4 tiles wide. Basically, build 2 Hellivators on each side of the jungle biome. (This works better if you have explored it beforehand and know where the temple is for example.

PS. It's fine if you cut off parts of the Jungle, you don't need all of that anyway. Do yourself a favor and only do a reasonable clearing or you'll go mad.

Also check the bottom of the underground jungle to ensure that there isn't a large cluster of stone before the layer of Ash in the Underworld.

In hardmode Chlorophyte will grow on mud, and spread a little to give protection to the Mud in the vicinity, so a sort of natural protection to stop it from getting too much out of hand.

If you do all of this, the Jungle Biome should be safe for the rest of the game. (kinda)


What if it's not? What if the V shaped corruption went right through the Jungle biome and it seems like I missed some rocks?! OMG the Mud is turning into DIRT!

The rate of spread is increased a lot in hardmode. Luckily once you beat the 3 mech bosses and Plantera it will slow down again to a much more managable level.
So if you are unlucky and the corruption part of the V strikes your Jungle you're going to lose parts of it.
As already pointed out in the guide, beating 1 mech boss unlocks the Steampunker and Clentaminator, using Green solution in the Clentaminator will remove the corruption, but what you will have instead is dirt and grass. And that ain't Jungle.Therefore it's kinda irreversible.*

When a Demon/Crimson Altar is broken there is a chance for a Stone Block to be corrupted in the Cavern Layer, this means that you should look for stones. Stones are the big villains in a diminishing Jungle Biome. *

So what can you do about that? Well you can turn those Stone Blocks into Pearlstone, the Hallow version. Hallow will not destroy your jungle.
So spraying that Solution at patches where you see stones will turn them into pearlstone and remove the risk of them ever turning corrupted. (Note, this seem to have changed. The Blue/Red/Purple Solutions now convert mud to dirt.)*

But we got the second best thing - Block Swap which means you can replace the Dirt with Mud right away once cleansed (or before). Don't have Mud? Grab some Dirt, stand in water - craft Mud Blocks. *
In the end, you should still isolate that part and look for the source.

Bombs and explosives is your real friend here in hardmode, don't take any chances, cut off the rot with good measure if something is off.


It's all gone!
If you really drag out on Hardmode, didn't do anything about the jungle and was unlucky with the V then there is a chance that your Jungle is broken by the time you go to fight Plantera. Very unlikely though.
But if it is, and you don't have any Jungle Grass Seeds, well then you need to go into another world to grab some I'm afraid. Because without a Jungle there is no Plantera and with no Plantera there is no progress.*

You can always create an artificial Underground jungle biome wherever it suits you. I've done plenty of Chlorophyte farms and encountered both Plantera Bulbs and Life Fruits to know it works. Just note to make it larger if you intend to fight Plantera there since she enrages if she deems the area having too few jungle blocks.
http://steamcommunity.com/sharedfiles/filedetails/?id=499192072

*: With The Labor of Love update there is now corrupted versions of Mud with jungle grass so this issue shouldn't be a problem anymore, just clear it like you normally would.

Useful items

Clentaminator:

The blessing of the 1.2 update and the best item to cleanse your undesired biomes is this amazing gun sold by the Steampunker for 2p (1p 60s with discount), she can move in after defeating one of the Mech Bosses. It's rather expensive and uses ammo called Solution to make it work. Since the Steampunker doesn't spawn until a mechanical boss is defeated in hardmodethis might take some time to get for some.

Ammo conservation:
Since the Clentaminator uses ammo you can make your purge more effective using ammo reservation gear. The highest ammo conservation you can muster will be approximately 60%. This is using the Ammo Reservation potion(20%), The Ammo Box(20%) and armor with reduced chance to consume ammo such as Cobalt, Mythril, Adamantite, Hallow, Chlorophyte, Shroomite, Vortex.

The Ammo box is sold by the traveling merchant for 15g, grants 20%.
Ammo reservation potions can be made using Bottled Water, Moonglow and Double Cod (captured in surface jungle). Grants 20%.

20% Necro armor set bonus, Cobalt armor set bonus, Mythril armor set bonus. Chlorophyte Helmet, Shroomite Breastplate
25% Adamantite armor set bonus, Hallowed armor set bonus, Vortex Breastplate

As you can see, the highest armor set bonus is 25%, This with the Ammo reservation and Ammo box results in a ~52% chance to not consume ammo but equipping the Vortex breastplate with the Chlorophyte Helmet results in the highest reduction (~62%), better than equipping a full set. Even the Shroomite Breastplate with the Chloro Helmet results in this phenomena, ~59% chance to not consume ammo.

Solutions:
The ammo used in the Clentaminator. I will simply list them here for convinience. Note that the buy price is 25s for each solution, none is more expensive than the others. (They cost 20s with the discount card or greedy ring, but note that 999 green solution will still cost you 1p 99g as opposed to 2p 50g so you save around 50g by using the discount)

Green Solution, this is what you will spray to fight the invading Biomes. Sold when nothing special is happening by the Steampunker. This turns Crimson, Corruption and Hallow into normal Dirt / Stone / Mud and Mushroom grass into Jungle grass.

Blue Solution, this turns Dirt and Stone into Hallowed grass and Pearlstone. Sold by a Steampunker in the Hallow Biome. You can either build a house there, temporarily turn the steampunker's house into a hallow biome using other items. Or by using the Queen Statue.

Purple Solution and Red Solution, This turns Dirt and Stone into Corrupted/Crimson Grass and Ebonstone/Crimstone. Sold by a Steampunker during Blood Moon and Solar Eclipse Events. Much like Vicious/Vile seeds and powder from the Dryad.

Dark blue Solution, this is used to turn Jungle grass into Mushroom grass. Sold by the Truffle.

New Solutions: White, Yellow, Brown: These completely change th biome. Snow, Desert, Forest.

New Items:

TerraformerDid you like the Clentaminator? Well the Labor of Love update (1.4.4) added this beauty. The Terraformer is a direct upgrade reaching further and has a wider beam of cleaning going on.
How do you get it? Just toss your Clentaminator into a pool of Shimmer after beating ML

Biome Sight PotionThe name says it all. This 5 min buff will highlight Corruption, Crimson and Hallow blocks.
Craft it with Water, Fireblossoms, Blinkroot, Moonglow and Grass Seeds

Useful Items 2

Other items

Long range Pickaxes:
When trying to remove corruption and other blocks it can sometimes be rather efficient to use pickaxes with longer range than normal:
Spectre Pickaxe + 3, the Lunar pickaxes +4, The best one for this use is the Laser Drill (+10) dropped by the Martian Saucer during the Martian Event.

Drill Containment Unit:
Or "the world clearer". A flying mount that only mines but does so at the same range as the laser drill and up to 4 blocks at a time. This is more for when you only remove everything instead of trying to cleanse it. Like the bombs. Really effective when making hellivators wide enough to stop spread but kinda inaccurate.

Digging Molecart
Another 1.4 item which is a Minecart that digs and lays Minecart Tracks from your inventory (at the cost of moving slowly). It uses the pickaxe in your inventory for power, can't be used on Sand or on the surface but it gets the job done if you want a chill way to mine horizontally.
Is bought from the Zoologist for 50g once the Bestiary is 60% filled out (312 entries) so Hardmode.

Sunflowers:
An item that slows down the spread of corruption during normal, the 2 grass below them cannot be corrupted if there is more than 1 present on the block. The real winner with this however is that each Sunflower makes things "normal" counteracting NPCs reactions to nearby corruption. It's more like a furniture in the way it is placed and picked up. Sunflowers can make a house in corruption suitable with enough blocks to counter it. You can buy these from the Dryad for 2s.

Seeds:
The Dryad sells seeds, the seeds can be placed on dirt on the surface to grow grass. She sells Grass Seed the moment you get her for 20c. During hardmode she also sells Hallow Seeds for 20s. During a bloodmoon she will sell Crimson Seeds or Corruption Seeds as well for 5s. Place her in Echo Mist (Graveyard) to make her sell the seed of the other corruption kind.
Jungle grass seed is used to spred jungle grass and mushroom grass spreads glowing mushroom on Mud. These are not sold however and needs to be found.

Powder:
In normal mode you have access to Purification Powder to cleanse areas back to purity and you can even spread corruption with Vile Powder or Vicious powder. Purification powder is sold by the Dryad for 75c. The other two for 1s but only during blood moons.
This affects a small area in the direction you use it and is mostly useful to purge corruption early in pre hardmode. It is adviced to use to get rid of Ebonstone/crimsonstone to reach shadoworbs/crimson hearts. However, it is not a good solution when it comes to cleansing your world.


Explosives:
Very effective, but it doesn't reset spread... It removes the blocks entirely (and background walls). Useful especially when mining ores like meteorite but also demonite and crimtane. Why? Because the early pickaxes can't break them. For this you mainly have 2 items. Bombs and Dynamite. They can both be bought from the Demolition man. The cost of Dynamite is only 20s now (it used to be 1g) and bombs can be bought for 3s. Bombs can also be aqcuired if you hook up bombs statues with a switch or timer.

Scarab bombs:
Another 1.4 addition. This bomb explodes in a straight line depending on your location. You can preview the explosion pattern with a blue glow. Crafted with Bombs and Sturdy Fossils (gained from jamming Fossil Blocks into an Extractinator)

Explosive Ammo
Besides the thrown explosives there are also explosives fired from rocket launcher type items: . Rocket 2 has the same area of effect as bombs and Rocket 4 of dynamite. They can only be bought during Blood moons and Solar Eclipses however from the Cyborg. So that requires that Plantera is defeated. Rocket 2 is sold for 2s 50, and rocket 4 is sold for 5s. So it's more economical than using bombs (slightly) and dynamites(very much).
In 1.4 Cluster Rocket II and Mini Nuke II were added which makes huge explosions of the same power as dynamites. The Cluster drops shrapnel on impact that explodes in turn. The mini nuke is just a silly large explosion.

Creating a Corruption free world

One of the most tedious challenges in the game is to accept the mission the Dryad gives you when she nags about how corrupted your world is.

What is the reward for succeeding? None.
With The Labor of Love update there is now an Achievemnt for this. But the changes to the spread system also means that this will be more managable to succeed with.
Below is advice on how to achieve this with as little pain as possible.

PS: Note that sometimes the Dryad's numbers are off when you talk to her, log out and in again into the world to get the updated values.

Revised version is coming. But short form below:
* Get the Clentaminator, or event better the Terraformer
* Get a whole bunch of Green Solution, like 2000 or more depending on your world size
* Get armor and items that buff your Ammo Persevation
* Get a Flying Mount (Bonus points for the Drill Containment Unit)
* For faster succession, bring the boss summoning items you need

Fast Version:
Spawn a Journey Mode small world with Crimson.
Turn off spread.
Hope that it only has 1 Crimson Biome.
Find it and start cleaning it up with explosives.
Clean the tiles under the altars with a Clentaminator
Defeat a boss of your choice
Build a house for the Dryad.
Talk to the Dryad
Success


1. Clean the place
Find the corrupted biomes in your world and clear them out.
(You might want to defeat the boss there first though to save yourself the trouble later if you intend to use the world for anything)

2. Dig Hellevators
Use your amazing arsenal of tools to divide your world up into managable parts to prevent spread from getting out of hand in other places when taking care of another.
(2-4 is usually enough if you already brought the tools and items along for this)


Remember the V shape pattern that the corruption and hallow spawn in from the underworld.

3. Prepare the Jungle
Just level the underground jungle a bit to make it easier to traverse, make sure to leave flat ground with jungle grass on it so that Plantera Bulbs will spawn there later.

4. Kill bosses etc
Do whatever you want before entering Hardmode. Find NPCs, kill bosses to unlock NPCs. I don't know what you want to do with this world...

5. Defeat WoF
Self-explanatory really, defeat that thing and get ready for the stressful part.
(If you want to rush Plantera, then make sure that you time this so that it will beEvening/ Night after your victory so you can use the Mecha Trio Summon items right away)

6a. Clear the new spread
Option a. Spot the V spread and start cleaning out the new spread first.
(Take note of the angle and how your Hellevators are placed)

6b. Defeat the Mecha Trio
Option b. Go defeat the Mecha Trio first to start the spawning of Plantera Bulbs
(This is my personal choice)

7. Locate Bulbs and Defeat Plantera
Go into your explored Underground Jungle and find a Plantera Bulb and defeat Plantera. This should slow down the spread to a much slower pace.

8. Check the Oceans
Quickly check the Oceans to ensure that there isn't corruption making it's way out of bounds that way.

9. Check leftover spread from the V
Backtrack and look at the places where the V spawned, chances are that you missed something and there is new spread.

10. Utilize your gear
Check with the Dryad about your balance, pop a Biome Sight Potion, spray and pray with your Clentaminator until you have cleared the last piece of corruption from your world. (Also note, you may need to log out and in again for the Dryad to give you the correct numbers.)

11. Missing a few %?
Not sure if this has changed, but in the past the spread could reach the parts of the map which are out of bound, as in, outside what the camera can show. So you may want to spray the Clentaminator along the edges of the world facing out.
Look for corrupted walls? Might be a thing.

Also feel free to use a tool such as https://terramap.github.io/index.html to load up your world file so that it generates a map for you to look at.


In the past there were dicsussions about things that would count as corrupted blocks or not.
After just getting the achievement let's get some things straight.
Demon/Crimson Altars do not count, I can see like 8 of them right here.
Shadow Orbs/Crimson Hears do not count, I kept 2 of them just in case.
Ore does not count
Statues with crimson/corruption themes do not count.
It's just the blocks.

Got the achivement in this here map with these things remaining.

Changes in 1.4

1.4 (Journey's End)was released May 16 2020. with it came some changes. I will list them here for a while before adding them to the guide itself.

It seems like all the new mini biomes like the Oasis and Living tree is making the world more squeezed together. Many people find the Corrupted biomes adjacent or inside their snow and jungle biomes.

Pylons

Having happy NPCs in on a point will let you buy pylons that works like interconnected teleports.
There is a Hallow Pylon so reason to make one of those.
Corruptions and Hallow will override the other biomes in the case of Pylons. A Hallow Desert is now needs a Hallow Pylon, not the Desert Pylon.
This also gives more incitement to keep your world pure.
If the corruption overtakes the area near your Pylon it will no longer work. Say your Underground Pylon for example. (MrEpicDwarf )

Journey Mode

As I and many others assumed. Journey Mode gives you the option to stop spread. Which is great for people who just want to focus on building without the stress of the world being overrun. Just open the admin panel (bunny) and toggle the "Spread" option.

Other stuff

- Celebration MK2 with Mini nukes II as an alternative to explosives and all that jazz. It's really something.

- Sunflowers now block Hallow as well.

-The sand blocks will not convert at the same crazy speed anymore

- Fixed a bug where corruption would win the spread fight against the crimson

- The 2 new bosses spawns in the hallow.

- New fishing rewards, so might be a point to keep some fishing spots intact, including Oasis.

-There will be a bestiary (collection of enemies defeated) this gives a reason to keep the Desert pure for the normal desert enemies as well as corrupting it with corruption, crimson and hallow for those versions if you want it 100%

-Block swap, you can mine+place blocks to replace corrupted blocks with uncorruptable blocks witrhout having to clear the area.

- Some of the new special seeds have special rules for the spread, such as having both Corruption and Crimson.


With the Labour of Love Update there are new pieces of useful information and changes

- Added Crimson and Corrupt Mud Grass. Corruption and Crimson no longer dry out Jungle Grass into dirt when corrupting it, but instead simply spread to the grass portion.

- Smashing altars no longer generates random blocks of infection in the world

- Added Terraformer, an upgraded version of the Clentaminator

- Added Biome Sight Potion, a potion that highlights Corrupted, Crimson and Hallowed tiles.

- New Achievment: And Good Riddance!
Completely purify all Corruption, Crimson, and Hallow from your world, until the Dryad is satisfied!

- Some of the new special seeds have special rules for the spread, such as the entire world being Corrupted.

So it finally happened, 7 years after publishing this guide the Corruption spread should no longer be an aspect of the game that makes people worried. My work here is done!

[DragonLover]

I'm really glad I found this, the spread of these biomes has been the one thing keeping me from playing the game more. It's so demotivating.

[Beta]

I ran some tests, and it looks like the corruption/crimson/hallow does not go to the underworld and go under the trench.

[JamesTDG]

Nice guide, but quick question, does converting crimson/hallow biomes into islands work just as well as (if not better than) converting my base area into an island?

[Bluedragon932]

just wait until 1.4.4 and we will keep mud when it is corrupted

[Bluecore102]

Steampunker: You don't need this guide, you need my clentaminator! Only 2 platinum!

[v7]

This guide is the best Terraria guide I've read, made me hungry from the time it took me to read it, it's so long and Very, Very well made!! Great job!

[Azzy Dreemurr]

Modern problems require modern solutions.

[Azzy Dreemurr]

Delete the world, that's how you stop the spread.

[lol no]

There are some for 1.3.5, don't know if their creators will update them for 1.4 though.

[Retsu]

or at least make it not spread?