Your grandmother sealed jam with wax or turned the jars upside down. Why not do that?

Paraffin, turning jars upside down, open-kettle sealing and oven canning all worked often enough that families used them for generations, but each one fails often enough to let mold in, and US canning guidance has dropped all of them. Paraffin shrinks and cracks over time; an inverted jar keeps more oxygen and a weaker seal; open-kettle jars get no heat after the lid goes on [1][2][3]. A short boiling water process with a two-piece lid replaces them all and costs only minutes. This page explains what each old method did, why it was retired, how British wax-disc and oven methods fit in, and how to bring a family recipe up to date.

The old methods at a glance

Paraffin: the method most people remember

Pouring a layer of melted paraffin over jelly in a glass was standard for much of the twentieth century. It looks like a perfect seal, and when it fails, the failure is hidden under the wax.

UGA's Elizabeth Andress lays out the problem: "Any pinholes, shrinkage or cracks in the wax paraffin allow airborne molds to contaminate and grow on the product," and leaks in the wax let product seep out, where it "will provide nutrients for molds to grow on the surface" [1]. Paraffin is "no longer considered an equally acceptable choice for any sweet spread, including jellies" [1]. Penn State puts it more plainly: paraffin or wax seals "are no longer considered safe," because the wax shrinks over time and temperature changes encourage mold and yeast [2].

Why that matters: USDA's advice for a moldy jar of jam or jelly is to discard the whole jar, not scrape the mold off [1]. UGA notes that patulin, a mycotoxin, has been found in a few tested jars of homemade apple jam and juice [1]. Mold under paraffin is exactly what this advice is about. More on mold is on mold and fermentation in jam.

There is no reason to add wax under a canning lid; anything left on the rim can keep the lid from sealing.

Turning jars upside down

Inversion means filling a jar with very hot jam, putting on the lid, and turning the jar over for a few minutes so the hot jam heats the lid and the headspace before the jar is turned upright to seal. It does produce a seal, which is why it is still common in recipes online.

UGA identifies three weaknesses [1]:

• It "works best with very hot product." Small delays in filling, lidding or turning the jar let the jam cool, and the method has no margin for that.
• "A larger amount of retained oxygen in the headspace may allow some mold growth if airborne molds contaminated the surface."
• "A weak seal may be more likely to fail during storage." Some people also find hot jam leaking out when the jar is turned over.

Commercial makers do use a version of this, called hot-fill-hold, with fill temperatures, hold times and closures set by a process authority. That is a licensed-kitchen method with its own controls; see hot-fill-hold and commercial filling. It does not make inversion a home method.

Open kettle, oven, dishwasher and microwave

Open-kettle canning is putting hot food into jars and sealing them "with no further heat treatment." Penn State says the heat may not destroy bacteria, so the food can spoil quickly or cause illness [3]. Inversion is a variant of the same idea.

Oven canning heats filled jars in a dry oven. Penn State calls it "extremely hazardous": heat moves into jars more slowly through air than through water or steam, and "the glass jars are not designed to withstand the intense dry heat and may shatter in the oven" [3]. Penn State's 2024 article on retired recipes adds that canning jars should not be used in dry oven heat at all because of thermal cracking [2].

Dishwasher processing has the same flaw from the other side: dishwasher water "is far below that required to kill harmful microorganisms" [3]. Oregon State's Extension answered a family who had processed strawberry jam this way with a clear no, adding that jars heated in a dishwasher can crack or shatter [4].

Microwave processing heats unevenly, and jars can explode in the oven or as you take them out [3].

British methods: oven-dried jars, wax discs and cellophane

If your recipe comes from a British book or a UK website, it probably tells you to sterilize jars in the oven, fill them to the brim with hot jam, lay a waxed disc on top and cover with cellophane or a twist-on cap. A classic British cookbook gives the method this way: wash the jars, dry them "in a cool oven at 140°C, 275°F, Gas Mark 1," fill them while hot "right to the top," lay a waxed paper disc waxed side down flat on the hot jam, and stretch dampened cellophane over the jar with a rubber band [5]. British retailers describe waxed discs as circles of waxed paper used "to help exclude air and prevent deterioration" [6].

This system depends on a hot fill and a nearly full jar. None of it has been tested to US home canning standards, and every element is one US guidance has moved away from: oven heat for jars [3], a closure that does not make a vacuum seal of the kind a two-piece lid does, and no heating of the closed jar.

To make a British recipe the US way:

1. Keep the recipe only if its proportions match a tested recipe for the same fruit, or replace it with one. See recipes you can trust.
2. Use canning jars with two-piece lids and 1/4 inch of headspace instead of filling to the brim.
3. Process in a boiling water or steam canner for the time the tested recipe gives at your elevation; see how long to process jam.
4. Leave out the disc, cellophane and oven step. If the jars must be sterile because the process is under 10 minutes, boil them; see do you need to sterilize jars.

If you would rather keep a British recipe exactly as written, keep the jars in the refrigerator and treat them as unprocessed jam.

What changed in the guidance, and why

None of these methods was banned in one stroke. They were dropped as laboratory research showed where they fail. Penn State describes the process as recipes being "withdrawn and updated" on the basis of new laboratory research, with So Easy to Preserve (first published 1984, now in its sixth edition) tracking the changes [2]. SDSU Extension's rule of thumb is that canning recipes older than 1994 may be unsafe [7].

For jam, the shift was from methods that rely on hot product alone to a short process of the closed jar. The water bath heats the lid, the headspace and the jar together, drives oxygen out of the headspace [4] and lets the lid form a vacuum seal as the jar cools. That closes off the two ways old jam spoiled: mold that got in at filling, and mold that got in later through a weak seal.

Bringing a family recipe up to date

A grandmother's recipe card is worth keeping, for the fruit she used, the flavor she wanted and the set she liked. What changes is the method around it. Compare the card against a tested recipe for the same fruit and pectin type, keep its flavorings where a tested recipe allows them, and use the tested recipe's proportions and processing. What you may and may not change without making a recipe unsafe is on what you can change in a tested recipe. The modern method, step by step, is on water bath canning jam.

References

1. University of Georgia Extension, Andress, Preserving Food: Processing Jams and Jellies, FDNS-E-37-1, revised June 2011. https://openscholar.uga.edu/record/23661/files/FDNS-E-37-1.pdf
2. Penn State Extension, Hassani, Why Some Old Canning Recipes Are No Longer Safe, updated September 13, 2024. https://extension.psu.edu/why-some-old-canning-recipes-are-no-longer-safe
3. Penn State Extension, Canners and Canning Methods that are Not Recommended, updated November 12, 2015 (copy hosted by UC ANR). https://oc4h.ucanr.edu/sites/default/files/2020-10/337831.pdf
4. Oregon State University Extension, Hyde, Safe canning practices for jam?, June 2013, reviewed 2024. https://extension.oregonstate.edu/ask-extension/featured/safe-canning-practices-jam
5. Josceline Dimbleby, The Cook's Companion, 1991, "Potting and Storing" (via ckbk). https://app.ckbk.com/reference/thec04086c15s001ss001e004
6. Waitrose, Cooking glossary: Waxed discs, undated, viewed October 2026. https://www.waitrose.com/content/waitrose/en/home/recipes/cooking_glossary/waxed_discs.html
7. South Dakota State University Extension, Kleine and Botzet, A Guide to Water Bath Canning (P-00255), 2022. https://extension.sdstate.edu/sites/default/files/2022-10/P-00255-v2.pdf