Cauldron

Cauldrons are an Engineer-only perk that allows the player to create unobtainable items. When an Engineer first reaches the appropriate rank, they receive a base set of recipes that everyone has access to, allowing them to create blocks such as grass or mossy cobblestone. However, there are multiple ways to obtain even more recipes. Some will be not as rare, and others will be one-of-a-king. It's up to the Engineer who receives the recipe whether or not they want to share it or keep it for themselves and make a fortune.

How to Create a Cauldron
You will need a 3x3x3 area to create your cauldron. On the bottom level, put all smoothstone with one lava source block (from a bucket) in the middle. On the second level, put one glass in the middle, surrounded by smoothstone. On the top level, put smoothstone and leave an open space in the middle. This is where you will throw your items.

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How to Use a Cauldron
Nothing could probably explain it better than this video. While in the video it shows smoothstone being used, I believe any other material can be used, but do so at your own risk.

Recipes
Please note that these recipes are only the few that are commonly known. Additional, secretive recipes will be given to Engineers through events, speed quests, or forum contests. The T1 recipes are the basis for the rest of the recipes, in that you need to have those ingredients to make other materials further down. This can make recipes further on much more expensive.

NOTE: All amounts are quantities of 1 unless otherwise specified.