Mortar
Mortaring is a mechanic by which loose blocks can be converted into their floating, non-loose, forms by applying sticky substances to them. To mortar a loose block, simply right-click on it while holding a mortar item (listed below). Blocks cannot be mortared while in your inventory; they have to be placed in the world first. This means that the loose block must have a supporting block underneath it before you apply the mortar. The exception is when placing loose blocks next to blocks that are already mortared - see temporary sticking below. Once it is mortared, it is no longer affected by gravity and the support can be removed.
Mortar items
The following items can be used as mortar. Right-clicking any loose block while holding one will consume the item and mortar the block.
| Item | Image | Properties |
|---|---|---|
| Clay | 
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The easiest form of mortar to obtain early on. However, it is difficult to come by in large quantities and usually preferrable to save it for making items such as brick ovens or planters. |
| Slimeball | 
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Can be obtained by killing slimes, which spawn above ground in swamp biomes or rarely underground. Building a mob trap in a swamp gives a large supply of slimeballs. |
| Nether Sludge | 
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Cooked from hellfire dust and potash, it requires Nether access and significant tech tree progress to create. Once you have the requisite infrastructure, it is cheap and easy to mass produce (the raw materials to make it are only netherrack and wood). |
Blocks that can be mortared
| Item | Loose | Mortared |
|---|---|---|
| Cobblestone | 
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| Bricks | 
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| Stone Bricks | 
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| Nether Bricks | 
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In addition, slabs and stairs made from the above materials can also be mortared.
Temporary sticking
If a loose block is placed next to a mortared block of the same type (e.g. a loose cobblestone slab next to any type of mortared cobblestone), it will stick for a few seconds before falling, giving you a chance to mortar it. This allows you to mortar a block without needing another block under it. It is also the only way to create upside down stairs and slabs, which normally can't be placed as loose versions.
