Here are two examples I actually did this evening. Both cleaned in denture cleaner.
I don't have before pictures, but can assure you they were crusty buggers. The #45 dial in the first picture has a few light hairlines and the #49 dial in the second picture has quite a few good hairlines in the upper half. Both came out very well after soaking for about ten minutes, brushing with a soft plastic brush, and then another soak for about 5 minutes... Then rinse and dry.
These are regular enamel dials. If your dials are glass enamel, the hairlines will not come out as well, but it will still greatly improve the look.
Possibly, your more accustomed to them being labeled hard or soft enamel. I realize in theory its a essentially glass, but American companies called soft enamel dials "glass enamel". Likely because it has more of a very shiney, glass like surface appearance.
Elgin Material Catalog dated may 1915.
I'm not so sure about 'hard' or 'soft, but the higher temperature enamels on the best quality English dials were usually known as 'Venetian', and the higher fusing temperatures necessary meant that they were enamelled on gold rather than copper. Divergence of language!