Alloy Furnace Recipes, Overheating, and Cooling
The 
Alloy Furnace is Eternal Starlight's ore doubler and its only route to Unrealium, and unlike a vanilla furnace it can overheat. It runs on fuel like any furnace, but every recipe also builds heat, and a coolant slot decides how long it can keep going before it stalls.
It is a large block: placing it fills a space two blocks across, three high and three deep, so clear the room before you set it down.
Getting one
The Alloy Furnace is not crafted. The Starlight Golem drops an Oxidized Alloy Furnace, and that is the source. The furnace is made of golem steel, so it weathers exactly the way copper does: scrape the oxidised block with an axe to clean it back to a plain Alloy Furnace, and it will slowly oxidise again over time.
To stop that, wax it. One Alloy Furnace plus a 
Honeycomb gives a 
Waxed Alloy Furnace, and 
Raw Amaramber works in place of the honeycomb if you have been mining in the Starlight dimension rather than keeping bees. A waxed furnace never changes appearance again.
Every recipe
Eleven recipes exist, and all of them burn for 400 ticks. 
Saltpeter Powder is the flux in most of them, and 
Starcore appears wherever a precious metal is involved. Both come from ores in the Starlight dimension and both smelt or blast out of grimstone, voidstone, eternal ice or haze ice ore.
Ore doubling:
- 1 iron ore plus Saltpeter Powder into 2
Iron Ingot, with a chance of 
Dimslag on the side - 2 raw iron plus Saltpeter Powder into 3 Iron Ingot
- 2 copper ore plus Starcore and Saltpeter Powder into 3
Copper Ingot - 3 raw copper plus Starcore and Saltpeter Powder into 4 Copper Ingot
- 2 gold ore plus Starcore and Saltpeter Powder into 3
Gold Ingot - 3 raw gold plus Starcore and Saltpeter Powder into 4 Gold Ingot
- 1 deepsilver ore plus Saltpeter Powder into 2
Deepsilver Ingot - 2
Raw Deepsilver plus Saltpeter Powder into 3 Deepsilver Ingot
Alloys:
- 4 Gold Ingot plus 2
Netherite Scrap into 1 
Netherite Ingot - 3 Deepsilver Ingot plus 2
Golem Steel Nugget into 1 
Golem Steel Ingot - 1 Deepsilver Ingot, 1 Golem Steel Ingot, 1
Malarite and 1 
Soul Dew into 1 
Unrealium Ingot
Two of those deserve a second look. The netherite recipe needs only two netherite scrap instead of the vanilla four, so every ancient debris you find is worth twice as much once the furnace is running. And the unrealium recipe is the only way to make unrealium at all, which makes the Alloy Furnace mandatory for the mod's endgame gear rather than optional.
Note the raw-versus-ore split. Ore blocks give a better ratio per block, but raw materials are what you actually collect without Silk Touch, so most players run the raw recipes and accept the slightly worse rate.
Overheating and coolant
This is the part that catches people out. The furnace tracks a heat level while it works, and when it overheats it stops. The coolant slot holds an item that both lasts a certain number of ticks and multiplies how long the furnace can run before overheating.
Coolants come in two grades. The ordinary ones give a 3x multiplier:
- Snowball, Snow,
Ice, 
Icicle, 
Ashen Snow and 
Ashen Snowball - 800 ticks each - Snow Block - 3,200 ticks
Packed Ice - 8,000 ticks
The cold materials of the Starlight dimension give 5x:
Glacite Shard - 1,200 ticks
Glacite - 7,200 ticks
Block of Glacite - 12,000 ticks
Blue Ice - 32,000 ticks
Blue Ice is the standout for sheer duration at 32,000 ticks, while Block of Glacite pairs a long 12,000-tick life with the higher 5x multiplier. A furnace fed blue ice or glacite blocks effectively runs unattended; one fed snowballs will need refilling constantly.
Snow is renewable and free in a cold biome, which makes it the right starter coolant. Switch to glacite once you are processing ore in volume, because the 5x multiplier is what keeps a busy furnace from stalling mid-batch.
A working setup
Place the furnace where it has room, feed it fuel and a stack of Packed Ice or better, and keep Saltpeter Powder stocked - it appears in eight of the eleven recipes and running out is the usual reason a batch stops. Wax the block once you are happy with how it looks, and keep the Starcore for gold and copper runs rather than spending it elsewhere.
Covers the mod's in-game guide topics: