What an emulsion actually is
Oil and water repel each other at the molecular level. Left alone, fat rises and water sinks — you see it in any unwashed roasting tin. An emulsion suspends one in the other, fat broken into tiny droplets small enough to stay distributed through a water-based liquid. The result is something new: thicker than either component, with a texture and richness that neither alone could produce.
The suspension doesn't hold by itself. It needs an emulsifier — a molecule that has one end attracted to fat and the other attracted to water. It sits at the boundary between each droplet and the surrounding liquid, preventing the fat from coalescing back into a pool. In domestic cooking, lecithin from egg yolk is the most useful of these. Mustard works too, modestly, because the mucilage in ground mustard seed has similar bridging properties. A classic vinaigrette stabilised with a small spoon of Dijon holds noticeably longer than an oil-and-vinegar mix left unaided.
The two emulsions home cooks encounter most are temporary and permanent. A vigorously shaken vinaigrette is temporary: given time, the droplets will find each other and the fat will separate again. Mayonnaise is permanent, or close enough that the distinction rarely matters in practice — the yolk provides enough lecithin, and the droplet size is fine enough, that it stays stable for days in a cool larder.
Why it splits
Heat is the most common culprit. Warm a mayonnaise or a butter sauce too aggressively and the fat liquefies faster than the emulsifier can manage; the droplets merge and the sauce breaks into a greasy puddle with watery liquid beneath. Beurre blanc — a butter sauce built by whisking cold butter into a reduction of white wine and shallots — is especially vulnerable, because butter is itself a solid emulsion of fat and water that starts to separate around 70°C. The working temperature for a beurre blanc is well below a simmer, and keeping it there is almost the entire technique.
Cold can also break an emulsion. A mayonnaise taken directly from the refrigerator and subjected to friction — say, vigorous folding into cold dressed leaves — can separate because the fat droplets have hardened and lost flexibility. In practice this rarely causes disaster, but it explains why a mayonnaise made at room temperature with room-temperature yolks is more forgiving than one made with everything cold.
Physical shock matters too. Adding oil too fast to a mayonnaise overwhelms the available lecithin before it can form a film around new droplets. The classic advice — a thin, steady trickle at the beginning, increasing only once the emulsion is established — exists entirely because of this. You are not being theatrical. You are giving the emulsifier time to do its work.
The two ways back
When a sauce breaks, two recoveries work. The first is mechanical: take a clean bowl, start with a fresh base — another yolk, a spoon of water, a little mustard — and whisk the broken sauce into it gradually, as if you were making it from scratch. The new yolk provides fresh lecithin; the broken sauce becomes the oil phase. This works well for mayonnaise and for any egg-based sauce.
The second is thermal: for a beurre blanc or similar butter emulsion that has split from too much heat, remove the pan from the heat immediately, add a splash of cold water and whisk. The water and lower temperature slow the fat droplets and give them a chance to re-disperse. It doesn't always succeed if the sauce has been overheated for long, but it works often enough to try before abandoning the pan.
Neither method is magic. Both work for the same reason: you are reconstructing the conditions — temperature, ratio, available emulsifier — that let the bridging molecules do their job. Understand that, and the split sauce is a problem to diagnose, not a catastrophe.