See:
- Andrew Sherratt in: Neil Asher Silberman, Alexander A. Bauer eds. The Oxford Companion to Archaeology, Tom 1, OUP 2012, pp. 403-404
- Borisilav Jovanović, Aleksandar Petrović - http://www.paundurlic.com/e_rglava.htm
- https://en.wikipedia.org/wiki/Metallurgy_during_the_Copper_Age_in_Europe