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Tongue only mentions higher dielectric loss with PVC, causing lower Q, compared to when styrene is used. Since Tongue is concerned only with the design of crystal radios, he does not mention the results of exposure to the elements (UV and moisture). This should produce a further increase in dielectric loss.
In principle, dielectric loss should cause a small reduction in inductance because a lossy coil form behaves like a shorted turn closely coupled to the coil. But the resistance of the lossy coil form would be very high, so I would expect the inductance reduction to be nearly undetectable.