Apologies if I've phrased this wrongly. I don't try to do this very often: that is, boil a sauce to evaporate the liquid (I understand this to be known as 'reducing the sauce'); however, when I do, I frequently end up with a skin forming on top of the sauce.
I have two questions related to this: this first is, is this expected / normal, or am I doing something wrong? The second: assuming I'm not, should I try to scrap the skin off, or just stir it back in?
As an aside, is there a guideline time for this (for example, 5 minutes per 10ml)? I usually leave it for too long, or get bored early and end up making a standard sauce using flour and butter.