Glass and ceramic urns are the styles most often sealed. You can place an empty plastic bag into the urn, then pour the remains into it and twist-tie the bag inside the urn. The lid can then rest on top and the remains will be secure. Alternatively, you can pour the remains directly into the urn and then seal the lid with silicone.
If you do choose to seal the urn, we recommend a simple clear silicone (for most urns). This results in an effective seal that can (should the need ever arise) be easily scraped or peeled off to allow access to the urn without breaking the memorial. We do not recommend more permanent types of sealants, such as glue or cement.