Complex sentence: Others, especially in the underdrawing, can only be seen with modern methods such as X-rays and infra-red reflectograms and photographs, which are able to record photographically pigments, depending on their chemical composition, which remain covered by later paint layers.
Split into simpler sentences: Others, especially in the underdrawing, can only be seen with modern methods such as X-rays and infra-red reflectograms and photographs. These are able to record photographically some pigments, depending on their chemical composition, which remain covered by later paint layers.