Admixture, alloy and adulterant are comparable when they denote an added ingredient that destroys the purity or genuineness of a substance.
Admixture suggests the addition of the foreign or the nonessential.
- pure Indian without any admixture of white blood
- love with an admixture of selfishness
- comic verses with an occasional admixture of mild bawdry
—Cowie)
Alloy derives its figurative implication of an addition that detracts from the value or perfection of a thing from an old literal application to a base metal added to a precious metal to give it hardness.
- there’s no fortune so good, but it has its alloy
—Bacon - he had his alloy, like other people, of ambition and selfishness
—Rose Macaulay
Adulterant, both literally and figuratively, implies the addition of something that debases or impairs a thing without markedly affecting its appearance. Consequently it usually implies the intent to deceive.
- interests . . . trying to upgrade consumer thinking on wool by classifying the new textile fibers as adulterants
—A. Adams - piety without any adulterant of hypocrisy