Here is another test similar to last year's experiments (A and B). I compare the same 2009 Spring Ali Shan Hung Shui Oolong stored in 3 ways:
1. (Left): a small ivory color porcelain jar made in Yingge (Taiwan) with industrial methods. The foil was kept below the lid. I filled the jar to the top and then vacuum sealed the jar in a plastic foil (similar to the one in the middle). Storage time: 15 months.
2. (Medium): Vacuum sealed plastic foil of 300 gr. Storage time: Since April 2009, when it was made.
3. (Right): a medium qinghua porcelain jar made with traditional methods. Storage time: 2 months.
There is a big difference between 1 and 2. The tea from the bag (2) still smells and tastes very similar to when I've got it. The roasting may have dissipated a little, but it has a nice freshness to it. In the ivory jar (1), the tea smells stronger and more oxidized, as I had noticed here. However, this strength is kind of crude and not very natural. The freshness seems replaced by more sour notes. This also translates into the taste, which is rougher, less smooth than (2).
The traditional jar (3), on the other hand, has a smell that is much closer to (2). This is normal, since the tea spent less time in it. However, I feel from the scent and taste that in this jar, the leaves are taking a different evolution than in (1). The tea smells finer and the taste is more elegant, smoother, while the freshness is preserved. This is proper aging.
This experiment explains why I have removed the ivory porcelain jars from my selection. They are bad fits for long term storage. I recommend to only use them for short storage periods. The qinghua jar isn't available, though, but I continue to work with several potters to achieve good Oolong jars. I believe that it's necessary to use more natural materials so that the jar won't emit bad, unnatural smells.
How can you tell if your jar is good or not for storage? A faster way may be to check what the cleaned and empty jar smells like. Does it smell bad, plastic, weird or clean and fresh?