Glenn,
For just a few pieces, you might use gloss black spray paint. Authentic japanning starts with a mixture of asphaltum, turpentine, rosin, and lead oxide. The mixture is carefully boiled to get a pourable consistency. It is applied with a brush, allowed to dry, and finally "stoved" or baked in an oven. Your kitchen will smell like an asphalt plant during the baking process.
If you spend some time on the internet, you will find some recipes and stoving procedures.
Jeff