The fossil record of planet earth is a biological time capsule preserved geologically in stone. Except for discovering the location of the fossils no other remedy for their preservation seems necessary. Stone is their time capsule.
The pulp content of paper that is predominantly wood fiber becomes brittle very quickly. Exposure to air and sunlight especially can turn newsprint into dust. Likewise, the pine box is never much of a barrier to the hereafter that comes first from beneath the earth.
Mummification was ancient Egypts time capsule for the ka by ensuring that an intact body might reach the afterlife. Rag content in the manufacture of paper similarly preserves the pages of a book for many generations.
When treated so as to be able to shield one behind an ultra-violet proof screen, glass protects ones eyes, and the contents of picture frames where paper, cloth, wood, and pigment are encased. A two- way mirror might represent the briefest of time capsules whereby we are able to spy on the on-going present, unseen.
The material of choice historically for the time capsule as we know it in modern times. While the timeless vanity of gold might be kept in a vault to insure the value of paper money, ultimately even this elemental treasure ends up inside a sealed metal capsule along with other human artifacts that we have deemed most significant amongst the cultural and historic bric-a-brac that we send into the future, buried in the cornerstone of a building or beneath the earth with those of us whose mortal coil is sprung.