For food storage I mainly use mason jars with stainless steel lids and food-grade silicone ring seals to keep them airtight. Twice while storing different types of loose-leaf tea, the silicone seals became oily and swollen even though they were not in contact with the tea at all. I'm assuming the silicone is somehow attracting the essential oils from the teas, but I thought silicone was inert and I can't figure out why it's doing this.
The first time it was earl grey tea, which I know has bergamot oil. After several weeks, I noticed that the silicone ring was covered in an oily film on both sides. It had also changed color (becoming more transparent) and expanded in size so that it no longer fit comfortably in the lid. The seal had never touched the tea, and neither the glass jar nor the steel lid had the oily residue on them. I washed the oil off the seal and let it air dry for a while. After a few days away from the tea it reverted back to its original color and size and feels normal again.
I didn't think about it much more until the same thing happened again while storing a loose-leaf herbal tea. This one had basil and chive oil in it. After a few weeks, similar experience: the silicone seal was covered in an oily film. Once again none of the tea had been touching the seal, and there was no oil on anything else. This time I didn't notice much size or color change, but it may not have been stored as long. Once again the silicone went back to normal after being cleaned and separated from the tea.
I assume the essential oils in the tea must be evaporating and then the silicone must be attracting and absorbing them somehow. Does anyone know why this happens? Should I be worried that it is damaging the silicone or causing it to leach?