Soot tags gather after fires in areas with low circulation. They are not, as commonly believed, ash covered spider webs.

oh, well then what the FUCK are they???

They’re made of sticky particles from a polymer or petroleum based fire, like burning carpet, drapes, upholstery, and clothes. Due to a static charge, they chain together and naturally gather near ceiling corners because the rising hot air pushes them into the cool spots by convection. 

Because they’re formed by static electricity, they can only be removed with professional chemicals and equipment. Attempting to remove them improperly will only break the chain before all the soot can be captured, leaving the remaining soot to spontaneously reform the webs later. Even worse, trying to wipe or wash them away can firmly adhere the soot to your wall or ceiling, which will permanently stain it. 

A natural phenomena that only coincidentally resembles the damned webs of transdimensional ghost spiders.