Moisture is one of the largest threats to long term storage, and over the years a variety of commercial products have been created to absorb moisture in gun safes and other storage areas.
I came across a unique DIY Sheetrock desiccant discovered during WWII as the Department of Energy at Oak Ridge experimented with devices to help with civil defense.
An Engineer by the name of Cresson Kearny designed a homemade device used to measure radioactive fallout. This device used materials easily scavenged, and it included a homemade drying agent made by heating common gypsum wallboard (Sheetrock).
Do NOT use calcium chloride, use gypsum Sheetrock only.
Obtain a piece of 3/8 inch thick gypsum wallboard approximately 12 inches by 6 inches
Cut off the paper and glue, (easiest done by wetting the paper)
Break the white gypsum into small pieces no larger than ½ inch
Heat the gypsum in an oven at its highest temperature (which should be above 400 degrees F) for one hour.
Heat the gypsum no more than two pieces deep in a pan.
Alternatively you can heat the pieces over a fire for 20 minutes or more in a pan or can heated to a dull red.
Anhydrite absorbs water from the air very rapidly, so quickly store in an airtight container while it is still hot. A Mason jar is ideal.
This works great for me as I have hundreds of pounds of Sheetrock desiccant available as I am still tearing down the trailer.