Since 2012 I have been actively researching and using niello in my work. Since then I have published several papers for both the Santa Fe Symposium as well as The Goldsmiths' Company Jewellery Materials Congress. These papers have covered the history of niello, its traditional composition, safe manufacturing and use, as well as development of a modern alternative to the traditional lead based recipes.
The papers linked below represent hundreds of hours of research and experimentation. They represent the best information and knowledge I have at this time. I hope the information and experience I have collected will be of use to you in using niello for your own projects.
I recommend reading all three papers to get the best understanding of how niello is best used and made. They all have important information to understand how niello was used historically, and why it works as it does. Both can be useful when troubleshooting potential issues.
2017 Santa Fe Symposium - Niello: An introduction to an ancient metal
Link to the Santa Fe Symposium site
2018 Santa Fe Symposium - Niello for the modern jeweller
Link to the Santa Fe Symposium site
2019 Goldsmiths' Company Jewellery Materials Congress - Niello 2.0