Breakage can occur for several reasons: Using commercial food jars rather than jars manufactured for home canning Using jars that have hairline cracks Putting jars directly on bottom of canner instead of on a rack Putting hot foods in cold jars Putting jars of raw of unheated food directly into boiling water in the canner rather, than hot water (sudden change in temperature-too wide a margin between temperature of filled jars and water in canner before processing). Thermal shock is characterized by a crack running around the base of the lower part of the jar, sometimes extending up the side. To prevent thermal breakage: Avoid sudden temperature changes, such as putting hot food in a cold jar, putting a cold jar in hot water, or placing a hot jar on a cool or wet surface. Keep jars in hot water until filled. Use a rack in the canner. Avoid using metal knives or spatulas to remove air bubbles or steel wool pads to clean jars. Internal pressure break is characterized by the origin of the break on the side. It is in the form of a vertical crack that divides and forks into two fissures. To prevent pressure breaks: Provide adequate headspace in jars for food to expand when heated. Keep heat steady during processing. Avoid reducing canner pressure under running water or lifting the pressure control or petcock before pressure drops to zero. Impact breaks originate at the point of impact and fissures radiate from the point of contact.To prevent impact breaks: Handle jars carefully. Jars that have been dropped, hit, or bumped are susceptible to breakage. Test new jars that may have been mishandled (to see if they break) by immersing them in room-temperature water, bring to a boil, and boil 15 minutes. Avoid the use of metal tools to remove air bubbles. Avoid using old jars. Jars have a life expectancy of about 10 years.