Several times I've made a basic marinara sauce and shortly before it's done I've added a little whole cream without a problem. Last I night I made the same thing but added half and half since I had some on hand. As soon as I began to stir it in, it started to seperate and instead of a nice creamy sauce, it had little grainy clumps in it. It was awful! What happened? Was it too hot? Or can't I use half and half in a sauce like that?