[dentstudent (OP)]

What element, product or, er, thing best retains heat? It would have to be something that doesn't generate it's own, and it should be a solid.

I'm expecting a harsh winter, and want to make a decent hand-warmer []

Let me just quantify it a bit - if there were a series of these "things" all heated to say 100°C, which would retain the heat the longest?

[syhprum]

I don't think 100°C would be a very suitable materiel for a hand warmer, what you want is some materiel that undergoes a phase change from solid to liquid at about 30°C giving up its latent heat.

[lyner]

A small amount of heat output and a lot of insulation around your hands is the most ideal. You have plenty of energy available in your body. The problem is your surface capillaries shutting off  and this makes your fingers cold.  If you just warm them up a bit and then insulate them - there are some brilliant gloves, these days - your own blood will keep them warm.
I remember being told by an experienced diver that  putting your hands into warm water  just before a cold dive was a really effective way of keeping them feeling warm for the next half hour as long as you had good gloves.
Of course, if you are in really extreme conditions, cold hands is the least of your worries.

[another_someone]

Going back to the specific question (rather than the perfectly valid alternative solutions to the underlying problem that have been suggested).

I don't know what would actually be the best materials as such, but there are as I see it 4 possible ways of retaining heat (not necessarily mutually exclusive).

Firstly, simply using a material with a high specific heat:

http://en.wikipedia.org/wiki/Specific_heat_capacity

Factors that affect specific heat capacity

• Degrees of freedom: Molecules are quite different from the monatomic gases like helium and argon. With monatomic gases, heat energy comprises only translational motions. Translational motions are ordinary, whole-body movements in 3D space whereby particles move about and exchange energy in collisions (like rubber balls in a vigorously shaken container). These simple movements in the three X, Y, and Z–axis dimensions of space means monatomic atoms have three translational degrees of freedom. Molecules, however, have various internal vibrational and rotational degrees of freedom because they are complex objects; they are a population of atoms that can move about within a molecule in different ways (see animation at right). Heat energy is stored in these internal motions. For instance, nitrogen, which is a diatomic molecule, has five active degrees of freedom: the three comprising translational motion plus two rotational degrees of freedom internally. Not surprisingly, nitrogen has five-thirds the constant-volume molar heat capacity as do the monatomic gases.[2] See Thermodynamic temperature for more information on translational motions, kinetic (heat) energy, and their relationship to temperature.
  
  
• Molar mass: When the specific heat capacity, c, of a material is measured (lowercase c means the unit quantity is in terms of mass), different values arise because different substances have different molar masses (essentially, the weight of the individual atoms or molecules). Heat energy arises, in part, due to the number of atoms or molecules that are vibrating. If a substance has a lighter molar mass, then each gram of it has more atoms or molecules available to store heat energy. This is why hydrogen—the lightest substance there is—has such a high specific heat capacity on a gram basis; one gram of it contains a relatively great many molecules. If specific heat capacity is measured on a molar basis (uppercase C), the differences between substances is less pronounced and hydrogen’s molar heat capacity is quite unremarkable. Conversely, for molecular-based substances (which also absorb heat into their internal degrees of freedom), massive, complex molecules with high atomic count — like gasoline — can store a great deal of energy per mole and yet, be quite unremarkable on a mass basis
  
  Since the bulk density of a solid chemical element is strongly related to its molar mass, generally speaking, there is a strong, inverse correlation between a solid’s density and its c_p (constant-pressure specific heat capacity on a mass basis). Large ingots of low-density solids tend to absorb more heat energy than a small, dense ingot of the same mass because the former comprises more atoms. Thus, generally speaking, there a close correlation between the size of a solid chemical element and its total heat capacity (see Volumetric heat capacity). There are however, many departures from the general trend. For instance, arsenic, which is only 14.5% less dense than antimony, has nearly 59% more specific heat capacity on a mass basis. In other words; even though an ingot of arsenic is only about 17% larger than an antimony one of the same mass, it absorbs about 59% more heat energy for a given temperature rise.
  
  
• Hydrogen bonds: Hydrogen-containing polar molecules like ethanol, ammonia, and water have powerful, intermolecular hydrogen bonds when in their liquid phase. These bonds provide yet another place where kinetic (heat) energy is stored.
  

Substance	Phase	cp	Cp	Cv
		J g-1 K-1	J mol-1 K-1	J mol-1 K-1
Air (Sea level, dry, 0 °C)	gas	1.0035	29.07
Air (typical room conditions)	gas	1.012	29.19
Aluminium	solid	0.897	24.2
Ammonia	liquid	4.700	80.08
Antimony	solid	0.207	25.2
Argon	gas	0.5203	20.7862	12.4717
Arsenic	solid	0.328	24.6
Beryllium	solid	1.82	16.4
Copper	solid	0.385	24.47
Diamond	solid	0.5091	6.115
Ethanol	liquid	2.44	112
Gasoline	liquid	2.22	228
Gold	solid	0.1291	25.42
Graphite	solid	0.710	8.53
Helium	gas	5.1932	20.7862	12.4717
Hydrogen	gas	14.30	28.82
Iron	solid	0.450	25.1
Lead	solid	0.127	26.4
Lithium	solid	3.58	24.8
Magnesium	solid	1.02	24.9
Mercury	liquid	0.1395	27.98
Nitrogen	gas	1.040	29.12	20.8
Neon	gas	1.0301	20.7862	12.4717
Oxygen	gas	0.918	29.38
Silica (fused)	solid	0.703	42.2
Uranium	solid	0.116	27.7
Water	gas (100 °C)	2.080	37.47	28.03
	liquid (25 °C)	4.1813	75.327	74.53
	solid (0 °C)	2.114	38.09

All measurements are at 25 °C unless otherwise noted.
Notable minima and maxima are shown in maroon.

The other option is to rely on phase changes (such as storing energy in some substance that melts at around 60°C).

Thirdly, using a reversible chemical reaction to store heat.

Fourthly, simply use a thermos flask (have a small surface of the material protruding from the flask, the material might be heated to well above 100°C, but since only a very small amount of the substance is exposed, so the rate of heat loss to the surrounding environment will remain low.

[lyner]

Electric, off-peak night storage heaters nearly all use some sort of refractory material which gets very hot and is kept in an insulating jacket. But I did come across one sort that used water. They were a bit bigger but had the advantage that, because the temperature difference between hot water and the room was relatively small, the actual heat transfer was less when the room got warmer - a sort of automatic regulating system.
Water does have a very handy specific heat capacity for something so common. But it's a bit special in a lot of  ways; it only needs a few trace chemicals to turn it into BEER - or US, for a start. And it's hardly at all poisonous.

[RD]

I'm expecting a harsh winter, and want to make a decent hand-warmer []

Self-Heating Electric Gloves

• Heats up to 40 degrees in 5 minutes
• Water resistant
• Great for the active outdoor person
• Safely concealed heating element warms the upper palm and radiates out to the fingers
• Cosy lining provides extra warmth and eliminates heat loss
• Requires 3 x AAA batteries in each glove (Order Code: L29BJ for high-capacity rechargeable batteries)
• Constructed using the most advanced heating wire
• Batteries stored in a discrete pouch
• Available in two sizes

These water resistant heated gloves are great for hiking, skiing and hunting. The safely concealed heating element in the gloves is designed to warm the upper palm area and radiate out to the finger area. They have a cosy lining for extra warmth which also helps to eliminate heat loss. These would make an fabulous gift for the active outdoor person!
Requires 3xAAA batteries in each glove (not supplied) Order Code: L29BJ for high capacity rechargeable batteries

http://www.maplin.co.uk/Module.aspx?ModuleNo=47714&&source=14&doy=search

[Soul Surfer]

Water is by far the material with the highest thermal capacity no other solid or liquid comes near.  For pedants lithium metal is almost the same as water. Hot water bottles have been used for years.  The reason for this anomalously high specific heat is that water has a very low molecular weight and is in effect a polymer to it stays a loquid to quite a high temperature.

The figure of 228 for gasoline in the table above must be wrong.  22.8 is more likely.

Water is just about the most amazing chemical there is but very few people realise this.

[neilep]

Forget all the above !!

BAKED POTATOES !!



bake em, wrap em, stick em in your pocket !!...then...ewe can eat them !!

[Karen W.]

OOOOOOOH I love baked potatoes.. But Hand warmers eh?? LOL I actuall heard my grandma talk about doing just that as a young girl!

[another_someone]

Water is by far the material with the highest thermal capacity no other solid or liquid comes near.

Does not liquid ammonia exceed the specific heat of liquid water (but then, ammonia has a lower molecular weight than water, it simply also has the problem of having a lower boiling point).

[syhprum]

Although water is a very good heat storage materiel surely the best way to store heat is by means of a liquid to solid phase change.
Again water is very good but it occurs at the inconveniently low temperature of 0°C, what we need is some materiel where this phase change occurs in the 30-50°C region, any suggestions?.
My great grandmother (born 1855) used to recommend a quantity of decayed wood (touch wood) smouldering in a tin can.

[another_someone]

Although water is a very good heat storage materiel surely the best way to store heat is by means of a liquid to solid phase change.
Again water is very good but it occurs at the inconveniently low temperature of 0°C, what we need is some materiel where this phase change occurs in the 30-50°C region, any suggestions?.
My great grandmother (born 1855) used to recommend a quantity of decayed wood (touch wood) smouldering in a tin can.

You mean like this: http://www.thenakedscientists.com/forum/index.php?topic=9619.msg118015#msg118015
or this: http://www.thenakedscientists.com/forum/index.php?topic=8763.msg103883#msg103883

OK, not quite the phase change you probably had in mind, but a pertinent phase change at around the right temperature.

[chris]

The stone blocks that I put into a storage heater we installed in our studio are pretty impressive retainers of heat. They resemble small paving slabs.

You install them with one block at the rear of the casing, then you place the electrical heater element in front of that, and then you add the second block in front of the element so the two blocks sandwich the element between them; then you pack some special foam-like insulation around the whole thing and encase them with a metal cover.

The blocks weigh a ridiculous amount, to the extent that I could barely lift the package containing the pair of them. I'd love to know what they are made of! They heat slowly, but then the heat release goes on for hours, so they obviously have the capacity to pack away a lot of energy.

What's the "recipe" for these things?

[wsktd7]

It would have to be something that doesn't generate it's own

Given that the cold hands you want to (re-)warm have a pretty sizeable heat capacity of their own, it seems that using anything based only on its heat capacity would make up for a rather poor, short lasting hand warmer - if not so extremely bulky/heavy that you would not want to carry it around with you.

But if in addition to heat capacity you would accept any kind of physical property change inside your solid material, then a pellet of ²³⁸PuO2, as used in radioisotope thermoelectric generators, sounds like a great candidate.

[Petrochemicals]

What element, product or, er, thing best retains heat? It would have to be something that doesn't generate it's own, and it should be a solid.

I'm expecting a harsh winter, and want to make a decent hand-warmer []

Let me just quantify it a bit - if there were a series of these "things" all heated to say 100°C, which would retain the heat the longest?

Something that is capable of retaining 100% of its heat will give of 0%  off its heat to you ! And this thermal capacitance is best judged by volume and weight, as light as you can make it within a set volume for a required heat/time period

Thermal capacitance is what you want for heat storing value. Something that will radate heat probably at about 15 to 20 degrees c, lower than your body temperature, and lower than your hands normal temperature, otherwise your hands can suffer uncomfortable side effects when thawing out your fingers.

What you want is the best materials for cobtaining energy at 20 degrees emmision, such qs in the response by Another Someone antd the thermos flask above.

[evan_au]

What material retains heat the best?

I vote for space shuttle tiles...

[yor_on]

This maybe, assuming that you're thinking of your home?
https://www.hunker.com/12003502/which-rocks-absorb-heat-the-best

If it's only your hands, then you already have had your answer :)
Btw: That's f**ng amazing Evan, never would have believed if you hadn't linked the video.

[Petrochemicals]

Evan,  so what temperature are they at then, and how does the visible infra red nor burn him ala barbecue ?

[evan_au]

so what temperature are they at then?

The narrator says that the bricks have been in an oven at "2200 degrees" but doesn't clarify whether that is Celsius (used by scientists) or Fahrenheit (used by the US public).

how does the visible infra red not burn him ala barbecue ?

The outer corners are in contact with air at room temperature, so they cool down very quickly (go dark).
You will notice that the first time, he doesn't hold it for very long...

The pointy corners represent a fairly small area of contact with your skin, so your blood at 37C is able to keep the temperature of your skin at a bearable temperature.

The material appears to be translucent to visible light, so you can see the glow from inside the block.
But it appears to have very low thermal conductivity, and be pretty opaque to infra-red radiation.

Due to the spectrum of black-body radiation, the amount of energy in the visible is pretty low - you will notice that the room is fairly dark so you can see the glow.
See the graph at: https://en.wikipedia.org/wiki/Black-body_radiation