I was making mustard croutons. I whisked together 2 parts warm melted butter, 2 parts olive oil, 1 part Dijon mustard, 1 part brown mustard (with whole seeds), 1 part sherry vinegar and table salt.
Since I had so much mustard I didn't think it would be necessary to do anything gradually (mustard being a great emulsifier), so I just dumped it all together and started whisking. Is that likely to have been the problem? Could it have been the warmth of the butter (not quite hot)?
Is there such a thing as too much emulsifier? The sauce never did emulsify by whisking, it just stubbornly stayed completely separated, albeit with ever smaller droplets of mustard/vinegar.
It probably wouldn't have mattered much since I was just planning to toss bread chunks in it anyway, to bake for croutons, but it bugged me so I whipped out the food processor. After processing the sauce looked emulsified, but it separated within perhaps 2 minutes. The croutons turned out great anyway, but for future reference I'd still like to know why I never did achieve an emulsion. I've made mayonnaise pretty much the same way, and it always turns out. What gives?