The 2 piece canning jar lids that the USDA recommends consist of a screw band and a flat lid. The flat lid has a sealing compound on the inside rim. I've also heard them called 'flats' or 'seals'. The screw band part is reusable but the flat lid is not. After use the sealing compound will become indented which might interfere with a new seal.
Take a flat lid that has not been used and one that has been used for canning and compare them. If you look at the inside of the lids where the sealing compound is, you'll notice that a used lid will show an indentation where it sealed to your jar. This indentation flattens the sealing compound and it may not seal the second time around.
There is a vacuum seal formed during the processing of your filled jars. The sealing compound you will find on the inner rim of canning lids sits against the jar and forms the all important seal.
The screw band fits over the lid and screws down to hold the flat lid in place. When placing your screw band you need to screw it down firmly but you do not need to be too forceful. Finger tight is fine.
As the food in the jar is being processed oxygen will be pushed out of the jar. As the food cools the lid will be sucked down and the rubber seal will form a tight seal protecting the food from any further contamination.
Where can I buy canning jar lids?
Canning jar lids can be found most anywhere canning supplies are sold. Your local Ace Hardware Stores are a great place to find canning supplies. If you use a lot of lids and want to buy them all at once, ask for a discount. I asked at our store and they gladly worked with me.
Amazon carries canning supplies as well.
Lids can be purchased with both the bands and the flats. Or if you already have screw bands reuse those and just purchase more flats.
And remember if you are placing orders online often you will get them cheaper per box if you buy several boxes at a time.
Use caution if you are buying lids second hand. The flats are not reusable.
Even if the seals you purchase are "new in the box" you need to inspect them. Make sure they are not bent and more importantly check the rubber sealing compound. It needs to be rubbery, soft and not dried out.
Personal story.... I purchased some seals at an auction. Now I LOVE auctions! My husband and I regularly scour through old boxes of "stuff". We enjoy it. We get some great deals at times.
This time was not one of those. I found a box full of jars and some boxes of seals. The seals were very old but they were in the original box and had not been opened or used. At first glance they looked like new. I thought I'd found a great deal.
However when I heated them before placing them on my jars I noticed that the sealing compound was peeling. Definitely not usable. The sealing compound was totally dried out. Very flat. No rubbery feeling to them at all. They were just too old.
If you like the idea of buying once and not ever again.... check out my review of Tattler canning jar lids
Using canning lids that have been simmered but not used?
Hi, sometimes I heat extra jars and canning lids just in case the recipe makes more than stated, Is it safe to use the lids after they have been simmered but not used in processing. Thanks Joanna
Yes it is safe to use canning lids that have only been simmered and not actually used on a jar.
There is only one reason you don't want to reuse standard canning jar lids. If you look closely at the sealing compound that you see in the rim of the lid you'll notice that after it is used it will be indented from sealing on the jar. This makes it difficult if not impossible to reseal.
Simmering the lid warms and softens the sealing compound but it only becomes indented after it seals to the lid itself. As long as that sealing compound is still intact, your canning lid can be used.
Is a jar with two lids stuck together safe?
I canned some dill pickles and discovered after the fact that two lids had stuck together on one of the jars. The top lid is sealed but I wonder about the lid underneath it. I did not want to disturb it as I thought that could break the seal. What do you think? Perhaps I won't know until I open the jar.
What I would do is test the seal just as if there were only one lid. Lift the jar off the counter by the edges of the lid. The seal should be able to support the weight of the jar. As long as it does I would think your seal is probably fine.
When you store that particular jar put it to the front so it gets used soon. Be sure and check for any other signs of spoilage. Bubbly product, funny smells. If there is any doubt just as with any jar be sure and throw it away.