What causes jars to break in a canner? Breakage can occur for several reasons: (1) using commercial food jars rather than jars manufactured for home canning, (2) using jars that have hairline cracks, (3) putting jars directly on bottom of canner instead of on a rack, (4) putting hot food in cold jars, or (5) putting jars of raw or unheated food directly into boiling water in the canner, rather than into hot water (sudden change in temperature; too wide a margin between temperature of filled jars