These blue metal bottle caps with oxygen liners are made for glass beer bottles.
These are pry off caps not to be used with twist top bottles. A great way to mark different batches or different brews.
To form a good tight seal a capper is required.
Three Piece Airlock
Star San Sanitizer - 16 fl oz (473 ml)
Cascade Hop Pellets - 1 oz (28 g)
Citra Hop Pellets - 1 oz (28 g)