For this achievement you'll need to find 6 doors which has the word 'VITRIFIED' painted across them in yellow and an intercom button on the wall next to them. Once you've found one of the doors, press the intercom button for it to count towards your total. Some don't have the 'VERTIFIED' on them, so look out for the intercom, but they're all in groups of 3.
The first 3 are once you've taken the lift up the tall metal rig, exit and u-turn to the left. You'll see across the gap a little walkway and 3 doors. Place a portal across the gap then one on the wall to your left. These doors across here
You may now want to refer to 'Master the Repulsion Gel' & 'Schrodinger's Catch' below before you look for the final 3 doors, seeing as they're later on in chapter 6 and there's still some achievements you'll need to get before you get to there. Once you've got them, see to the paragraph below.
The next 3 are in the same corridor you take to get the 'Ship Overboard' achievement. After a long way into the chapter 6 you'll exit through a door and be in an open area. If you look across the area you'll see a glass building with a banner saying 'CONTROL ROOM' across it in yellow.
Make your way up and in there and look in the left corner behind a cabinet. You'll come across hidden metal door which will automatically open as you get near. Walk through it and you'll see the remaining 3 doors and their intercoms.