More than 30 different brands of home canning lids are sold. Do not use any home canning closure that has not been scientifically proven. Don't use any questionable closure or technique recommended in newspaper, magazines, books or write-in advertisements. Do not reuse flat metal lids in home canning. Gasket materials are designed to soften sufficiently to provide an air-tight seal and maintain a vacuum in the jar when in contact with the jar rim. Variations in the shape of jar tops may prevent an adequate seal when lids are reused. Be sure all closures are perfect. Don't use any with dents or rust because these prevent airtight seals. Wash all lids and bands. Metal lids with sealing compound may need to be boiled or held in boiling water for a few minutes; follow the manufacturer's directions.

When Should Lids With Screwbands Be Tightened On The Jars?

Before placing filled jars in the boiling waterbath canner or pressure-canner, tightly screw down screwbands and leave in this tightened position. Do not tighten screwbands after processing. Tightening after processing can result in breaking the seal.

Why Lids Do Not Seal
There are several reasons why lids do not form a seal on jars after you process them. The jar and lid have one purpose: to make a seal at the time of processing. This seal does not guarantee that the food will not spoil; it only keeps out contamination. Each manufacturer uses a different composition for the sealing compound around the edge of the lid. One common reason why lids fail to seal is because you may not have followed the directions on the package. Other failures may be due to: If a lid fails to seal on a jar, remove the lid and check the jar-sealing surface for tiny nicks. If necessary, change the jar, add a new, properly prepared lid, and reprocess within 24 hours using the same processing time. Headspace in unsealed jars may be adjusted to 1 1/2 inches and jars could be frozen instead of reprocessed. Food in single unsealed jars could be stored in the refrigerator and consumed within several days.
	Under or overtightening of the screw band. Jars with nicks, cracks or chips on the sealing edge. Reuse of single-use lids or jars. Incorrect amount of headspace or failure to remove air bubbles before placing lid on jar. Faulty or insufficient processing that does not destroy the organisms that cause spoilage. The lids may seal at first, but then unseal later as spoilage occurs. Failure to clean the rim of the filled jar before sealing. Uneven processing temperatures can allow food, grease, or seeds to lodge under lids when liquid leaks from the jar during processing. Careless handling of jars and lids before and after processing. Tightening or loosening bands immediately after processing. Never take for granted that all home canning lids and caps are alike and use exactly the same way. This is a common cause of sealing failure.

Reusing Lids and Bands

No, don't reuse old lids. While it may look like there's enough sealing compound present on last year's metal lids, once the compound has been indented, it is unlikely to seal again safely. Screw bands can be reused.

Several older kinds of lids are no longer recommended because their failure rate is high. Throw away zinc lids, old metal bands used with rubber rings, and glass lids, or alternatively, store dried foods in these jars. Outdated jars and lids can still be useful for nonperishables, but not for canning.
One piece lids are not intended by canning purposes.

The flat metal lid with sealing compound and metal screw band is the most popular type of cap for home-canned products. To use, center lid on the rim of the jar with the sealing compound next to the glass. Screw the band firmly to hold the lid in place. Do not loosen. During processing, air is exhausted from the jars so that as the jar cools, a vacuum pulls the lid tightly against the rim of the jar. When the jar is completely cooled and sealed, remove the screw band. The vacuum and the sealing compound will maintain the seal.

The screw bands may be reused as long as they are not bent out of shape, but the fiat lids are designed for one-time usage. Be sure all closures are perfect. Don't use any with dents or rust because these prevent airtight seals. Wash all lids and bands. Metal lids with sealing compound may need to be boiled or held in boiling water for a few minutes; follow the manufacturer's directions.

Do not reuse flat metal lids in home canning. Every brand of lid has a different sealing compound, follow package directions exactly when preparing the lids for canning. Gasket materials are designed to soften under heat and pressure to provide an air-tight seal and maintain a vacuum in the jar.

What Causes Lids To Buckle?

Put on screwbands firmly so that they are hand tight. If they are applied too tightly, the flat may buckle. The buildup of pressure inside jars causes lids to buckle. This is a result of putting screwbands on so tightly that air can hardly escape from the jars during processing. Buckling may cause tiny pinholes in the flat. For this reason, the food should not be stored on the shelf. Within 24 hours of processing, foods can be refrigerated and used within 1 to 2 days, or reprocessed.