Painting pot black

[Thomas Budge]

Just received a 700ml Ti pot from Backcountry.com. Has anyone painted the exterior of this type of pot black and compared boiling times? With a Heiny pot I get a boil 20-30 seconds faster if it's painted. Not sure if I would get the same result since the Ti pot is a darker gray color and not as reflective as the Heiny.

[Jerry Adams]

I have Evernew Ti 900 mL

Painted bottom of pot with black barbecue spray paint. Masked the rest of the pot with masking tape and paper.

Years later it looks pretty much the same, a few small scratches.

Never tested it though

[Ultra Magnus]

How well did you test your heiny pot boil times? How well controlled were the conditions? How many times did you repeat the test?

I've been skeptical about the claims that paint could speed heat transfer. All I see is that paint is another medium that needs to be heated in the way of heating the bottom of the pot.

Also from my personal tests, I've found that even with well controlled conditions, i.e., my kitchen counter, 70f air temp, 70f water temp, boil times can vary +/-30seconds for no apparent reason.

BM

[Thomas Budge]

3-4 tests in garage. Not rigidly controlled, but every time a faster boil by 20-30 sec.

I wonder as well about paint slowing heat transfer. On the other hand, everyone knows that black is hotter in the sun.

Anyone?

[Ben H.]

Flames are at such high temperatures that radiation is the main form of heat transfer. As a result increasing the absorptivity (painting it black) out weighs the penalty of increased resistance to heat transfer (a thin layer of paint) through the pot. It is a balancing act, but it doesn't surprise me in the least that it can noticeably affect boil times.

It would be nice if someone could do a side-by-side comparison (i.e. two pots and two stoves). Presumably your tests were done sequentially in time (ie. test uncoated pot, coat pot, then test coated pot).

[Thomas Budge]

I used the same stove used with each pot, but I wasn't strict about water temperature or environmental conditions. Would be interesting to see results from multiple tests using two identical stoves and two pots (one painted and one not) and interchanging the pot/stove combination.

[Ultra Magnus]

hmm… I might have to try this. I was digging around in my garage the other night and I found a pristine keg can waiting to be upcycled into something useful. I think that might have to be another cook pot, with a black bottom…

BM

[David Thomas]

To add to what Ben wrote: Radiation from a object is proportional to the temperature TO THE FOURTH POWER. Hence, it becomes a big facter at high temperatures (say, over a flame-spreader plate that glows orange).

While you want high absorbtion of infrared light, what we percieve is absorbtion of visible light. Shiny metal surfaces have low absorbtion in both ranges. Some metal surface are essentially mirrors in the infrared. I can thermally image shiny, 120F metal in a 70F room and get a reading of 75F. It mostly see the surroundings, not the hot metal. Generally, visibly black surface emit/absorb more infrared, but YMMV.

An interesting test would be to have a large pan (aluminum backing dish for instance)with different paints and coatings in different regions of the bottom. Fill the pan with hot water and use a hand-held non-contact thermometer to measure the apparent temperature of each region. A "hotter" region isn't hotter (because the water is all the same temperature. A "hotter" region is emitting more infrared. Hence that same coating would absorb more infrared and be a good choice for coating the bottom of your pot.

Ben is also right that a thin layer of paint won't effect convective/conductive heat transfer much (hot gas to paint to metal to water) because most of the resistance to heat transfer is in the air film below the pot.

[David Thomas]

I couldn't resist. I painted an aluminum baking dish in black, white and red spray paint, leaving one quandrant unpainted, shiny aluminum.

I then cut a hole in a cardboard box and inserted a hairdryer (VIdal Sasson 1500 watts):

And inverted the painted aluminum baking dish onto the cardboard box:

I used a non-contact infrared thermometer (Craftsman Model 82327) to measure NOT THE TEMPERATURE, but the infrared EMISSIONS off of each section:

I collected 4 readings from different locations in each quadrant, repeated, and then rotated the pan 180 degrees. I did that so if there was uneven heating from the flow of hot air underneath, the left was now on the right, the far side now on the near side. And collected another 32 readings for a total of 64 readings. It was striking how consistent the data points were from one round to another.

[David Thomas]

Averaging 16 data points for each paint color (eyeballing it, the standard deviation looks to be about 3-4 degrees F for painted sections and 7-8 degrees for the shiny section) gives the following:

Black: 197F
White: 197F
Red: 185F
Unpainted: 98F Yes, 98F like body temperature. I didn't drop a "1" on that.

Again these are measurement of EMITTED INFRARED, not the true temp of the metal. I assure you the unpainted section was quite hot!

What does this tell me?

1) Paint color doesn't matter because we're talking infrared and all these paints are pretty "black" in the infrared. More precisely, they all have high emissivity in the infrared. Although white and black have more than red, I'm guessing because the purer red color may only have one or two dye molecules in it, while black and white either have more kinds of dye molecules or simply many more electron transitions going on in whatever dye is used.

More importantly:

2) The unpainted section was much less of an infrared emitter. MUCH less. That's good for the sides of a pot which out onto the cold, cruel world. But the view from the bottom of the pot is a hot burner. You want high emissivity there. "blacker" bodies emit AND absorb more infrared. A flat black car gets hotter in the sunlight than a white or chrome car would. A shiny metal object doesn't cool as fast on a clear night as a black object.

Bottomline: I'm painting some pot bottoms black. I'll test them before and after, side-by-side, switching burners each time.

[Thomas Budge]

Nicely done, David. Looking forward to the burner test.

p.s. an infrared thermometer is not part of my home repair arsenal. What else do you use that for?

[Ultra Magnus]

Could you do me a favor and do one more hair dryer test? Do you have some scotchbrite on hand or something abrasive that you could use to make a portion of that aluminum baking pan a satin or flat finish?

Thanks!
BM

[David Thomas]

>"What else do you use that (non-contact infrared thermometer) for?"

Thomas: Let me put is this way: it's on the top of the tool box. When they first hit $79 I got one. Then another when they got to $39. At $29, I got one for the car glovebox. Now they are $5-$7 including S&H on eBay.

http://www.ebay.com/itm/GM300-Non-Contact-IR-Infrared-Thermometer-Laser-Point-/270882520645?pt=LH_DefaultDomain_0&hash=item3f11db3e45

Some of it is work related: I can check a temperature of a pipeline or gas piping, concrete radiant floor or soil temperature VERY quickly. And because I can do it so quickly and painlessly, I might get 10 data points instead of one and look for a pattern and sometimes find one. I can tell temps with my hand from 0 to 160F within a few degrees, but some of that expertise comes from all the data collection. My day job involves doing stuff that Fortune 500 companies can't figure out for themselves, so there's always a steep learning curve.

But really, I use it more while dinking around the house. I can find a stud in the wall during cold weather due to the greater heat transfer. I can check cylinder head or exhaust temps on an engine and find a cylinder that is running lean or rich. I explore just how cold the car gets below ambient air temps on a clear, still night (as much as 15F colder, hence the dew and frost on those nights). I bake wedding cakes for friends (5 tiers, feeds 150, usually a hazelnut-chocolate torte) and the guanche chocolate glaze really wants to be 91 or 92F to pour it. 93 F and it's too runny. 90 and it's too thick. The list goes on.

"Honey, you were asking me for a stocking stuffer idea. . . ."

[David Thomas]

BM: As soon as I started to Scotchbrite it, I knew what would happen. Because there was a black residue that came off – the aluminum oxide layer – so it was a little bit shinier. Shinier means a better mirror and therefore even more like a mirror reflecting room temps.

So I used a piece of 150 grit sandpaper to really rough it up another section. And collected another 24 data points. (this didn't run as long as the last run so look at this data separately from the data in my previous post).

Plain: 81F
Scotchbrited: 79F
Scotchbrited and sanded: 85F
Sanded: 84F

My interpretation:

Scotchbriting it polished it, made it shinier and therefore even closer to room temps. i.e. better for the sides of a pot, worse for the bottom of your pot.

Sanding gave it a definite roughness and may have reflect more of the infrared from the nearby flat surface (the painted and unpainted portion of the pan or maybe some nearby objects catching some of the hot air flow.

My suggestions: Don't scotchbrite or sand the bottoms of your pots. Paint them black. Scotchbright or sand the sides of your pots, especially if there is soot or food residue there. Scotchbrite if you want it shiny. Sand it if you want a more no-slip surface.

[David Thomas]

I said "I assure you the unpainted section was quite hot!" and that seemed kind of lame, so I thought, "How to prove that?"

I licked my finger and wetted the pan across each paint/paint or paint/no-paint boundary. Then watched how faster the spit evaporated away.

It evaporated away fastest from the unpainted section – it was bubbling a little at times, it never boiled on the painted sections.

Clearly true: the unpainted section was HOTTER than the painted sections yet (see previous results) the unpainted section was emitting MUCH less infrared even though it was hotter. I'd estimate by about 5 to 7F. Comparing 675R* to 669R to the fourth power, the hotter unpainted section should have emitted 3.6% more IR, but it emitted far less because of its lower emissivity.

Why was it hotter?: I'm pretty sure because the heat into the bottom of each section was the same. Same air temps, same air velocity, same color (unpainted). While the heat loss off the top was much more for the painted sections, cooling them more than the unpainted section.

Reminder: This test is the REVERSE of what happens on the bottom of your pot. However emissivity is the same in each direction so the big loser in this case (black paint) when its environment is colder is also the big gainer when next to a hot burner.

*R = degrees Rankine, an absolute temperature scale used by Americans and Brits who remember when humans used to walk on the Moon. Akin to degrees Kelvin that you young whippersnappers use. But in a more convenient, 5/9 size.

[Ultra Magnus]

"Why was it hotter?: I'm pretty sure because the heat into the bottom of each section was the same. Same air temps, same air velocity, same color (unpainted). While the heat loss off the top was much more for the painted sections, cooling them more than the unpainted section."

Very interesting, and this explains something I have observed first hand in the past, but never really given a whole lot of thought. Way back in my younger years, I had the miserable job of working at a tractor/trailer junk yard cutting up old semi's for scrap, in Phoenix, AZ. The summers were hell, and one of the first things I learned (should have been to wear a hat, young and dumb) was to never leaver your tools in the sun. They got darn hot, really fast, and the chrome tools stayed hot for a very long time. I only had to burn my hand a few times before I started paying attention to the movement of the sun to keep them in the shade as the hours went by.

I'll for sure be doing some back to back tests…

BM

[Ben H.]

excellent set of tests Dave… you certainly did run with that. I did a double take when I saw your posts.

Do you know of any readily accessible spray paints with a higher absorptivity?

[David Thomas]

Ben: Thanks for your kind words. Looking at the temperature as interpreted by the IR thermometer versus how I saw water evaporate / boil, I think the thermometer assumption of .95 emissivity is pretty good. There's not a lot of improvement to be had between .95 and 1.00 especially if you already have a can of black spray paint in your garage.

Potentially, the solar industry has some spiffier coatings – I remember thermal solar panel manfacturers raving about their coatings in a past life when I installed 80 such systems. But they'd be looking to absorb in the visible and not emit in the IR. For our pots, we don't care about visible and we want high emissivity in the IR which I seem to have gotten with off-the-shelf white and black paint.

I'd just look for high-temperature paint. Like for car engines.

But the process did give me an idea. Anyone gone through the patent process and wants to share another one? I think it's a slam-dunk, but I hate filling out paperwork.

[Jerry Adams]

You use those for cooking

See if your pan is hot enough to put the food in

Camping World has them for like $60

Use spray paint intended for barbeque – readily available and cheap

[Thomas Budge]

Do you suggest painting everything below the water line or just the bottom of the pot?

[Bob Gross]

"Use spray paint intended for barbeque – readily available and cheap"

How much weight is that going to add?

–B.G.–

[David Thomas]

Thomas:

I'd suggest just painting the bottom of the pot, not the sides at all.

There's a concept of "view angle": How much of your view sees hot versus cold objects.

For the pot looking down, 180 x 360 degrees, there's some pretty hot stuff – hot steel bits, campfire coals, etc. The objects around the bottom of the bottom are warmer than the pot so it should be black so it absorbs more radiant heat.

For the side of the pot, they look out at the same terrain and scenery and sky that we do. Most of the objects being ambient temperature, the clouds cooler, depending on their elevation and deep space on a clear day or night, quite cold (4 degrees Kelvin, I think). The sides' surroundings are colder than the pot, so it should be left shiny so it emits less heat.

A pot within a windscreen probably should be painted black up to the height of the windscreen. Let me puzzle out how to answer that particular question.

I suspect that painting the bottle black will help the most on a white gas stoves, less on a butane stove and much less on a alcohol stove. But I'll be testing that in a few days. I just bought four 4 identical stainless steel canisters as Walmart that are comparable in thickness and diameter to BP pots and I'll modify three of them in different ways and try them on different stoves.

So for now, if you're leaving for a thru hike tomorrow, I'd say: paint the bottom black. If you've leaving next week, I hope to have more definitive answers.

[David Thomas]

BM said, "and the chrome tools stayed hot for a very long time."

That is a very astute observation and one that I am going to steal. I try to not give K-12 students and junior engineers an equation without giving real world examples. Black objects getting hotter in the sun is a common experience. I've used white lines to cross a parking lot barefoot on hot sunny days.

Black versus white or, more so, black versus shiny metal cooling more quickly is less commonly noticed. But is just as true.

[Jerry Adams]

"Use spray paint intended for barbeque – readily available and cheap

How much weight is that going to add?"

negligible

even for gram wienie

paint layer is very thin

[C Nugget]

Awesome!! I look forward to more results :) giggling too… Who needs Discovery Channel when you have a forum like this!!! Would different metals react the same or??? Thinking Titanium vs. Anodized Aluminium.. or is thickness more of a priority? What about the non-stick coating inside the pot some make the inside shiny and some(ceramic) make it flat black.. Would that affect it as well?? Would a larger surface area on the bottom of the pot increase heating as well?? What about verses a more slender tall pot of the same volume?

I sense a can of worms… but interesting… must be a forum on some of this…

-christy

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