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One day, my mom decides to light the house on fire, and after we put the fire out, all of the cobwebs in the house are black. Obviously, this is because of the smoke. Later, some renovations guy tells her something about how the cobwebs aren't really cobwebs and when a house fills with smoke, there is some weird chemical formation that makes a black cobweb-like-substance that just happens to show up right where cobwebs would be. Basically, the guy told my mom that those black cobwebs were chemical formations from smoke instead of cobwebs that became black after being in smoke. My mom will not believe me when I say that the renovations guy is wrong and they were obviously cobwebs. I ask her "What happened to the cobwebs we already had?" and "Why would a renovations guy know that it is a smoke formation if there are cobwebs in every house?"

I can't prove to my mom that she's wrong, so I'm asking a bunch of chemistry people to confirm that smoke doesn't stick to walls and form cobwebs.

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Awkward. It just so happens that the renovations guy is right. The term you need to google is "smokeweb".

The ionisation process of burning (particularly) synthetic materials like plastics, rubbers and polymers, creates charged smoke particles which can attract towards certain surfaces, and attract to each other creating chains, and ultimately web-like formations. They form under certain conditions called wet smoke, which involves low heat, or smoldering type environments. Smoke webs are typically sticky and pungent and will smudge easily and show up in particular areas where there is limited air flows, typically corners and edges of ceilings, in curtain folds, cabinets etc.

Spider webs will burn up very quickly in a real fire.

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