Cobwebs that can be burned are mostly full of little spiders and make you move slowly once you in them... To burn a Cobweb push
on the dpad for your lighter. Now simply walk straight into a Cobewb and it start to burn away.
Note that not all can be burned even they look like they could be...
If you really want to speed it up do as follow...
Start the first level ASHES and as soon your out of the train you find yourself in a hall full of Cobwebs. Its directly at the beginning...
Now push
for your lighter and burn down all Cobwebs thats possible, there should be about 6 or 7... After that push
and select LOAD LAST CHECKPOINT...
Repeat till achievement pops...
Remind: You can also do the achievement here for using 10 lever switches...
Anna will tell you to use one in the same room, you cant miss it.
Same works for all other cumulative achievemnts like kills, open safe boxes, turn of lights...
Kind Regards