Melting milk or white chocolate requires more careful attention. If these chocolates have any contact with moisture or if they are melted over water that exceeds 95 to 110 degrees, they tend to tighten, seize or lump quickly. Too much heat affects the milk proteins in the chocolate (a process known as denaturing), causing it to become lumpy.
If any kind of chocolate tightens or seizes into a fudgelike mass during melting, before you add it to another mixture, add small amounts of clarified unsalted butter or vegetable shortening, about a teaspoon at a time. Most of the time it is moisture that creates this unworkable state. If the chocolate is overheated, however, nothing you do will restore its original flavor or gloss.