* since plasma is made up of ions, the container has to be completely airtight and the seal between the "lid" and the "container" has to be fused shut.
* there material has to withstand temperatures of several thousand degrees Celsius (http://en.wikipedia.org/wiki/Plasma_%28physics%29#Temperatures)
* the plasma has to be made from the gas of a single element (like Neon), meaning that the culture has to be capable of extracting tons of various PURE elements from different sources.
* Not compounds. One of the elements on the periodic table: [en.wikipedia.org]
I haven't seen anyone say what element they thought the Egyptians were extracting. The airtightness is another problem, too... since ionized gas will escape easily from containers that are "airtight" by regular standards.