Thompson Enamel company sells a product called P-1 - Overglaze Black and P-3 Underglaze Black. They come both in an oil base or dry powder. The Underglaze Black is perfect for darkening your recesses then wiping away the excess. Let it dry thoroughly then sift or wet pack with an enamel over. I would not use Liver of Sulphur as it will probably react chemically against the enamel when fired in the kiln.
The P-1 Overglaze black can be painted on the surface of the enamel and fired.
Thanks so much for the advice I will order some and give it a try.
I was to experiment with some basse taille enameling. I intend to create the pattern using a rolling mill, and I I would like to bring out the pattern by darkening the recesses. I would normally use liver of sulpher to darken the piece and then sand off all of the high points, leaving the recesses portions darkened. However, I'm uncertain as to whether I can then enamel the piece. Does anyone have any experience with this?