40d:Specific heat

The various SPEC_HEAT tokens control a material's specific heat capacity.

Specific heat is not a temperature. It is, more or less, the amount of energy it takes to raise the temperature of the material from one temperature to another. Gold, for instance, has a very low specific heat, so it takes very little energy to raise its temperature by one degree. Water, with a very high specific heat, takes 32 times as much energy to raise its temperature by one degree. For more information, see [1] [2] [3].

SPEC_HEAT appears to be in units of J kg^-1 K^-1 at STP; i.e. 1000 times the normally reported c_p in joules per gram per kelvin, or exactly the more rarely reported c_p in joules per kilogram per kelvin.

It is not understood why the game uses the °C or K scale here, while using the °DF = °F + 9968 scale elsewhere.

Sample specific heats as used by Dwarf Fortress

The following values are from Dwarf Fortress version 0.28.181.40d, though most also apply to other versions as well.

Material	cp in J kg-1 K-1	DF uses
Adamantine	n/a	7500
Aluminum	897	900
Copper	385	385
Carbon (Graphite)	710	409
Carbon (Diamond)	519	409
Gold	129.1	129
Iron, cast	≈500	450
Iron, pig	>500	500
Iron, pure	443	450
Iron, wrought	480-500	450
Nickel	444	444
Zinc	388	390
Alcohol, ethyl, anhydrous	2440	2440 (default for plant alcohol)
Alcohol, ethyl, 100 proof	3856	not in raws
Meat (beef, carcass, not frozen)	2850	4181
Charcoal	≈ 1000	409 (hardcoded)
Cheese (Cheddar, aged 60 weeks)	3012	4181 (0.31), 2000 (40d)
Coke	850	409 (hardcoded)
Glass, crystal	≈ 500	700 (hardcoded)
Glass, silica	≈ 840	700 (hardcoded)
Granite	790	800 (default for all stone)
Sand	835	835 (hardcoded)
Silk (silkworm?)	≈ 1380	not in raws
Silk (hornet)	≈ 1500	not in raws
Silk (spider, viscid)	≈ 1400	420 (0.31), 1000 (40d)
Water, liquid, at STP	4218	4181 (hardcoded)
Water, solid, at STP	2114	4181 (hardcoded)
Wood [†]	1700 to 2900	420 (hardcoded prior to 0.31)

[†]  The specific heat of wood varies somewhat between different species. However, the variance is dominated by the moisture content of the wood. Moreover, wet wood behaves very differently below freezing. See [4]

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The default value for SPEC_HEAT depends on the material and on the version of Dwarf Fortress in question.

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[5] and [6] have tables with other materials’ specific heats.

References may report specific heat in joules per gram per degree Celsius. Because the Celsius and Kelvin systems use the same scale, differing only in their zero points, this is exactly the same as reporting in joules per gram per kelvin.  J/g°C is equal to J/gK

Sometimes references will give specific heat in calories per gram kelvin, or calories per kilogram kelvin. To convert calories per gram kelvin to DF units, multiply by 4184. To convert calories per kilogram kelvin to DF units, multiply by 4.184.

Older references may give specific heat in B.T.U. per pound per degree Fahrenheit. 1 B.T.U. per pound per degree Fahrenheit is by definition equal to 1 calorie per gram per kelvin. Btu/lb°F is equal to cal/gK. So just multiply by 4184 to convert to DF units.

Sometimes references will report specific heat of liquids or gases in joules (or calories) per mole per kelvin. The symbol for this is C^p (with a capital C). This is less useful for our purposes, as you will need to know the molecular weight (in grams per mole) of the compound to convert it.

An example: the molar heat capacity of ethyl alcohol is 113 Joules per mole per degree Centigrade. The molecular weight of ethyl alcohol is 46.07 grams per mole. Divide 113 J/mol°C by 46.07 g/mol, resulting in 2.45 J/g°C. Multiply by 1000 g/kg, resulting in 2450 J/kg°C. That is our desired result in DF units.

Note that specific latent heat is a different concept; do not use those values for SPEC_HEAT.