I have two of the vintage Century lamps.  17 inches tall.  Both have been wonderful til I used 40 watt bulbs purchased bulk online.  They both got too hot and the lava would only float.  I've redone both.  One turned out just fine.  The other has the bubbled lava.  I have included pictures of each lamp.  

40 watt bulbs may be to hot for this older version. They seem to take a 30w bulb. There should be an inner label showing the bulb list, also those original bulbs were made shorter. The bulb you bought were they 40w A15 type and are they frosted coated. Clear 40w bulbs also run hotter.

Thank you to you both!  Such a pleasure to meet other lava lamp lovers!  <3