Blackjack – Unique! BlackJack goes almost clear when hot, but self-strikes back through brown to near-black depending on how thick you use it. BlackJack likes an oxidizing flame, but does not reduce with gray streaks like most black boro colors. BlackJack is not suitable for thin stringer work where you want the stringers to look like a dense black, but it’s the perfect color for very specific things, like sculpting without the graying. Because this glass goes clear when hot then self-strikes, it is useful in applications where, say, you want to mask a weld.