Ask a spectator to wrap his own quarter in a plain napkin you give him. The magician doesn't touch either the napkin or coin until the spectator finally hands it to the magician to demonstrate the freezing. The napkin helps protect from the cold that will occur. [Note: there are no chemicals on the napkin and none in the magician's mouth or on his hands]
Summoning a change in his body temperature, the magician begins to blow freezing air onto the napkin, causing the napkin to become ice cold. He unwraps the quarter and, leaving it sitting by itself on the napkin, without even touching it, he blows on the coin.
The quarter frosts over white and freezes visually. When picked up by a spectator the coin is bitter cold and still white from frost.
Frozen was generated by the mind of Adam Grace. It is workable and practical. Our team noted that in every street performance the spectators had no answer except that Adam must have really changed his body temperature.