Dialectric Constant


At a hamfest yesterday, I bought a Barker and Williamson CX 62 C 
variable air condenser.  It's 8"x6"x6" and made with ceramic and 
polished stainless steel plates that are 1/2" apart.  The craftmanship 
is superb and it's literally a work of art.  It's a shame these 
beautiful components are no longer crafted.  And, it was a great buy.

Prior to buying it, I checked it and the capacitance varies from 0.01 
to .04 nF.  Yes, nanofarads.  My plan is to increase the maximum 
capacitance to 20 - 40 nF by changing the dialectric and spacing 
between the plates.  I am building a 3/8" acrylic housing for it.  

Changing the spacers between the plates is simple.  I can even add a 
few more plates.  Question is what dialectric should I choose that will 
have 500 - 1000 k and good dialectric strength?  Either a gas or liquid 
dialectric would do or even application directly to the plates is a 
possibility.  According to R. Hull's recent post capacitance resides in 
the dialectric only, rather than the plates.  Maybe this is a place for 
Kristin's XeS6 or what ever that dialectric was.  Any thoughts?

I cannot find the recent posting of modern dialectric constants and 
their strengths.  Would someone please repost it publically or