Why didn’t the alchemists realize the “gold” they made was bogus?

Two hundred and fifty years before Miriam bat Isaac’s time, Archimedes proved that King Hiero’s crown could not possibly be pure gold but rather a mixture of metals. He compared the weight of the crown with an identical volume of pure gold. The scales tipped in favor of the gold. Thus the crown had to be a mixture of gold and less dense metals.

So, why didn’t the alchemists similarly realize the gold they made was bogus?

They did not recognize density as an identifying property of a substance. Instead they identified a substance by its color. When a material they synthesized had a golden color, they assumed it to be gold. When it didn’t show any of the other properties of gold, such as its malleability or chemical stability, they reasoned that with improved methods and the Grace of G-d, they’d eventually be able produce a better sample. And so the early alchemical literature, such as the Leyden Papyrus X, featured methods for tingeing base metals with sulfur and mercury to impart a golden or silvery color.