Dark spots on the walls?

Hey All,

Let the wild speculation begin! My mom lives in an old victorian house that has been converted into apartments. Her rooms are on the main floor. She burns candles when she is home. I went there last night to help her take down some decorations taped to the walls. As the tape came off, I could see a different color and assumed I was pulling the paint off the walls, but really it was soot. Then I looked closer and noticed that there was a fine layer of soot all over the walls. As I was admiring the soot I noticed circular areas about 1/2 an inch round, regularly placed on the ceiling and upper part of the walls of concentrated soot.

Then it dawned on me that those were the nail or screw heads under the surface texture of the dry wall.

Then I started trying to think of ways to explain the phenomena of higher soot concentrations on the areas direclty over the nail or screw heads.

Is the soot magnetic? Is the soot attracted to cooler areas of the wall directly over the screw or nail heads. Is there an indentation in the wall and the soot only looks darker?

What’s the straight dope?

Wow. My first thought is that she must be burning hundreds (thousands?) of candles to put out that much soot, or that she’s been doing it for, like, fifty years. Therefore, I would suspect an alternate source for the soot. Is there a wood-burning fireplace or stove in her apartment?

Soot is just pure carbon and AFAIK is not magnetic.

< wild speculation >

WAG – I would speculate that there is an ever-so-slight temperature difference between the paint over plain drywall and the paint over top the metal nails. So there’s a miniscule bit more condensation moisture over the slightly cooler spots where the nails are. The soot from the candles is sticking to the slightly-more-damp dots.

Otherwise you might have some very slight “nail pops” that are accumulating more dust etc. If you put a bright light on the floor against the wall and shine it upwards, you may see a much more dramatic effect.

< /wild speculation >

As per this Argonne National Laboratory- Home Energy Site candles can cause soot. However, it is very uncommon as noted in the cite.

Also as per the cite - the patterns of soot deposition are effected by static charges, Thermophoresis, and Gravity.

[QOUTE]*Thermophoresis, or “thermal precipitation,” is a physical settling effect in which particulates in the air stream plate out on surfaces that are colder than the airstream. Different materials acquire or lose heat at different rates; therefore, some items will always be colder than other surrounding items. Particles will “stick” to colder surfaces and will become noticeable as a sooty stain. *


So I suspect your nail heads are causing thermophresis.

Basically read the cite, it pretty much describes whatever there is to know about soot (IMHO).

Actually, it doesn’t take much to leave a fine film of crud on walls. The wall just gets a little grey so one day you think to yourself “hm… the paint job is looking a little faded or something.”

My father moved into a brand new, freshly painted apartment. When he died two years later from a smoking-related illness, the cigarette smoke had left quite a film over the walls. Running a damp sponge was quite awe inspiring. (I can only imagine the man’s lungs!) After ony two years, the film left behind from the smoke was pretty shocking.

SandWriter probably doesn’t mean that the walls are black/grey, they probably just “don’t look vibrant.” But peel off some tape or wipe it with a wet sponge and you’d go “Ack!”

There’s wall cleaner stuff that is recommended before you paint and for when you really need to wash/wipe down the walls. The suff is great for revitalizing the paint job in a home where there is a smoker, fireplace or rabid candle fan.

You guys are great! Now I can impress my mom with Thermophresis (pronounced… Therm-o-four-ee-sis?).

She does burn a lot of candles, and she has been living there for over 12 years…

Thanks for the answer!

Sandwriter, do you know if the texture of the wall has been modified in any way over where the screw was inserted?

Just speculation, but if small spots of the wall were plastered to cover the recessed screw/nail before painting this may change the texture of the wall. Texture would change rate of depostion.

Hey Antechinus,

I ran my finger over a few spots and detected no difference in texture. I also laid my face on the wall to get my eye along the wall to see if I could see any dents or anything, but it looked smooth.

And it wasn’t just one screw, it was on every head holding up the ceiling drywall and wall drywall above 6 feet. I don’t think my mom even noticed it cause her eyesight isn’t that great.