(What did you think it was going to be about, ya perv? ^_~)
Many vintage fountain pens have a sac inside of them to hold the ink. By the time these reach a modern collector, the sac is usually shot and needs to be replaced. Two materials are used in modern replacements: rubber (latex) and silicone. Both have their place, and both have advantages and disadvantages.
Rubber is the traditional material for pen sacs. It's flexible and has fast spring-back, and is the less expensive of the two options. Unfortunately, it has a limited lifespan due to a long-term reaction with the ink. It also releases gases as it ages, which can discolor the materials some vintage pens are made out of.
Silicone is a recent and still-developing addition to the repairer's arsenal, and thus there are a lot of myths about it. Some people believe it is less flexible than rubber. Personal experiments have shown this is not correct with the sacs currently available from Wood Bin, although it may have been true when silicone first hit the market, or with other suppliers' sacs. Silicone springs back more slowly than rubber, but this can be handled by allowing a few more seconds for a pen to completely fill. It does not react with ink, and so has a longer lifespan than rubber. Most importantly, it does not releasing discoloring gases, making it invaluable for restoring early discoloration-prone pens.
Given the advantages of silicone, and small price premium over rubber, why not use it everywhere? The pen community debates this in vague terms, often quoting the myth that it does not collapse as readily. However, as far as I know, only Laura and I did actual physical experiments with it. We found that the currently available silicone sacs are at least as flexible as rubber and collapse fully, but they do have a problem: ink sticks to them in a way it doesn't to rubber.
In the above picture, two sacs were filled with colored water and put mouth-down into absorbant tissue. The rubber sac emptied in a few seconds, but as you can see, the water clung to the inside of the silicone sac and stayed there. This is the real reason that silicone should not be used in pneumatic fillers. They collapse fine when empty and fill completely, but then cannot be purged of ink, even when cycled repeatedly. Furthermore, I have no reason to believe that lever fillers would fare any better. For this reason, I suggest that silicone only be used in pens that have discoloration concerns, or are too delicate to take apart repeatedly for future sac replacements.
Copyright Kati d'Esprit 2008