Steve,

Congratulations on a beautiful carved Head Stone. The shape is outstanding.

At my shop we use Extira for the concrete casting industry in my area. It's resistant to swelling while setting in wet concrete for a few hours. I've made lots of letters with it but it does have life. Over time it will begin to flake apart and become soft. Even when sealed up it will come apart. I'd guess your carving to last a few years depending on the climate.

The photo shown by Scott is exemplary.

Joe