Painting pot black

[C Nugget]

sorry bout the double post…

It was slow loading!!!

ooops

[Tim Zen]

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[Ben H.]

David –

I am surprised that white paint would have an emissivity of 0.95 in the IR spectrum. When I saw that you got similar results with white and black I thought there was room for improvement, but perhaps not. You could stick a thermocouple on the pan and confirm your results.

Christy – The type of metal certainly plays a significant role. Anodized coatings are what a lot of satellites use to control radiative properties of parts. There is no convection in space, so radiation really does dominate. I wouldn't think the radiative properties inside the pan play a significant role. Temperature gradients are low and convection dominates.

In terms of pot shape, you have to realize that radiation from the stove will go in all directions. The more area "visible" to the hot flame/burner the more radiation will be absorbed. Likewise putting reflective surfaces (like a windscreen) in the areas you don't want to get hot will increase the amount of heat absorbed by the pot.

[James Klein]

Thomas, good quick and dirty test and thanks for the heads up on the cheap IR thermometers.

I would say that we care plenty about visible emissivity for the bottom of a cook pot — probably on par with how much we care about IR emissivity — the flame does have a color after all.

I would go for black, high temp paint.

[Andrew F]

One other easy way to get your pot black is to cook over a wood fire. No paint required.

Andrew

[David Thomas]

Ben: You sound like a rocket scientist. I clean up after them (Lockheed, perchlorate in the groundwater, etc).

>"Anodized coatings are what a lot of satellites use to control radiative properties of parts." Is that to increase emissivity? Metals work like a mirror (hence their use in mirrors). I'm guessing metal oxides work like paint pigment (hence their use as paint pigments!) and are close to black-body behavior, but you tell me.

I did stick a thermometer on the pan, it just wasn't a mercury thermometer. It was a patch of spit. The spit that straddled the white and black sections evaporated in the same way, at the same rate. Red was a bit different. Unpainted was much different in that it went faster and actually boiled a bit. So unpainted was definitely hotter for having less radiant cooling. But white and black paint – the ones I used – had the same IR emissivity at 8um (peak radiation from 100C metal). Would expect them to have similar IR emissivity at 2um (peak radiation from a 1000C flame/steel)?

I'll be testing four identical pots against each other to boil water. I'll have one unpainted, one black and one white, among other modifications. And I'll test them over white gas, natural gas, propane, butane and alcohol. I think radiant contribution could vary a lot over that range of fuels, but we'll see. My plan is to start with ice water as an easily acheivable constant starting point (removing the ice just before the test).

After an initial round on unpainted / white / black and seeing how different fuels compare, I hope to focus on maybe propane (cheaper) if it parallels butane behavior and get into heat fins and fixed wind screens.

I suspect one outcome of this is that I'll know enough to make "The Spaghetti Pot From Hell" with the color, fins and integral windscreen to cut boiling time in half on the home stove.

[Ben H.]

Indeed I am a rocket scientist! Though I work mostly with LO2/LH2 rockets, so I don't think I am responsible for any of your messes :)

Yup, you got oxides down as far as I understand their use (I don't work a lot in radiation though).

What I meant by adding a thermocouple is to measure the emissivity. Difference in temperature from the thermocouple and IR thermometer gives you a good indication of emissivity. If there is a good match, there is not much point exploring alternative coatings.

What you plan on doing sounds an awful lot like what I imagine Jetboil did when developing their system.

[David Thomas]

>"I work mostly with LO2/LH2 rockets, so I don't think I am responsible for any of your messes"

Ben: Yep, anything you spill is gone in 3 seconds. One way or another!

ChemE Berkeley, 20+ years working with Civils and Geos (they do rocks, dirt and concrete) so I do all the wires, pipes, motors, bio, and chem. You?

[Paul Ashton]

Hiram Cook made some empirical tests of coated and uncoated pots for boil time. Found no significant difference. Check them out on youtube

[David Goodyear]

Why paint,

+ 1 on letting nature do it for you. Go for a blackened bush pot with a healthy coating of creosote. Better for the environment that the crap coming out of a spray can. :)

All in fun,

Dave

P.S. I realize that I probably can't hike in California – as I would be importing the toxins bonded to the bottom of my pot.

[C Nugget]

Ben..

Sorry my science is not up to snuff for this thread and some things are going above my head. I am keen to be enlightened though.

To help me understand…
An aluminium pot would be thicker after being anodized? But the anodized process would help heat the pot faster or slower? Or, the function of anodizing a pot more to harden metal(making it safer to eat from) and create a non-stick benefit instead of increasing one's boil time?

So taller pots with a narrower base would take longer to reach boil time than a wider base pot of the same volume and type of metal because of a lower surface area on the bottom? Am I on the right track or lost in space?

I have a tall pot (http://www.rei.com/product/784114/gsi-outdoors-pinnacle-soloist-cookset) and noticed that the alcohol stove flame seems to creep around the edges. I left it behind last trip for a lighter wider pot but missed the ease of use of mine.. (I did a very non-scientific test between the two and the lighter wider pot won out for boil time). If tall pots take longer to boil then why are they on the market??

Is there any way to help with the efficiency of a tall pot and alcohol stove?? I figure the faster the boil time the less fuel used.. but maybe this is not the case either.. Would I benefit from painting the bottom black or soot up the bottom to help increase boil time?? Or… would some kind of collar work in addition to my windscreen to help in heat exchange. Or… perhaps I should just look at getting a wider based pot.. :(

My stove: ThermoJet MicroLite (where the pot stand is the windscreen)

Trying to convert discussion data to real life works for my cooking set-up… Ak!!

-christy

[Ben H.]

Hey Christy,

I'll do my best to answer your questions. One thing to realize is the science can only get you so far. Science can point you in the right direction, but in the end it is the real life data that matters.

"An aluminium pot would be thicker after being anodized?"

The anodizing process reacts the top layer of aluminum to form a hard aluminum oxide. It should have a pretty negligible affect on thickness of the pot. Anodizing has many useful properties as you mentioned, but in terms of this discussion anodizing can change the optical properties of the pot. A nice black anodized pot should absorb more radiant heat from the stove. The confusing part though is, it is not really important how black it appears in visible light. What is important is how black it is in the infrared spectrum. In general things that are more visibly black are also more black in infrared, but note in David's tests the white paint was "as black" as the black paint.

"So taller pots with a narrower base would take longer to reach boil time than a wider base pot of the same volume and type of metal because of a lower surface area on the bottom? Am I on the right track or lost in space?"

yes and you are on the right track.

"If tall pots take longer to boil then why are they on the market?? "

People like the form factor of the tall narrow pot.

"Is there any way to help with the efficiency of a tall pot and alcohol stove??"

I believe a lot of windscreens increase the efficiency of heat captured from a flame. Many people are happy with their caldera cone.

"I figure the faster the boil time the less fuel used.."

if you are only talking about the pot side of the equation that is true. To make thing more confusing, if you are talking about the stove the the opposite is true. The pot is able to capture more of the heat from a nice low flame. As a result, a stove that pumps out the heat, will use more fuel (per cup of boiled water) than a stove that pumps out less fuel. The hot stove will get the water boiling faster but use more fuel doing it.

"Would I benefit from painting the bottom black or soot up the bottom to help increase boil time??"

I think David's test would indicate the answer is "yes"

"Or… would …."

I am sorry I don't have enough practical knowledge on the subject to say what would help you the best. Painting the pot is pretty easy… designing an effective heat exchanger would be more challenging.

"Or… perhaps I should just look at getting a wider based pot."

I have read enough anecdotal evidence on these forums to say that is probably the case. :)

Don't get frustrated by all the science. I am an experimentalist and by that I mean I believe the science is useful for understanding experimental results and for guiding future experiments. The actual experiments are what leads to better stove/pot/windscreen combinations for real world use. Best of luck, and let me know if you have any other questions. I love to get nerdy!

[David Olsen]

How will similar mechanics work with tarp fabric?

Say you want a tarp that stays cooler in hot sun. I've experimented and have some
answers, but always learn something from this group.

Dark color, light color, what would you scientists expect and what have you
engineers found?

[Keith Selbo]

The guy at minibull design tested blackened vs natural pots and found no significant difference in boil times. That made sense to me because the hot gasses from the flame contact the pot directly, and the heat transfer is by convection/conduction not radiation.

Measuring boil time wastes a lot of time. I've done a lot of stove experiments and found that the temperature rise is linear with time in almost every case (wood stoves being the exception). That means that you really only need to measure the temperature rise over a fixed time interval to get degrees per second. No need to wait for a boil or control the starting temperature to make your comparisons.

[Bob Gross]

"How will similar mechanics work with tarp fabric?"

You want it to be cooler in hot sun. Make the top surface reflective aluminum.

–B.G.–

[James Klein]

"That made sense to me because the hot gasses from the flame contact the pot directly, and the heat transfer is by convection/conduction not radiation."

Radiative heat xfer will occur as long as there is a temperature differencial btw the stove output & pot bottom AND there is nothing btw the two to block the radiation.

If there wasn't a temperature differencial the pot bottom would likely melt.
If there was something blocking the radiation you wouldn't be able to see the flame (though the combustion products probably block some of the infrared radiation).

[Keith Selbo]

There will be radiant transfer, but based on the results we've seen so far it is certainly not the dominant mode and likely doesn't justify painting the bottom of the pot black.

[Rand Lindsly]

"I figure the faster the boil time the less fuel used.."

Ben's right in saying that this is not true…..but did I want to kinda explain it a little differently. If you pump heat out of the stove really really fast, the pot won't absorb the heat as fast as you are pumping it out. Quite a bit of that heat is lost to the atmosphere. The experiment you can perform to test this is to hold your hands near the top of the pot/windscreen. If you feel heat coming off, that is heat that is not going into boiling your water….and consequently fuel that is used to create the heat that isn't being put into your water/food. So, while it seems counter-intuitive, the faster the boil time, the MORE fuel is used…..because you are using a good bit of that to heat the campsite too.

Since he brought up the Caldera Cone, that is an example of a system that holds the hot stove exhaust next to the pot for longer, so when the system exhaust comes out the upper vents, it is not much hotter than the water in the pot. Pretty efficient.

…but I'm just a cottage guy….so clearly no innovation here….

Rand :-)

[Jerry Adams]

You have to weigh canister before and after

Record temperature before and after

Carefully measure amount of water (weigh it is good)

Before and after wipe any condensation off the canister

Repeat several times to make sure it's consistent and you're not basing your opinion on some uncontrolled variable that you're not aware of

Measure black pot, not black pot, not black pot again, black pot again to cancel out any error due to canister being less full

etc.

if the difference between black measurements is as big as the difference between black and not black then you have to measure more to average or just conclude no measureable difference

[Jerry Adams]

And it would be good to take pictures with ultra-fast "explosion" camera

Nevermind, that's a different thread : )

[David Thomas]

>"Would I benefit from painting the bottom black or soot up the bottom to help increase boil time??"

Cross-posted from another thread where this Q came up:

I'll write this up as a new thread in a few days with photos and all, but here's the punchline:

Four pots identical pots. One unpainted, three with different paint jobs.

Each timed on each of four different stoves. 1000 g of 4C water to a rolling boil.

Average (of 4 stoves) with no paint: 5:50 (350 seconds)

Average (of 12 runs, 3 pots on 4 stoves) with paint: 5:24 (324 seconds).

Saving 7-8% of your fuel for a 2 or 3 grams increase in weight? You'll come out ahead on the second liter you boil.

It's a no brainer – DO IT! Paint the bottom of your pot. Painting the sides didn't help or hurt in my set-up WITHOUT windscreens.

Caveats: These four stoves were propane stoves. I plan to re-run with white gas and with with alcohol. I suspect the effect will be greater with WG and less with alcohol, but we'll see.

[David Thomas]

>"I've done a lot of stove experiments and found that the temperature rise is linear with time in almost every case"

Keith, I like that. A lot. During my wide versus narrow and painted versus unpainted tests, I did a lot of work to maintain constant starting temps, room temperature, etc. And just what is a "rolling boil" varies several seconds.

Whereas if I throw a thermocouple in the center of each pot and time a 10 degree temp rise (or record temp rise in one minute) and that captures everything I need to compare one set up to another and to calculate fuel savings.

Thanks

[C Nugget]

@ Ben experimentalist and beyond,

Thanks for simplifying some of the science for me. Very helpful to have things broken down a bit more. I will not given up when the jargon hits.

@ Randy,

Thanks for another view point and sarcasm on innovation. Haha! If I didn't already have a stove set up, I'd be interested in the caldera cone. I was looking for ways to improve upon it… I am curious about the boil times and fuel use of your set-up with my version pot(GSI pinnacle soloist).. though I believe my pot may be the problem. At the end of the day when your hungry you just wish the water would boil faster and for longer.

@ David,

Awesome testing data that is useful.. If I don't get rid of my pot it's going an infrared version of black… of course I'll be testing the boil times before and after that.. non-scientifically of course.

About your temp gauge.. 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?? I tire of looking at a 1.5" thermometer and guessing… but I can't help but wonder exactly. Any suggestions? Or, maybe just a longer thermometer.

Could you post the link to the thread for soot vs paint… Sorry bout the cross post.

thanks all y'all

[Paul Wagner]

Not being a scientist, I can't comment on some of the findings here—but I appreciate the info.

My wife is a professionally trained chef, who points out that it is important to match the size of the pot with the size of the burner for best results.

And now she will finally let me paint the bottom or our backpacking cook pot black!

[Dirk Rabdau]

This thread reminds me warmly of time spent hiking in the company of engineers. I didn't understand a lot of what they discussed, but they were ecstatic to explain to a liberal arts kind of guy the finer points of thermodynamics. Truthfully, I think what I came to appreciate is that there is a lot of good in the world we owe to engineers.

I do really enjoy this thread. Thanks David and all for the explanation and supporting data. It's greatly appreciated.

Dirk

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