Home Canning: Common Causes Why Your Lids Won't Seal

I use the two-piece, self-sealing lid system when I home can. I use metal canning jar lids with rubber seals, and metal, threaded bands to hold the lids in place. The sweetest sound you can hear when you're canning fruits and vegetables is the pinging of the metal lids. When the processing is over, I remove the jars from the canner and set them on towels spread out on my counters. I then cover them with additional towels. The towels help hold the heat in, and keep the drafts off the hot glass. Then, as the jars cool, I wait to hear the pings. Those noises tell me the metal lid on each canning jar has sealed. What causes the pings? As each jar cools down, a vacuum forms inside. It pulls the lid down tight against the glass and seals it in place. Sometimes a canned jar doesn't ping/seal, and that tells you something went wrong. There are common causes why your lids won't seal when you're home canning.

1. Improper Use of Canning Jar Lids
Read and follow the preparation instructions on the boxes of the metal lids. Different brands of lids require different preheating times. The rubber seals on the lids mold to the shapes of the jar rims so they seal. Once they've been used, they can't be reused.

2. Using Damaged Rings
The metal rings, which hold the lids in place during the canning process, can be used over and over again. However, rusty or bent rings can prevent your lids from sealing properly. Inspect used rings and replace them as needed.

3. Tightening the Metal Rings Too Much
When you screw the ring down onto the canning jar lid, tighten it only hand-tight, according to the University of California Cooperative Extension. If you overtighten the ring, the air can't escape. Don't re-tighten the rings after you have processed the jars, either.

4. Using Damaged Canning Jars
Another common cause why your lids won't seal is that you've used jars that have nicks or cracks in them, especially around the rim. Damaged jars don't allow air-tight seals to be formed during the home canning process.

5. Using Non-Standard Canning Jars
The best choice for home canning is to use conventional, Mason-type jars, according to the Michigan State University Extension. If you re-use mayonnaise or pickle jars, for example, you run the risk of your lids not sealing. (You also risk jars breaking.) Non-standard jars such as these don't always have thick enough rims or deep threads to ensure a good seal.

6. Messy Rims On Canning Jars
After I fill my canning jars, I use a clean, damp dish cloth to clean off each rim. This prevents any food that may have spilled onto the rims from interfering with the lids sealing. Even the smallest food particles can keep the lids from sealing properly.

Resources
http://www.food-skills-for-self-sufficiency.com/glass-canning-jars.html
http://ceplacer.ucdavis.edu/files/76.PDF
http://web1.msue.msu.edu/imp/mod01/01600497.html