Skimming: foam, fat and a clear broth

How to tell it is going right

• It is held below a rolling boil for the whole time. Agitation drives fat and fine protein into the liquid as an emulsion, and nothing skimmed afterwards will take them back out.
• The grey foam that rises in the first minutes is lifted off and discarded rather than stirred back in.
• Fat is taken from the surface where it gathers, the pot moved off the centre of the heat if that is what it takes to collect it in one place.
• The liquid is visibly clearer at the end than it was at the start: it is possible to see down into it.
• Nothing lifted off goes back in. What was skimmed is discarded, not tipped back once the ladle is full.

Where it goes wrong The pot was allowed to boil, usually while something else was being attended to. Fat and protein emulsify into the liquid under agitation and cannot be lifted off afterwards, so the finished broth stays cloudy and greasy however patiently it is skimmed from then on.