sample taken from a Toefl exam

>Just as painted designs on Greek pots may seem today to be purely decorative, whereas in fact they were carefully and precisely worked out so that at the time, [sic] their meaning **was** clear, so **it** is with Chinese pots.  

>([langlib.com][1])

Hi everybody, my question is about the antecedent of "it", and what is the reference of "was"?
 


  [1]: http://www.langlib.com/Mokao/TOEFL/Analysis/MTAtUjEx