Heat

The second half of Feedback is heat: melting, smelting, making steel, tempering, raising steam. It runs on a few simple rules.

Temperature: Tu

Temperature is measured in Tu. Without an instrument you only get words, the way a smith reads metal by its glow:

Word	Up to
Warm	200 Tu
Hot	480 Tu
Faintly red	580 Tu
Dark red	730 Tu
Bright red	930 Tu
Glowing orange	1100 Tu
Yellow	1300 Tu
Pale yellow	1400 Tu
White hot	1500 Tu
Brilliant white	1600 Tu
Too bright to judge	above that

The words crowd together where metalwork happens, but steel's 60 Tu window still sits across the yellow/pale yellow line, so by eye you can get close and never quite land it. Carry a Thermometer and those turn into figures.

Some landmarks: water boils at 100 Tu, wood chars at 300, lava is 1200, steel is made at 1350–1410, and iron burns at 1470 in a steel batch.

A fire is a temperature, not an amount

A fire doesn't "add heat points". It has a temperature, and whatever sits on it climbs toward that temperature and slows down as it gets close. It can never get hotter than the fire under it.

So:

A crucible over a charcoal fire (1220 Tu) settles just under 1220 Tu, however long you wait.
Lava is a fire fixed at 1200 Tu that never changes. A crucible over lava sits just under 1200 Tu forever. That's very stable, but it's too cool for steel.
To get hotter, you need a hotter fire, and that means air from a Powered Bellows: full air makes a fire 1.5× as hot.
Insulation helps a vessel settle closer to its fire and hold its heat once the fire is cut. It can't take it past the fire.

Heavy vs. light vessels

How fast something heats up and cools down depends on how heavy it is:

A light vessel (a Small Crucible) moves fast: it closes most of the gap to its fire in about 2 seconds. It responds quickly, but it's twitchy and overshoots.
A heavy vessel (a Large Crucible) takes about 20 seconds for the same. It's hard to get going and hard to stop, but it holds a temperature almost by itself. A Refractory Lining doubles a small crucible's sluggishness.

Some processes care how fast the temperature is changing as well as what it is (JEI shows this as Max rate). Steel allows 5 Tu a tick. A small crucible on a full blown fire is still climbing at about 9 Tu a tick inside steel's band, so it simply can't make steel at full blast; a large one climbs at under 1 Tu a tick.

Hot items stay hot, and cool down anywhere

When you take something out of a hot vessel, it comes out at that temperature and starts cooling straight away, at a steady 1.5 Tu per tick (30 Tu a second), every item the same. It cools in your hand, on the ground, in a chest, in a hopper, even in an unloaded chunk. Hover over it to see how hot it is.

Because the cooling is steady, you can work out your time exactly. Steel straight out of the crucible at about 1400 Tu has about 10 seconds before it drops under its 1100 Tu working limit, and 17 seconds before it's too cold to work at all (900 Tu). Hot pieces also hurt to hold: see Hazards.

Ways to change an item's heat by hand:

Drop it in water: it's cooled instantly. For hot steel this is quenching.
Right-click water with it in hand: it snaps to room temperature.
Right-click lava with it in hand: it's heated to lava temperature instantly. It's crude and uncontrolled, but it needs no machine.

Room temperature

Everything cools toward the room's temperature: 20 Tu in the Overworld, 40 Tu in the Nether.

Processes

Melting, smelting, steel, ceramics and tempering are all thermal processes. JEI shows each one with:

Tu: the temperature band where it works.
Optimal: where it goes fastest.
Hold: how long it takes if you keep it exactly at the optimum. Further from the optimum, it takes longer.
Max rate: if listed, how fast the temperature may be changing while it works.
Spoils: above this temperature the batch is ruined instantly (for example, iron burns to Burnt Iron).

If you drift out of the band, progress slips back as fast as it would have gained; it doesn't reset. Drift too hot and you risk the spoil line. The vessel doesn't know any of this. It keeps heating whether your batch is ready or not.

While you're away

A vessel in a chunk that unloads doesn't freeze. When you come back, it's brought up to date as if you'd been there: its fire burns the fuel it had (unblown; nobody was working the bellows), the vessel follows the fire while it was lit, and cools toward the room once it went out.

What isn't caught up is progress. A batch in the crucible doesn't finish while you're away; it picks up where it left off, at whatever temperature the crucible is now.

The blocks

Block	Role
Firebox	Burns fuel under a crucible or boiler
Heating Coil	Electric heat under a vessel
Crucible	The open, measurable vessel for steel, melting and alloys
Furnaces	Furnace, Smoker, Crude Blast Furnace, Improved Furnace, Kiln, Annealing Furnace
Powered Bellows	Makes a fire hotter
Insulation / Damper	Keep heat in / let heat out
Bimetallic Strip	A built-in thermostat
Thermometer / Temperature Sensor	Measuring
Heat Exchanger	Moves heat between two blocks
Boiler / Steam Engine / Condenser	Heat into rotation, and the water back
Pressure Vessel	Stores compressed air, and explodes