All spices are sensitive to heat and moisture, which can cause the caking that you have described. To lessen the chances of this happening, we suggest the following:
• Store spices in a dry, cool cupboard rather than over the sink or range, as these are areas of high moisture.
• Keep your containers tightly sealed.
• Do not sprinkle spices or herbs directly into a steaming pot of food, since this will allow excessive moisture to enter the bottle.
It is normal, as spices age, for them lose their natural moisture content and harden. Often a sharp tap or loosening the contents with an object such as a chopstick, or the handle of a spoon can loosen them and make easier to use.
Onion and Garlic products are particularly susceptible to caking, as they tend to absorb moisture from the air.