The peculiarity of the 1 L canning jars resides in the mastic within the capsule, a thicker and more elastic mastic than normal mastics. It in fact has the characteristic of being permeable to gases (and thus air) impermeable to liquids (and therefore water). Thanks to this, during the boiling process, the air within the jar expands when heated and escapes through the pores of the mastic without water entering. This way, a vacuum is formed which is visible through the lowering of the central and flexible part of the capsule determining the characteristic metal snap.
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