Once fond crosses from deep brown to black, deglazing doesn't fix it — it dissolves the burnt fond straight into the liquid, carrying scorched, acrid compounds through the whole sauce. If the pan is only dark brown and the smell coming off it is nutty rather than sharp, you're fine. The bitterness starts at black, not before.
Why burnt fond ruins the whole sauce
Fond is what's left stuck to the pan after searing — proteins and sugars fused to hot metal, holding most of the flavor that browning built. Left on the heat a little longer, those same compounds keep changing. Brown turns to dark brown, dark brown turns to black, and black is bitter. It's one continuous process, not a switch that flips at some exact temperature.
Deglazing doesn't discriminate. The liquid lifts everything stuck to the pan at once, the good fond and the scorched bits together, and dissolves them into a single liquid. You can't strain a burnt sauce back into a good one afterward. The bitter compounds are dissolved, not floating as particles — there's nothing solid left to skim out.
Catching it before the liquid goes in
Smell is the fastest warning you'll get, and it beats color because a dark pan can still be fine. Browning fond smells nutty and a little toasty. Burnt fond smells sharp and acrid, closer to a struck match than to food — if you catch that smell, pull the pan off the heat before you add the liquid.
Color matters too, but only at the extreme. A deep mahogany-brown fond, close to the color of dark coffee, is exactly what you want. Fond gone matte black, or fond with actual black flecks flaking off it, has crossed into the bitter range and won't be saved by anything you pour in next.
Can this batch be saved
If the burn was mild — the fond went a shade past deep brown rather than fully black — a knob of cold butter whisked in at the end helps. Fat coats the tongue and blunts bitterness, and a little acid, a squeeze of lemon or a splash of vinegar, does something similar by shifting how the bitterness registers against everything else in the sauce.
If the fond went properly black, there's no version of this that ends well. Wipe out the pan, wipe out the burnt bits, and start the sauce over with a fresh knob of fat before adding the liquid again. The meat itself is usually still fine to serve — it's the drippings that carried the burn, not the meat sitting on top of them.
Getting fond you can actually use
Fond builds fastest right where the pan runs hottest, which is usually the center. That's also where it burns fastest, so food left sitting dead-center on high heat browns unevenly — dark in the middle, pale at the edges — while the middle keeps cooking past the point you wanted.
Medium-high heat, not the highest burner setting, buys you time to catch the color as it happens instead of finding out after the liquid's already in the pan. And leave the food alone. Meat that's genuinely ready to release lifts off the pan on its own; food that's still stuck is still building fond, and forcing it early tears fond off unevenly and leaves the rest to keep browning toward burnt.