We have spectroscopically observed, as suggested by Sanford et al. (1996), a transition from discrete wire implosions to the implosion of a more continuous plasma shell in aluminum-wire-array z pinches driven by a 7-TW electrical generator. The time history of magnesium dopant K-shell X-ray emission depended on the interwire spacing, as well as whether the dopant was introduced as an alloy in or a coating on the aluminum wires. We diagnose the final assembly of the hot, dense K-shell emitting core, a region of the pinch not accessible to laser interferometry or optical/UV emission measurements.