ammonium chloride wouldnt survive a melt. Alum residue in a jar would produce a sulfur smell as it would free sulfur in the process of reacting with the copper. Moisture would have come from the cooling process if anything at all. I can get into the chemistry if you like but the alternatives, a true ammonia like smell would mean the copper melted was actually C14500 machineable beryllium copper. If it was melted WITH the use of a flux then an acid reaction from the jar would produce Beryllium chloride. So lets hope not.

Most likely is you melted dirty pipe with salt residues, and if you did that you likely also melted mineral residues and thats where the reaction came from in the jar with sufficient moisture and bimetal contact. This is the most likely scenario unless you melted bare bright or virgin copper product.