Amber Boston Round Bottles protect contents from UV rays and are ideal for light sensitive products Attached black phenolic (thermoset) caps with Polyseal cone liners form an exceptionally tight seal and offer a good chemical barrier. This liner molds itself around the sealing areas of the finish as the cap is screwed on, virtually eliminating leakage, evaporation, contamination, binding and backing off. Great for everything from liquids, to soaps, to macro-invertebrates.