The Purifier is a type of world-generated machine that allows you to make changes to the biome that you find it in. Purifiers are found on three biome types: temperate, desert, and deep.
How to Obtain
You can't obtain this item; it exists in the world. It can't be mined or destroyed, and once a part is added to it, that part can't be removed.
How to Use
A purifier has seven parts scattered around the world in large green industrial chests that can be found in Bunkers and Dungeons. Unlike other containers that drop items into your Inventory, clicking on an industrial chest will generate a brief message on the screen and then automatically add that piece of the Purifier to the machine.
On the minimap, you can see how many parts have already been found. When the count reads 7/8, you "find" the final piece by going to the Purifier itself and clicking on it to activate it.
Once activated, the purifier in a Temperate Biome will begin slowly removing the acidity from the air. Like the parts count, the acidity level of the world is displayed on the map. As the percentage of acidity decreases, the color of the sky changes from green to blue. It takes several real-world days for a world to be fully purified. On a fully-purified world, the rain is no longer acidic, butterflies are able to spawn, and it becomes possible to grow flowers from bulbs and greenery from compost.
The machine in a Desert Biome is also called a Purifier, and the mechanics of repairing it operate the same way, but it is actually a humidifier that removes aridity from the air, making it possible for players with lower Survival Skill levels to survive without having to consume jars of water. Unlike Temperate worlds, fully-purified (or humidified) Desert worlds cannot support butterflies or any growth from compost or bulbs other than cactus bulbs.
Purifiers were implemented in June 2012, shortly before the public release of the game.