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The pickling process noted above was first used by the Romans. It allowed them to debase the silver denarius yet still produce perfectly silver-looking coinage. The 22k blank is blanched in acid, which results in the surface copper leaching out and only the gold remaining. Then when it is struck, the surface will look gold, but the coin still is made of an alloy. If the coin remains in pristine condition, it will continue to look gold, but if it is sufficiently circulated, then the wear will rub past the more pure external gold surface and into the alloy, where the coin can look more rose gold in color. The pickling described explains 100% of this phenomenon. The Royal Mint could do 18K Russian gold and the pickling would still yield a yellow gold coin. The imperial Romans got a 20% silver Denarius to look just like the 95% Denarius of the Republic with this method. The acid removes surface copper and leaves the gold.