Why Do Mason Jars Have Two Lids?

mason jars lids
5/5 - (1 vote)

Mason jars – these retro, pint-sized glass jars with iconic dome lids and screw bands evoke wholesome images of backyard fruit harvests and homemade jams. 

Their sturdy, lidded design has preserved countless batches of food for over a century. But why the two-part lid? Why not just a single screw cap like modern jars?

Key Takeaways

  • Mason jars were designed for canning food and have two-piece lids for safe and effective home canning.
  • The two-part lid allows air to escape during canning and creates a vacuum seal for long-term food storage.
  • The screw band can be reused, but flat lids with the sealing compound are not recommended for reuse in canning.
A row of mason jars filled with colorful pickled vegetables on a countertop

A Legacy of Preservation

Mason jars were originally designed for preserving and canning foods. In the late 1800s, John L. Mason patented a molded glass jar with a reusable metal screw-on lid. This created an airtight seal, allowing people to store food for long periods of time.

Before Mason jars, preserving foods required actively heating jars in a hot water bath to create a vacuum seal. Mason’s innovative threaded jar and lid allowed people to easily seal and store preserved foods at home. 

The airtight seal prevented air from reaching the jar’s contents, stopping the growth of mold and bacteria. This allowed fruits, vegetables, jams, pickles, and other foods to be preserved for months or years while maintaining safety and flavor.

Today, while Mason jars are no longer the only option for home canning, they remain popular due to their versatility, durability, and nostalgic American design. However, their original purpose was to let farmers, homesteaders, and families preserve the summer’s bounty to enjoy throughout the winter. The ability to store canned goods transformed home cooking and made once-seasonal foods available year-round.

Parts of a Mason Jar Lid

A close-up of several Mason jars with flat metal lids and screw bands in various colors

The two-piece lid on a Mason jar consists of a flat metal lid and a separate screw band.

The flat lid is made of thin rolled steel with a rubberized flange on the underside, which acts as the gasket, and a porcelain lining on top. This creates a tight seal when used with the screw band.

The separate screw band threads onto the jar over the flat lid. As it’s tightened down, the screw band compresses the flat lid’s rubberized flange to create an airtight seal. The screw band screws on evenly around the jar and compresses the flat lid down when closed.

Why Do Mason Jars Have Two-Piece Lids?

Mason jars have two lids for a specific reason: safe and effective home canning. During canning, food is filled into the jar, and the flat lid with the gasket is placed on top. When heated during water bath canning or pressure canning, the air inside the jar expands. 

The two-piece lid design allows this air to escape through the gap between the lid and the jar rim. Once the desired processing time is complete, the headspace cools and contracts, creating a vacuum seal. The rubber gasket on the flat lid forms an airtight seal with the jar rim, preserving the food inside.

A single lid wouldn’t allow this air to escape effectively. This could lead to dangerous situations like the jar bottom blowing out or the lid buckling due to pressure build-up. The two-part lid system in Mason jars prevents these risks and ensures safe home canning.

It’s worth noting that beyond food preservation, mason jars can be used for decorative purposes as well. 

Reusing Lids

While the screw band can be reused many times if it’s not rusted or bent, flat lids with the sealing compound are not recommended for reuse in canning. The sealing compound on these lids is designed for single use. 

During canning, the heat softens the compound to create an airtight seal. Reusing the lid can compromise the seal due to a deformed gasket or worn-out compound, potentially leading to food spoilage or even dangerous bacteria growth. It’s best to always use new lids for canning to ensure a safe and effective seal. 

Wrapping Up

The charming, vintage allure of Mason jars carries a story of innovation and practicality that has changed the way we preserve food at home.

The ingenuity of the two-part lid system, the durability of the glass, and the versatility of the jar itself are what make Mason jars a must-have in every household. It’s no wonder that Mason jars are still a staple in many kitchens today.

Whether you’re canning your own fruits and vegetables or simply looking for a way to store leftovers, these jars are sure to come in handy.



  • Dacia Flowers

    Dacia Flowers is a graduate of the University of Nevada, Las Vegas (UNLV) with a major in Cultural Anthropology. Since graduating in 2019, she has expanded her expertise and applied her anthropological background in the practical domain. As the Administration Manager at CMJJ Gourmet since August 2022, Dacia handles crucial day-to-day operations, ensuring the warehouse's optimal functionality, timely delivery of orders, and managing invoices. Her commitment to excellence and in-depth understanding of organizational dynamics stem not just from her academic prowess but also her personal passion. Beyond her professional responsibilities, Dacia has a keen interest in reading, loves to travel, and stays active through regular exercise.

    View all posts

Leave a Comment

Your email address will not be published. Required fields are marked *


Get the inside scoop on all new products, sales, and articles

-served up fresh in your inbox each week.

Scroll to Top