What causes jars to break?
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 steel wool 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 breaks:
- Avoid the oven method for processing home-canned food.
- Provide adequate head space 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 at room temperature.
The impact break originates at the point of impact and fissures radiate from the point of contact. 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 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 very old jars. Jars have a life expectancy of about 10 years.
Other causes of jar breakage include:
- Using jars with cracks.
- Putting jars directly on the bottom of the canner instead of on a rack.
- Putting screw bands on so tightly that air cannot escape during processing. As a result, pressure builds up inside of jars causing them to break.
Note: Although most commercial (one-trip) pint and quart size mayonnaise jars can be used to can acid foods in a boiling-water canner, expect more jar breakage and seal failures. Don't use commercial jars in pressure canners--excessive breakage is a problem.
Barbara Willenberg, Nutritional Sciences, University of Missouri-Columbia
Last update: Tuesday, May 05, 2009