Preserving mushrooms requires almost no special equipment, although for large-scale work it's best to build or buy dehydrators. Nutrients and flavors decrease if mushrooms dry in temperatures over 140 degrees. Drying mushrooms at room temperature preserves the mushrooms but does not kill insect eggs--maggots could hatch during storage. Not all mushrooms dry with equally good results, but some, including boletes and morels, may even improve in flavor when dried.
Inspect the mushrooms to determine freshness. Slice the mushrooms through the center of the cap and stem, discarding any with discolored flesh or insect damage. Rinse the mushrooms clean.
Line a cookie sheet with baking parchment and place the sheet on the upper rack of the oven. Set an oven thermometer on the center of the sheet. Leave the oven door ajar and set the oven to its lowest baking temperature. Adjust as necessary to keep the temperature on the cookie sheet between 130 degrees F and 140 degrees F. Don't depend on the oven thermostat for an accurate temperature reading.
Slice large mushrooms into pieces 1/4-inch thick. Cut small mushrooms--with fleshy caps no thicker than 1/4 inch--in half through the stem. Arrange the mushrooms in a single layer on the cookie sheet. Replace the cookie sheet on the top shelf of the oven.
Turn the cookie sheet 180 degrees after 1 1/2 hours. Turn again after three hours, and turn over the mushroom slices and pieces to ensure even drying. Rotate the sheet again after 4 1/2 hours.
Check the mushrooms for moisture after six hours of drying. When warm, mushroom slices should bend slightly without breaking. At room temperature, properly dried mushrooms should snap without bending. Rotate and shift the mushrooms if more drying is needed, and put the sheet back in the oven. Check again every hour until the mushrooms test dry.
Cool the mushrooms to room temperature before storing the dried mushrooms in air-tight glass jars. Place a packet of silica gel in each jar to absorb any remaining moisture. Store the jars in a dark place--light will degrade the quality of the food.