If I had to guess, I think burnishing pottery to make it water tight is a myth started by a modern potter examining the
exterior of pottery from the Mississippian culture. On the basis of "Form follows Function" the early potters are probably copying
the surface of an object they're familiar with - the common gourd. We know they had and used gourds since they made clay copies of
them that are "gourd effigy pots". The paste they used was made up of clay, organic materials, sand/dirt, and crushed shell so you
can see that the pottery was quite porous. Unglazed pottery has one use, to hold more liquids than leak out.
All the pit-fired unglazed pottery I've eaten from has the common trait of tasting like burnt clay...regardless of how many times
they're used. Any bowl used for food gets moldy, whether they are rinsed out or not. The mold issue can be remedied by refiring the pottery.
All the pots where drinks were made became micaeous, from the natural inclusions of mica in the clay. It seemed ok to drink since the liquids
were brought up the boiling temperatures on the pit fire.
In the Southeast United States, when the Mississian culture collapsed pottery became smooth but not burnished until about 1775 when
burnishing returned, probably to copy the glaze finish from European trade items.
Different cultures make the pottery for different reasons. Some South American Amazonian cultures grind tree resins into a powder and when the pots come
off the fire, the mixture is dusted onto the exterior making them appear glazed. In Peru, the clay has so much lead ore in it that when
the pottery is fired it has a natural lead glaze...I think it is called plumbate pottery. Some people who use this pottery live to as old as 34.!
But since you plan to use pure clay and an electric kiln, its wide open for you to experiment with burnishing and such.
Make lots of things with notes on what steps were taken.
Good luck to your endeavers,