Sericin may be in a fairly stable conformation when coating the silk fibers (acting as the "glue" in silk fiber networks). Such stability could arise from the forces between the sericin proteins (like hydrophobic interactions), and just treating with hot water may work but is probably a quite ineffective way to remove all sericin proteins from the fibers. Adding chemicals like various soaps or salts can reduce the interaction between the sericin proteins and yield a more effective denaturing/change of conformation of the sericin, which leads to a more effective removal. 

ATR-FTIR is applicable to most materials that can be analyzed with FTIR in transmission. There are a few studies for how the FTIR spectra depends on the sericin/silk fiber ratio: 

Production of silk sericin/silk fibroin blend nanofibers: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3212049/ 

Using FTIR spectroscopy to detect sericin on historic silk: http://link.springer.com/article/10.1007/s11426-010-0050-y