Michigan State University Extension
Preserving Food Safely - 01600738
There are several types of breaks that occur. Each break looks different and has specific causes. If the reason for jar breakage can be determined, faulty procedures can be corrected so this problem does not occur.
Thermal shock is characterized by a crack running around the base of the lower part of the jar and sometimes extending up the side. To prevent thermal breakage:
-Avoid sudden temperature changes, such as putting a hot jar on a cool or wet surface or putting hot food or liquid in a room-temperature jar. Keep jars in hot water until filled.
-Use a rack in the canner.
-Avoid using metal knives or spatulas to remove air bubbles or steelwool pads to clean jars. They may damage the glass and make it susceptible to thermal shock.
The internal pressure break is characterized by the origin of the break on the side. It is in the form of a vertical crack which divides and forks into two fissures. To prevent pressure break:
-DO NOT USE the oven method for processing home- canned food.
-Provide adequate headspace in jars for food to expand when heated.
-Keep heat steady when processing.
-Avoid reducing canner pressure under running water or lifting the pressure control or petcock when pressure drops to zero at room temperature.
The impact break originates at the point of impact and fissures radiate form the point of origin. To prevent impact break:
-Handle jars carefully. Jars that have been dropped, hit or bumped in transit or at home are susceptible to breakage. Test new jars that may have been mis-handled 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 very old jars. Jars have a life expectancy of about 10 years.