The B818 is a 4 oz cobalt blue PET plastic boston round bottle with 24-400 neck finish. The boston round bottle is distinctive in its classic shape. Characteristic of the boston round are its rounded shoulders and rounded base. Polyethylene terephthalate (PET) is marked with the resin identification code of 1. PET is of the polyester family and has high clarity, good chemical resistance, high impact resistance, and medium rigidity. PET is also light-weight and is used for products containing essential oils. Avoid using PET containers to store products that contain essential oils greater than 5% or fragrance oils greater than 15%. If your oil percentages exceed these respective levels, please consider using glass. This item has a 24-400 neck finish. The first number refers to the diameter (in mm), and the second refers to the closure depth, style and thread. This container can be paired with a closure with a matching finish. This container is cobalt blue, an electric and vivid blue. In the case of glass, cobalt blue is made by mixing cobalt oxide into the base glass. Plastics are colored by using pigments. This cobalt blue color provides some UV protection for light-sensitive products.