siliceous melt is related to the glass but also porcelain, which creates a solid metal protective coating smooth and glossy character. Temperature kiln firing is right at the interface of glass and porcelain. Although enamel brittle, it can no discoloration and gloss survive for hundreds of years.
One big advantage enamel baths, their resistance to aggressive chemicals and high hardness.
For enamel baths is perhaps a simple correction enamel if accidentally break.