Thank you for this very detailed explanation, Min. I honestly appreciate it so much. I feel a bit stupid, firing the underglaze to vitrification temperature - I know that glazes don't take well on vitrified clay so I'm not sure why that completely went over my head. With regards to these pendants, they actually look really nice as they are without an overglaze, so for this batch I will leave them as is:
20190923_074045 by Rachel Brown, on Flickr
If I wanted to repeat this process to accomodate a transparent overglaze in the future, do you think the following would work:
1. Bisque fire casts to Cone 05
2. Paint on underglaze and fire to cone 015 to burn off impurities
3. Paint on overglaze and fire to cone 7 (will mature at 6) to create final, vitrified piece.
I don't think I could do liambesaw's suggestion of painting onto greenware, as the pieces are so fragile before bisque firing - it is very easy for example to chip off an ear on a pendant once dried.
With regards to crazing, the supplier has recommended this exact overglaze to pair with the slip I am using from the same supplier. It is designed specifically for porcelain. So hopefully it will be okay, so long as I fire to the correct temperatures.