Cork

Natural cork is inert plant tissue and the closest thing to a non-question on this site: its suberin-rich cell walls make it barely permeable to fluids, and there is nothing in it to migrate. The whole safety question is the binder. In agglomerated cork the matrix is conventionally polyurethane derived from petrochemical sources, and it is a thermoset resin, not a glue line you can ignore - which means a composite cork gasket is a cork-and-plastic composite and we treat it as one. Bio-based binders exist but are a research subject, not the default. The other thing people arrive asking about is cork taint. TCA (2,4,6-trichloroanisole) is a fungal metabolite formed when filamentous fungi methylate 2,4,6-trichlorophenol, and human noses detect it at roughly 1.4 to 4 nanograms per litre in wine, a few parts per trillion; between 1 and 5 percent of bottles are estimated to be affected. The literature treats it as an organoleptic and economic problem rather than a toxicological one - it ruins the smell of a wine long before any dose question arises. It is a curiosity here, not a reason to avoid cork.

Cork does not melt and has nothing to leach when warm - it is one of the better-behaved materials in a hot path. It is a genuine thermal insulator, and it is boiled for at least an hour during processing, so hot liquid contact is not an issue. What heat does do is dry it: repeated dishwasher cycles or a hot dry cabinet drive moisture out of a cork seal and it loses the springback that makes it seal. Agglomerated cork adds its binder's limits to cork's own.

Natural cork shows continuous, irregular bark structure - faint growth striations, occasional lenticel channels running through it, and no visible particle boundaries. Agglomerated cork is unmistakably granular: you can see discrete chips of cork with binder between them, and cheap versions look like coarse breakfast cereal pressed into a cylinder. Microagglomerated technical stoppers sit in between, with very fine uniform grain and a machined finish. For a lid seal or gasket the practical question to ask a brand is simply whether the cork is solid or agglomerated, and if agglomerated, what the binder is - a brand that says "cork" and stops has almost certainly specified a polyurethane-bound part. Do not read a smooth, dyed, fabric-backed "cork" sheet as solid cork; those are usually thin cork veneer laminated to a synthetic backing.

Cork deforms substantially under compression without fracturing and springs back, which is what makes it work as a seal in the first place, but that recovery is what wears out. A cork gasket is done when it stays flat instead of springing back, or when it starts shedding grains at the edges. Keep it from drying out and from long soaks; hand wash and air dry rather than running it through a dishwasher.

Natural cork is plant tissue and biodegrades. Cork recycling programmes exist and work, but they are one-way: recycled cork is never used for stoppers again, only for flooring, insulation, boards, and sports and industrial goods. Agglomerated cork is not the same disposal case - the polyurethane matrix does not biodegrade with the cork granules, so a composite part behaves like the filled thermoset it is.