Preheating the scrap in an old cast iron frying pan on top of the furnace for ten minutes or so befor adding to the pot will generally drive off the moisture that excites the tinsel fairey.
Keep an eye on it while it preheats and if it is wet enough to visibly steam cook it some more till it doesn't and then more yet. Also a cast iron ladel that has been laying around on the concrete floor in the garage will soak up enough moisture to be dangerous if you don't pre heat to dry it out befor dipping in the melt, same goes for ingots stored where it might be damp, dry them out befor adding to your casting pot, same for stirring spoons etc.
In other words pre heat to dry out ANYTHING that will be dipped or dumped into the melt.