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Challenge: soot mystery

  1. Apr 7, 2012 #1


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    OK fizziks guys, here's a puzzle.

    Took this pic at a friend's house today. They have a wood-burning stove in the room. The ceiling and wall are blackened with soot where ever there is a stud. I can't think of any explanation.

    Some deets:
    - the room is a closed-in porch, probably uninsulated or at least poorly-insulated
    - there's a wood-burning stove just to the left of the pic. It's been in the house for decades.
    - those soot-markings are quite clearly coinciding with studs. They're 16 inches apart and have cross braces.

    Why would soot prefer to settle on the walls only where there is a stud behind it?

    Attached Files:

  2. jcsd
  3. Apr 7, 2012 #2
    I can't think of any electrostatic reason.

    Thermal conduction through the wall is higher where the studs are. Are these studs in outside walls?
  4. Apr 8, 2012 #3


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    Yes. It's an enclosed porch. Roof and walls are exposed.

    I am dubious about your assertion that the studs are better conductors of heat that the gaps between them. I know 4 inches of air is a good insulator, but so is 4 inches of wood.

    That being said, my best hypothesis is also that it has something to do with heat differential, I think the air gaps will be colder. Though I don't know how that would result in the observed phenom.
  5. Apr 8, 2012 #4
    I agree that the air gaps should probably be colder, unless there is some insulating material in there.

    Could it be related to humidity? The wooden studs might be prone to absorb humidity, whereas the air gaps, if ventilated, would dry out better.
  6. Apr 8, 2012 #5


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    Are you sure (have you verified) those darkend areas are indeed accumulations of soot?
    Could they be discolorations from some other cause?
  7. Apr 8, 2012 #6


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    I think the thermal reason is the one to go for. But, rather than conductivity, I think it must involve thermal capacity. The areas attached to the studding will change temperature slower than the bits in between. Hence the temperature will be more different from that of the air beneath for longer. After the fire has been lit, the resulting differential temperature lag could produce more condensation of water vapour or, probably more importantly, condensation of oils, distilled from the burning wood. This would then tend to collect more dust, tar and soot particles over the years. There is always more smoke in the room just after a fire has been lit because the chimney has not yet started to 'draw' well so this period would be when there is more grot in the air and also when the temperature differential is greatest.
    Last edited: Apr 8, 2012
  8. Apr 8, 2012 #7
    sophiecentaur's explanation sounds entirely reasonable to me.
  9. Apr 8, 2012 #8
    Since it is DaveC, I will offer the following observations:

    1) It is not actually the stud lines that are showing, that is coincidental. The boards are, of course, nailed at the studs, not in between.

    2) The marking actually outlines the ceiling boards as can be seen from the transverse markings as well as the longitudinal ones. There may well not be any studs behind these transverse markings.

    3) There is likely some material property of the joint filler used between the boards that is causing this. It may have become deliquescent.

    go well
  10. Apr 8, 2012 #9


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    There isn't a lot of point in speculating about the construction - the true facts are available and Dave could help us with that (I thought he already had). But, the separation between the lines on walls and ceiling are much much more like stud and joist separation than standard plasterboard size (900mm X 1200mm). Plasterboard joints aren't usually a problem when taped and skimmed correctly.

    If we're looking for a non-thermal explanation then wallpaper width would seem to be about right for the pattern. But the lines are too thick and even, I think, to be due to 'chemicals' at the seams.

    If it has to be chemical then perhaps the timber for the studs was treated and the treatment chemicals are leaching out into the board. The actual colouration could still be largely due to stuff in the air ending up in some areas more than others. Are they smokers? - I remember people used to need their rooms redecoratied very frequently when everyone smoked in a household.
  11. Apr 8, 2012 #10
    I can think of two possible reasons. The first belongs to sophie already and is most likely correct, given the probable lack of insulation. The studs would act like cold plates used in oldtime fruit cellars to control humidity, water vapor condenses on those areas and the soot sticks . The second could be a moisture behind the walls condition. Drywall manufacturers now produce water and mold resistant products to combat this problem in high moisture areas. Are the surfaces painted with a latex or oil based paint? Wallpaper? The situation could be hazardous if the soot is in fact black mold. The reason the mold would grow in a pattern matching the studs is because the studs behind the walls are absorbing water from a leak/improper venting ect. and the moisture is wicking from the wood into the drywall. This could be the case given the conditions of the structure. Poor or no insulation, converted porch with no heating or cooling system, 5 outside walls(alot of surface area for water to leak through in the rain), there could also be a leak in the vent pipe (combustion of materials usually produces co2 and h2o).
    Last edited: Apr 8, 2012
  12. Apr 8, 2012 #11


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    It would be insanity to cut the panels in such tiny pieces.
    1] Each panel would be 16 inches wide.
    2] They would have to have been cut to different lengths, since the horizontal lines are not aligned. One might be 3 feet up, then the next might bw 5 feet up. This may not be readily apparent in the pic. But I noted it explicitly.

    No, they are most certainly studs, not panel edges.
  13. Apr 8, 2012 #12
    My first thought was of a more mechanical nature. Soot is made up of fine particulate matter. Ambient noise may cause slight vibration of the drywall which essentially shakes off some of the particulates. Along the lines where the drywall is nailed to the studs it responds less to ambient vibration.
  14. Apr 8, 2012 #13
    Whatever is going on, insulating with a proper moisture barrier [facing] inside is almost certainly the appropriate solution.
  15. Apr 8, 2012 #14


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    I like this explanation.
  16. Apr 8, 2012 #15


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    I'm not sure the boundary conditions would be right to explain the positions and shapes of the patterns. You' d be needing suitable solutions to the wave equation for rectangular vibrating plates and I don't think the normal modes would support those particular shapes. Also, I think the residents would be wearing ear protectors. Haha.
  17. Apr 8, 2012 #16
    I do like mrspeedybob's approach to it, although I dont think its applicable in this situation.
  18. Apr 8, 2012 #17


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    Is this in Toronto? I see that you have 3 months of sub-freezing weather during the year. This might be a clue.

    And I disagree with you that 4 inches of air is a good insulator. 4 inches of unobstructed air is horrible insulator. There's a reason most thermal pane windows have a 0.6" gap between the panes.

    And wood kind of sucks as an insulator also. 0.7 - 1.4 R per inch? That's an R value of 4.9 at best for a 2x4. Nearly 3 times the conductance of my R-13 wall insulation.

    I've a wood burning Franklin stove in my living room (they leak like a sieve!), that I've used for almost 20 years during the winter, and I've no such lines on my ceiling or walls.

    With so few clues/parameters, it's difficult to deliver little more than a hypothesis.

    Mine is that the ceiling and walls do contain insulation, and the thermal conductance of the studs is causing cold spots, which leads to condensation [STRIKE]points[/STRIKE] lines, which leads to soot lines. (In agreement with Sophie and the Jedi's choices, as far as I can tell.)

    or perhaps it's not soot at all... perhaps it's the Black Death!

    My entire bathroom is covered in that stuff.... :redface:
  19. Apr 8, 2012 #18
    For a while this winter, I lived in a place where i could look across the street and see a sloped shingled roof (facing east), which usually accumulated a heavy frost every night. The morning sun would hit the roof by about 7:30 am. By about 8 or 9 am, I could see clear lines on the roof where the frost has sublimed. These lines appeared to be directly above roof joists. It wasn't clear whether the frost sublimed because of a heat leak, or because of stored heat capacity in the joists themselves.
  20. Apr 8, 2012 #19


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    sophie, you may be overthinking it. It wouldn't have to be standing or resonant waves. It is plausible that the panels where they are secured to the joists will tend to shake less than anywhere else.
    Yes, but how?
    That's what BobS was claiming. He suggested conduction is better through the studs than through the air:
    Yes. I think thermal conduction is surely the key to this. But snow is one thing, soot is another.

    I see two outstanding questions:
    1] Are the studs going to be warmer than the air or vice versa? Or is there a more complex reaction occurring over time?
    2] How does thermal change result in changes in soot accumulation?
    Last edited: Apr 8, 2012
  21. Apr 8, 2012 #20
    If for whatever reason the panels are colder above the studs, then you'd expect these areas to condense more moisture (or volatiles evaporated from the fire). Soot would in turn stick to that. Sophie already pointed that out.

    However, I believe that 4 inches of air would conduct more heat than wood, due to convection.
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