cross-section from Fall of Gog and Magog

Sample analysis to identify artist materials

While examining cross-sections from the murals, conservators identified an unpigmented coating on several of the murals. Was this an original coating or was it applied in a later restoration? Why was it there and what was its composition? Should the conservators remove it during the cleaning?
Cross-section from Fall of Gog and Magog.The arrows point to the coating layer. The paint layer on top of the coating suggests that the coating was applied by Sargent. (2003 photo)
The Triumph of Religion