Finally manage to preserve the pitchers together with its shape and colours. Tried a few ways recommended by enthusiasts around the world. Found that the freeze dried method works the best. It works very well on pitchers with harder and more sturdy walls. N. mirabilis pitchers crumple a little in the process. The pictures below are pitchers that has been freeze dried for around 5 months and have been placed outside under room temperature for about 2 weeks. Lets hope the colour is maintained always.
Had also tried to bury the pitchers in powdered washing detergent. The result was no good as the pitchers turn brown and it was difficult to clean the detergent from the pitcher as the detergent has hardened. Perhaps moisture was absorbed from outside as I did not cover the container. The same results shows on pitchers that I tried drying using silica gel mixed with baking powder. On top of that bumps appear on the walls of the pitchers because of the silica gel. Unless if you can get your hands on powdered silica gel.
Anyway, here's the pictures of the dried pitchers:
Freeze dried:
Washing detergent: As you can see there is still detergent in the pitchers. The pitchers are very brittle at this point of time and I dare not dig the detergent out. Pitcher colour has turned brown.