Painting pot black

[Keith Selbo]

David,

Glad I could help especially because your findings are particularly interesting. The findings of other experimenters I've read have been most notable for their imprecision and have gone both for and against blackening.

I eagerly await your further findings.

Keith

[Ryan Smith]

Finally, now I know why my cookies burn on the bottom if bake them in dark pan. Sheesh.

Ryan

[Ultra Magnus]

I've done some tests, though not scientific enough to post hard data, between my 1.3l ti pot and my Heineken pot (wide vs. narrow). With everything else being more or less equal, water boiled in the wide pot around 2 mins faster (9mins vs 11mins). But- the stove also burned out of fuel faster. I'd need to do more consistent controlled tests to collect enough data to get some consistent averages, before I could say what is exactly going on. I credit that to the larger pot reflecting more heat downwards, causing more thermal feedback to the stove (alcohol) increasing the rate of vaporization. I don't know how it would affect a canister stove, but then again, I can't say that I can recall anyone using a canister stove on a beer can pot anyway…

BM

[David Thomas]

>"I'm looking for a light weight way to test the air temp while hiking.. Would a non-contact infrared thermometer be a way to go??"

Christy-Lynn: It wouldn't give you air temps. Opaque solid objects give off infrared that corresponds to their temperature. Shiny metal objects reflect the IR of their environment. Materials transparent in IR would allow IR to pass through them, but most glass and clear plastic is transparent in the visible but not in IR. (e.g. eyeglasses noticeably shield your eyeballs from the radiant heat from a campfire).

So a non-contact thermometer would give you an air-temp reading only if you pointed it some non-metalic object in the air (piece of paper or plastic or cloth). You'd want it to be out of the sun, so you could shoot the front or back of a companion's backpack or strap, depending on which one was in the shade.

Some digital watches have digital thermometers, but you'd want it off you wrist to get a decent air temp. And some are also alarms, altimeters, pulsemeters, etc. It's even a watch.

Some kitchen thermometers are digital and pretty small. And have a waterproof probe so you could also check water or snow-pack temps.

But I can't think of a lighter option than squinting at that 1.5" zip-pull / thermometer.

[Ross P Hemphill]

Just thought it worth mentioning that the height of the pot above the burner is a major variable in efficiency.

Also thought I should mention that you can make your own "Caldera Cone" for an arbitrary pot.

[David Barnas]

Wow this was nerdy fun, I seriously was interested in the scientific discussion going on here. I made a Heineken pot recently (from one I found in a recycling bin a few years ago) and noticed some people on YouTube were painting their pots black with high heat paint. I was wondering the reason for it and came across this. I just got schooled! Thanks everyone for all the work that went into this info :-)

[Ryan Smith]

Oh yeah David. There are A LOT of nerdy threads around here on all sorts of topics. Just search for your desired topic and you will find one eventually.

Ryan

[David Gardner]

Disclosure: I make and sell brushed aluminum beer can pots. Originally I brushed the cans for aesthetic reasons. I continue to brush them because of test results like those below.

One huge variable that is difficult to control is relative humidity, unless you have both a humidifier and a de-humidifier, and a hygrometer to measure it. I get a difference of about 30 seconds in boil times in 65% vs. 85% humidity.

At any rate, I did 5 boils each using the cans pictured below over the course of two and a half hours.

Same air, fuel and surface temperature: 63.5* F.
Same quantity of water: 500.0 grams (500.0 ml).
Same quantity of fuel: 12.62 grams (16.00 ml).
Same relative humidity: 68%
Doing it in a two and half hour period with no apparent change in the weather, I presume essentially the same atmospheric pressure as well.

"Boil" measured as 212* F with a digital thermometer.

I did one burn for the series of cans (one shiny, one black, one brushed), then a second burn in the same sequence, etc. Used a series of air-temperature ceramic tiles for the resting surfaces, to avoid any surface temperature variables.

Here are the results I got:

1. Shiny original can:

6:56 (416 sec)
7:02 (422 sec)
7:06 (426 sec)
6:58 (418 sec)
7:05 (425 sec)

Average = 421.4 sec = 7:01.4 min
Maximum deviation = 10 sec = 2.4%

2. Bottom of brushed can blackened to top of windscreen by soot from crappy de-natured alcohol brand (do NOT buy E-nrg alcohol, it burns dirty and stinks too):

7:20 (440 sec)
7:15 (435 sec)
7:24 (444 sec)
7:18 (438 sec)
7:22 (442 sec)

Average = 439.8 sec = 7:19.8 min

Maximum deviation = 9 sec = 2.0%

Clean brushed can:

6:36 (396 sec)
6:29 (389 sec)
6:38 (398 sec)
6:31 (391 sec)
6:40 (400 sec)

Average = 394.8 sec = 6:34.6 min

Maximum deviation = 11 sec = %2.8

I expected the brushed can to absorb heat faster than the shiny can, but I did not expect that the soot-blackened can would be slowest. I had thought that because it was black on the bottom, it should have been the fastest. Perhaps the soot acts as an insulator, or emits IR faster than it absorbs it (is that even possible?), or….?

I did not have any high-temp black paint to try a can with just the bottom painted black. Will try that soon, when I get the same air temperature and humidity. If I was really smart, there is probably some formula that lets you account for those variables and get comparable results, but I don't know what it is.

[edited to add disclosure]

[Steve B]

Just an aside from the good discussion, but Esbit burns hotter than high temperature grill paint can withstand, and the paint will bubble and get messy (even more messy than normal with Esbit)

Steve

[Bob Gross]

I've seen high temperature grill paint that failed because it was never applied correctly. Some people just spray it on and let it dry before use. Most of the grill paint that I've seen has instructions that say to spray it on, let it dry, and then place it in your oven at a certain temperature to bake it on, and that cycle is repeated at another temperature. Then and only then is it ready to actually use.

–B.G.–

[. Kirby]

The nerd factor in this thread reaches a whole 'nother level. Love it.

[Delmar O’Donnell]

We are truly blessed with some smart nerds here! Sometimes I get the feeling that half the posters are engineers. I'm not an engineer, but I love these arcane discussions. I feel smartified after reading them.

Nerds: Keep it up! Ignorant people like me are depending on you for enlightenment.

[Glenn S]

Caution: Thoughts in Progress

Well, after reading and re-reading this thread, it seems to be an accepted fact that shiny metal reflects IR waves. I get that.

What I DON'T get, is why then, simply masking off the shiny metal, eliminating its reflective properties (painting it?) is attributed to some kind of gain by the material used (paint). The color of the paint doesn't matter, because it isn't adding any influence at all, it's simply blotting out the reflective properties of the metal.

I think people are barking up the wrong tree by looking for something to attract more IR energy, when simply preventing shiny metal reflection is where the action is. It's like everyone is thinking out of both sides of their brain.
"Reflective windscreens and baseplates? Shiny metal! Check!… Reflective pot bottoms? Shiny met… Wait, what?"

At least that's my read on all this.

And as far as just measuring degree rise by a graduated time marker, that's just shorthand for measuring boil times. It tells nothing of fuel used or efficiency. Weighing fuel after a snuff won't work, because the first 30-45 seconds, as the stove warms up, are NOT linear. So for true efficiency comparisons, it's my belief that a full burn needs to be conducted, not just a reading from a linear timeline, unless boil times are the only interest.

[Jerry Adams]

Flames and gasses are hot

they emit IR

Black pot will absorb these better and get a bit hotter

Reflective pot will reflect them away

The flames and gasses will also emit IR away from pot. If you have reflective windscreen, it will reflect the IR back to the pot.

I suspect this will be a fairly minor effect.

Good thinking about testing. I think you have to have a measured amount of alcohol. Burn it all up. Measure temperature of water before and after. If the water doesn't quite reach boiling, that will give you efficiency. Carefully measure amount of water the same for each test.

For example, if it was 16 g alcohol, start temp 5 C, end temp 85 C, then it would take:
16 g * 100 C desired temperature increase / 80 C actual = 20 g to achieve 100 C increase

Then, if you had a configuration of stove/windscreen/pot/… that heated the water, for example, 90 C, you could just scale up the amount of alcohol to get it to 100 C, if that's what you wanted.

Maybe 90 C desired temp difference is good enough because you don't really have to bring to boil to kill bugs and redhydrate food.

[David Gardner]

I have tried all kinds of painted/chemically darkened Fosters pots and have not been to measure any improvement over brushed cans (disclosure: I make and try to sell brushed cans). In theory it seems it should work, but black also emits IR as well as absorbs it, so that may be the reason. Especially if the can is black above the top of the windscreen.

Here are some of the variations I have tried:

[Ben H.]

Reflective or dull surfaces is all about engineering where you want the heat absorbed. Flame temperatures are high enough that IR plays a significant role in the overall heat transfer. It gets confusing when you tinker with things on a system level. We don't want the windscreen or ground absorbing the heat that has been generated (shiny!!!). We want the pot/water to absorb as much heat as possible (dull….). But… from a system perspective its not so simple. If the stove absorbs more or less fuel it can change the flow rate of the stove… is that good or bad????

In terms of the best way to conduct these experiments, I agree that an overall system approach is good. It is the overall system efficiency that matters (including start-up and how quickly it comes up to steady-state operation). That being said, if I was back in school and was going to present this data to my thesis advisor (I shudder just thinking about it…) I would put the stove or an accurate scale and some thermocouples in the liquid and hook everything up to a data acquisition system and calculate flow rate, liquid temperature and efficiency over time (plus some spatial knowledge about temperature).

[Marko Botsaris]

Just a few points form the physics professor side:

(1) Any effect will be minor – it might or might not be measurable. It will not be very significant. However, I know on BPL even small effect down to quantitatively unmeasurable ones are significant to the community.

(2) "Black" has almost nothing to do with it, as has already been discussed experimentally above. IR is much longer wavelenght light than visible light. Probably any paint, or even just a rough surface, will be "black" to IR. Light will stop reflecting normally off a surface (and will be mostly absorbed) when it starts to have irregularities (roughening) on the same scale as the wavelenght. Also, when you add a layer of paint of ANY visible color (if it is the right thickness) it may cause more IR to be absorbed. It is less the color of the paint than the thickness, and how it disrupts the scattering of IR off the surface. Black paint will either either have an identical effect to white paint, or only very slightly different – so (visible) color will be a minor variation on an already minor effect of having ANY paint.

(3) A body that ABSORBS more IR will EMMIT more IR, very strongly dependent on the temperature. So if it is a "cool" surface such as a pot filled with boiling water then a bit more heat might flow into it if it absorbs more IR. But the possible net effects of painting or not painting surfaces that will be much hotter are not so straightforward, and the effect will be nullified entirely if the heat of the surface it the same as the heat of the source.

(4) A surface like an aluminum foil windscreen, while shiny, most likely absorbs as much IR as it reflects, and it primary job is to coral the hot gas, not the photons. Yes, even in your baked potato that is its primary function.

Probably other people have said all of this before both in this and the numerous other thread on this subject.

[James Marco]

Marko, you make some good points. There are a couple I would add.

a) The insulative effect of a layer of paint. Thick coatings of "black" paint ("black" is a generic term to cover the black body radiative effect of a pot) could be worse than no paint. While conducive to picking up more IR than reflective surfaces, it may insulate against the conductance. Conduction makes up the large majority of a pots performance, hence the interest in surface area and heat exchangers. IR will have a slight performance value in the range of around 5-10%. This may be why your paint shows no increase in performance. A very thin coating, would work as well or better than a thick one and it should be almost transparent, ie, more of a wash than a paint.
b) The brushed aluminum has the effect of increasing the suface area of the pot (think small heat exchangers.) Any coating over this will loose this brushed effect. You may be working to oposite effects by covering them from conductance.
c) Brushed aluminum will act as if it is polished aluminum that has been painted thinly. IR is picked up, the molecule becomes exited. The electrons move to the outer shells. They loose a quanta of radiation and drop back. This is the same amount of energy that has been input, but it is in a random direction. It may simply vibrate the molecule slightly (what we interpret as heat and what we want.) With your brushed aluminum, there is a higher probability the IR will be reabsorbed rather than lost. Note that there are specific ranges at which this works best, too. A thin coat of paint will add more frequencies to be absorbed, hence more heat to the system.
d) The reflectivity of Aluminum is quite good. So, it makes sense to polish the unheated portions (heat screen, top) and brush the heated portions (bottom, 50% of the sides.) Ti is not a great reflector or conductor, it probably won't help.
e) The entire IR to heat to IR is statistical. Sometning gets hot by absorbing more heat than it radiates. This does NOT mean that it does not radiate.This is why very small pots do not heat as well as larger ones. And, by the same token, why very large pots don't heat as well as the smaller ones(they radiate too fast.) There is an optimal range of sizes for pots for the size of burner/heat screen you use.

[Marko Botsaris]

Yep, all true. Especially when there is a small effect other factors that were neglected can easily be more important. Since heat diffusion is much more important the IR even a small change in thermal conductivity could easily make bigger difference that any change in IR absorption. When you paint the surface of the metal you are placing a barrier between the hot gas and all those free electrons in the metal that conduct the heat so well.

Increased surface area from roughing point is well taken. This is what happen when we focus too much on one factor and forget about the unintended consequences – sometimes they are in the same direction, and sometime in the opposite direction.

[Author]

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