What is the reason behind the 2% increase in silver metal composition just after melting the raw silver alloy (55% silver)? I determined the silver composition from an alloy obtained from a jeweler to be 55% and wanted to extract silver out of it. However, upon melting around 5 kgs of alloy, the silver composition went up to 57%. I noticed similar increase for another 2 times (different alloys of silver, i.e. different samples) 

Could it be something related to oxidation/reduction of silver or any metal resulting in the increase in composition while in the furnace? 

What are some of the analytical tests out there I could use to accurately determine the elemental composition and the compounds present in the original alloy vs the one just after melting it?
Zinc is used in some silver alloys; Zn would be easy to lose into dross or vaporization.