One of the reasons oven canning is popular is that it requires no special tools; however, as of 2015, oven canning is unsafe. The method leaves room for environmental differences and user error. Using a boiling water bath or a pressure canner are the only reliable methods.
Killing botulism requires heating the food in the canning jar to 240 degrees Fahrenheit. Even though the oven dial is set above this temperature, without pressure, water boils at approximately 212 degrees Fahrenheit so the internal temperature does not reach the safety zone. Oven canning of low acid foods allows the spores to continue growing.
Dry oven heat affects foods differently than the moist heat of a boiling water bath. With the water bath, the temperature of all the food in the jar stabilizes with boiling water, providing a safe canning method for high acid foods. In the oven, the outside of the jar reaches the designated temperature, but the coldest parts of the food never reach it.
The safety of oven canning goes beyond the quality of the food. Jar manufacturers warn that heating jars in an oven is likely to cause the glass to crack or the jars to explode due to uneven heating. The exploding glass can cause injury to people near the oven.