Why is rock or metal often cold to the touch but wood or plastic is not?

Question

In a room at normal room temperature, certain materials, such as metal, glass, ceramic, or rock, will feel cold to the touch, but others, such as wood or plastic, do not so much.

Which physical properties do the former materials have in common that cause them to be cold to the touch but the others lack? The fact that wood is porous? Solid plastic pieces still do not get/feel as cold even though they are not porous. Most sources point to thermal conductivity, but how can that alone be the answer? Rock and rock-like materials (such as glass and ceramic) have a very poor thermal conductivity whereas metal has a very high thermal conductivity.

I know that because rocks take a long time to heat up and cool down in a fire, but metal takes not very much time. Which physical properties (might be more than one) determine whether a material at ambient temperature will be perceived as cold or not?

Answer 1

What we perceive as an object being "hot/ cold to the touch" is related to the rate at which heat is transferred from the object to your hand. In the case of touching an object with your hand, the physical properties that determine the rate of heat transfer are the thermal conductivity of the object as well as the difference in temperature between the object and your hand. So, for a piece of metal sitting at room temperature, there is a difference in temperature between your hand and the metal (your hand being at body temperature which is a decent bit higher than room temperature) and the metal has a high thermal conductivity. This means that when your hand comes into contact with the metal, heat is transferred quickly from your hand to the metal and you perceive it as "cold to the touch". Material like rock or wood may have a lower thermal conductivity than metal but if it is still at a lower temperature than your hand it will still be perceived as "cold". Though, it will be perceived as less cold than the metal due to the lower thermal conductivity (even though they may be at the same temperature). Here is a more detailed reference that explains the concept of heat transfer at about high school level: https://teaching.smp.uq.edu.au/fiveminutephysics/index.html#topic=PHYS1171-Physical-Basis-of-Biological-Systems&lecture=Heat-Transfer.

Answer 2

In a room at normal room temperature, certain materials, such as metal, glass, ceramic, or rock, will feel cold to the touch, but others, such as wood or plastic, do not so much.

When you touch something initially at room temperature there will be heat transfer away from your skin to the object because the temperature of your skin is higher than room temperature. How "cold" each object will feel depends primarily on the heat transfer rate from the skin as that determines how quickly the skin cools (decreases in temperature). That, in turn, will depend on the thermal conductivity of the material being touched. The thermal conductivity of rock and rock like materials (k = 0.92 W/$m\cdot K$), fall between the extremes of metal (203 W/m.K for aluminum) and plastic (average of about 0.25 W/m.K). It will also depend on what area of skin is in contact with the object. The object will feel cooler on the skin of the forearm (very thin skin) than the finger pad (thicker skin).

Another important factor to consider is the heat capacity of the object. If the objects of different thermal conductivity are all massive relative to the skin, then thermal conductivity alone determines the coolness. However, if the heat capacity on a given object is low compared to the skin that may determine the relative coolness.

For example, aluminum foil has high thermal conductivity but low heat capacity for the area touched. So aluminum foil at room temperature will not feel as "cool" as a chunk of aluminum at room temperature. Even though they have the same thermal conductivity, the amount of thermal energy available from the foil, because it is so thin, is much less and its temperature will rise quickly to that of the skin. For that matter, the aluminum foil may not even feel as cool as a chunk of rock, even though its thermal conductivity is higher, because its heat capacity is so low.

Hope this helps.

Answer 3

I can confirm that the granite slab feels cooler than paper or wood, despite having a greater temperature.

I experimented with the following materials: A steel handle, a granite slab, a wooden chair, lens from a solder stand, and a paper cup. See that the temperature of the granite slab and steel handle is in fact higher than the rest of the materials. The temperatures were measured using a Fluke 62 max IR thermometer (I don't have any associations with the company, this thing was just lying there in the lab). The feeling was measured using my forearm. The forearm was properly recalibrated after each measurement by idling for one minute.

Object	Temperature ($Celsius$)	How it felt	Thermal conductivity($Wm^{−1} K^{−1}$)
Steel handle	26.4	Very very cool	45
Granite slab	26.6	Very cool	~3.1
Wooden chair	25.2	A little bit cold	~0.1
Solder Lens	25.9	A little bit more cold	~1
Paper cup	25.8	Warm	0.05

The following orderings were made by comparing each material in pairs.

The temperatures were roughly the same ($25.98\pm0.49^{\circ}$C) for all these objects since they exist in this air-conditioned room for as far as I remember. So the main factor contributing to the coldness here should be the thermal conductivity.

The materials ordered by thermal conductivity (high to low):

Steel>Granite>Glass>Wood>Paper

The materials ordered by how cold they feel (cool to warm):

Steel>Granite>Glass>Wood>Paper

The order of coolness seems to match nicely with the order of thermal conductivity. Note that I looked up the thermal conductivities after performing the experiment, in order to remove confirmatory biases. Bob's and Nic's answers explain well the thermal conductivity part and how coolness is related to the skin's heat transfer rate rather than actual temperature (Also see this related question).

This experiment seems to confirm that their explanations are on point.

I would also like to add that the feeling of coldness also has to do with the surface roughness and porosity of the material. Smooth materials would have a higher contact area with the skin, allowing for better heat transfer.