Keep empty tins and jars fresh by popping a sugar cube inside before sealing. This is an old antique-dealer’s trick and works on all manner of sealed containers in glass, ceramic and metal, including biscuit barrels and decanters. In the biscuit barrel, it also has the advantage of keeping your biscuits crisp.
Sugar is a biocide, so nothing can grow on it, and it absorbs moisture and prevents bacteria build-up. Make sure the object is dry before you add the sugar, but if the worst happens, you can just rinse out the liquified sugar with warm water.