Charles now needs high-grade fuel for his eyepieces, and asks you to go to the District of Ash to fetch the materials.
Go to the entrance for Maldura: District of Ash (Heroic) at -108.79,1.53,-46.81 and enter.
Gather high quality coal from the ore cart that's across the entrance area from you. There's some agro in the area so stay alert.
Go back to the door and down the stairs beside it. Pick up the gear laying next to the destroyed clockwork.
Go up the stairs from the entrance and look to the right, where you'll find a barrel of industrial strength solvent. Again, be aware of agro at the top of the stairs.
Return to the Forge of Brell and craft "Liquid Fuel" using the high pressure stem vessel near Charles. You'll need 3 thalumbral roots, 3 malduran lumber and 5 thaumic coal in addition to the stuff you gathered.
Give the liquid fuel and the gear to Charles to receive your reward.