If gold in the crown was mixed with a less-valuable metal like bronze or copper how would that affect its density?

Mixing gold with a less-dense metal like bronze or copper would decrease the overall density of the crown. This is because the density of the crown would be a weighted average of the densities of the constituent metals, with the less-dense metal diluting the overall density. The exact impact on density would depend on the proportions of gold and the less-valuable metal in the mixture.