The crucible is one of the most basic items in Thaumcraft. It is used to create basic Thaumcraft items as well as to send aspects to an infusion altar. It is created simply by placing a vanilla cauldron and using a Casting Wand on it. To use it, however, it requires to be filled up with a bucket of water. The water has to be refilled after each use. It will take a few seconds for the water to boil after a fire is made or lava is placed underneath. When the water starts to bubble, it is ready to use. One thing to note is that you can get the corpus essentia if you are to jump into a crucible with boiling water but it will also hurt every moment you are in the crucible as you are in fact boiling.
To craft the crucible, you must simply use your wand on a cauldron placed in the world.
The crucible is an important tool for any thaumaturge. It allows him/her to render physical objects into their component Essentia and recombine it again as he/she wishes.
The crucible needs to be placed over a source of constant heat (such as small pool of lava or a burning flame) and filled with water. The water has to be refilled after each use. Once the water starts boiling you simply throw any item you wish to break down into the crucible and they will be broken down into their component Essentia. The crucible can only hold 40 objects, so be cautious when creating large quantities of something. You can then use a wand on the crucible and if you got the ratio and amount of Essentia right you might be able to craft something.
Any leftover Essentia is simply spilled into the local aura where it forms Flux. If you sneak while clicking with a wand, you will not attempt to craft something and directly spill the Essentia into the aura. This may seem counterproductive... for now.*
However, do keep in mind that if you throw in Rotten Flesh or Brains into the Crucible while making Magic Tallow, you risk causing a Giant Angry Zombie to appear, which can kill you in 3 hits while wearing Iron Armor. It is unknown if spilling the leftover Essentia will also cause this to happen.
If you are planning on experimenting with the crucible it is a good idea to keep scribing tools and paper handy in case you discover something.
(*See Arcane Alembic)