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Gently clean the exterior of the chaga with your hands or a soft-bristled brush. Make sure there are no insects or residual tree bark left on the chaga, or else it could develop mold. Larger pieces of chaga will take longer to dry out and could possibly develop mold. Use a knife to cut the chaga into pieces so they will dry out more efficiently.  The pieces do not need to be uniform in shape, but they should be relatively similar in size. You can cut the pieces thinner if you want them to dry more quickly. Try to use a grated tray so air can flow on all sides of the chaga pieces. Leave them on the windowsill for 3 days until they feel hard and dry.  Do not use the oven to dry out mushrooms. If you have access to a dehydrator, set the temperature to 120 °F (49 °C) or lower to dry the mushrooms. Dump all the pieces of the chaga into the jar and keep it in a cupboard or closed pantry. Chaga can be stored throughout the year.  Most times, 1 grapefruit-sized portion of chaga will be enough for a full year unless you use it multiple times a day. The chunks can be ground into a powder with a mortar and pestle after they have been dried.

Summary:
Remove any tree bark from your chaga. Break chunks of chaga into pieces 1 by 1 in (2.5 by 2.5 cm) or smaller in size. Place the pieces on a tray on a sunny windowsill to dry them. Store the pieces in an airtight glass jar in a dark place for up to 1 year.