The "National Gallery Technical Bulletin" offers a record of research carried out at the National Gallery, London. Published annually, it covers all aspects of the conservation, technical study and care of the paintings in the collection. Drawing on the combined expertise of curators, conservators and scientists, it brings together information about artists' materials, working practices and techniques. This is the 23rd volume of the bulletin. It features: the cleaning and restoration of Fra Angelico's predella panels in the National Gallery; the technique of Garofalo's paintings in the National Gallery; two paintings by Charles-Francois Daubigny; the blackening of vermilion - a study of the process through detailed analysis of paint samples; the "MARC II" digital camera and the scanning initiative at the National Gallery; enhanced image processing of X-rays; and adhesives for the repair of early panel paintings - an evaluation of their physical properties.