If recycling is enabled in the configuration file (it is by default), a very large list of recipes designed to allow reclamation of some resources from badly damaged items not worth fixing. In short, the pile of badly battered gold and chain mail armor zombie drops, or that adamantium sword a skeleton dropped with only a few uses left, now have a better purpose than being pitched in lava. They can be fused to get a block of ore, sometimes two.

Also, more than tools and armor can be recycled. That extra clock or compass, or old pair of shears, can also be recycled.

The basic form of the recipe is:

catalyst -- [coal/charcoal]
[Item] ---> [ore* output] <--- [Gravel block]
------------------[Fuel]------------------

* Note: as of Minecraft 1.17.1, recycling outputs are either gems or raw metal or stone, instead of ores. This is to prevent infinite duping of materials by way of using a Fortune pickaxe to mine the (formerly) resulting ore block for multiple raw metals or gems.

The majority of items use coal or charcoal as a catalyst. This actually has a history in alchemy, where powdered charcoal was added to powdered metals to remove oxidation. Most items are fused into a gravel block, producing an ore block gems or raw metal. Some more valuable materials have different requirements.

For all items, the general rule is that if it contains seven primary materials (iron ingots, diamonds, etc) will produce two ore blocks raw metals. In the list below, items in italics produce two recycled items (raw metals or alloy chunks).

IMPORTANT NOTE for pre-MC 1.14.4 versions: If an item produces two ores/ingots when recycled, it requires two input blocks and two catalysts (lava buckets being an exception). So, when recycling a chainmail chestplate, it requires two coal / charcoal lumps and two blocks of gravel. This is not the case for 1.14.4 and later.

The materials covered are vanilla Minecraft materials,the metals in Simple Ores2, Fusion metals, and Netherrocks metals. Fusion alloys, which do not have ores, recycle into a large chunk (which is smelted into an ingot).

The list for all items includes when available (and so will not be listed below): helm, chest plate, leggings, boots, sword, pickaxe, axe, shovel, hoe, horse armor (vanilla version).

Note: Leather and Wood recycling removed, because it makes no sense to recycle flammables using an alloy furnace --Sinhika

Material: Stone .................. Recycles into: Smooth stone block
Catalyst: coal / charcoal ........ Second item: gravel block
Other recyclable items: smelter

Material: Iron (+ Chain armor) ... Recycles into: Iron ore raw iron
Catalyst: coal / charcoal ........ Second item: gravel block
Other recyclable items: empty bucket, cauldron, compass, iron door, flint and steel, empty minecart, hopper, minecart & hopper, hopper, shears, anvil (anvils produce 4 ore)

Material: Gold ................... Recycles into: Gold ore raw gold
Catalyst: coal / charcoal ........ Second item: gravel block
Other recyclable items: clock

Material: Diamond ................ Recycles into: Diamond gem
Catalyst: bucket of lava ......... Second item: coal ore block
Other recyclable items: ---

Material: Copper (SO) ............ Recycles into: Copper ore raw copper
Catalyst: coal / charcoal ........ Second item: gravel block
Other recyclable items: copper bucket (also smelt-able in current version), copper door

Material: Tin (SO) ............... Recycles into: Tin ore raw tin
Catalyst: coal / charcoal ........ Second item: gravel block
Other recyclable items: tin shears

Material: Mythril (SO) ........... Recycles into: Mythril ore raw mythril Catalyst: coal / charcoal ........ Second item: gravel block
Other recyclable items: mythril bow, mythril rod, mythril furnace

Material: Adamantium (SO) ........ Recycles into: Adamantium ore raw adamantium
Catalyst: coal / charcoal ........ Second item: gravel block
Other recyclable items: adamantium shears

Material: Onyx (SO) .............. Recycles into: Onyx gem
Catalyst: lava bucket ............ Second item: netherrack block
Other recyclable items: onyx bow, onyx rod, onyx door, onyx shears, onyx furnace

Material: Bronze (SO+FF) ......... Recycles into: Large bronze chunk
Catalyst: coal / charcoal ........ Second item: gravel block
Other recyclable items: ---

Material: Steel (FF) ......... Recycles into: Large steel chunk
Catalyst: coal / charcoal ........ Second item: gravel block
Other recyclable items: ---

Material: Thyrium (SS+FF) ........ Recycles into: Large thyrium chunk
Catalyst: lava bucket ............ Second item: gravel block
Other recyclable items: thyrium bow, thyrium rod

Material: Sinisite (SO+FF) ....... Recycles into: Large sinisite chunk
Catalyst: lava bucket ............ Second item: netherrack block
Other recyclable items: sinisite bow, sinisite rod

Most of the Netherrocks recycling recipes use the following pattern:

catalyst - [Netherquartz]
[Item] ---> [raw metal/gem output] <--- [Netherrack block]
----------------[Fuel]------------------

Material: Malachite .............. Recycles into: Malachite Ore raw malachite
Catalyst: Nether quartz .......... Second item: Netherrack block
Recyclable items: helm, chest plate, leggings, boots, sword, pickaxe, axe, shovel, hoe

Material: Argonite ............... Recycles into: Argonite Ore raw argonite Catalyst: Nether quartz .......... Second item: Netherrack block
Recyclable items: sword, pickaxe, axe, shovel, hoe

Material: Ashstone ............... Recycles into: Ashstone Ore ashstone gem
Catalyst: Nether quartz .......... Second item: Netherrack block
Recyclable items: sword, pickaxe, axe, shovel, hoe

Material: Illumenite ............. Recycles into: Illumenite Ore raw illumenite
Catalyst: Glowstone Dust ......... Second item: Netherrack block
Recyclable items: helm, chest plate, leggings, boots, sword

Material: Dragonstone ............ Recycles into: Dragonstone Ore dragonstone gem
Catalyst: Lava Bucket ............ Second item: Netherrack block
Recyclable items: helm, chest plate, leggings, boots, sword, pickaxe, axe, shovel, hoe

Material: Fyrite ................. Recycles into: Fyrite Ore raw fyrite
Catalyst: Nether quartz ............ Second item: Netherrack block
Recyclable items: helm, chest plate, leggings, boots, sword, pickaxe

Items listed in italics produce two outputs.

Note that some items that can be smelted can also be broken down by crafting, smelting, or breaking, possibly with better results.

Original text of documentation by akkamaddi, from Minecraft Forums