The Crossed Furnace Wall is a black building-block ornated on all 6 sides with an interrupted cross and lava-colored stripes along the 4 ledges that glow in darkness/in the night since R36.
It can be crafted in your Crafting Menu (default key "Q") or at any Crafting Table in worlds created before R22 on September 16th 2015, after the recipe has been unlocked upon buying it in the Store as a part of the "Furnace Decorative Pack" (as of R25 in November 2015).
To craft 8 (blocks of) Crossed Furnace Walls, you'll need (as of R25):
- 2 liquid Lava-blocks, which require a Lumite Mining Cell to scoop
- 2 Iron Slabs made of Iron Bars in a Processor
- 2 Globs of Goo made from Mold in a Processor or looted/harvested from many creatures including Pigsies
- 1 Iron Rod made of Iron Bars in a Processor or found in Iron Treasure Chests or in Diamond Treasure Chests