Canned food stored in a warm place near hot pipes, a range, a furnace or in direct sunlight may lose some of its eating quality in a few weeks or months, depending on the temperature. Dampness may corrode cans or metals lids and cause leakage so the food will spoil. Jars do not need to be sterilized before canning if they will be filled with food and processed in a boiling water bath canner for 10 minutes or more, or if they will be processed in a pressure canner. Jars that will be processed in a boiling water bath canner for less than 10 minutes, once filled, need to be sterilized first by boiling them in hot water for 10 minutes before filling them. This can be dangerous because the temperature will vary according to the accuracy of oven regulators and circulation of heat.
Dry heat is very slow in penetrating into jars of food.
Also, jars explode easily in the oven. Yes, two layers can be processed at once, in either the boiling water bath or pressure canner. Make certain that the water covers the tops of all jars by one inch in a boiling water bath canner. The pressure canner should have two to three inches of water in the bottom. Yes, it is very important to allow steam to escape for 10 minutes before closing the valve or placing the weight on the vent.