Mühlethaler, B. and Thissen, J., 'Smalt', ‘Studies in Conservation’, 14, 2 (1969), pp.53 - 7 provides a list of occurrences of smalt in easel paintings in chronological order within each school of painting.
Glovanoli, R. and Mühlethaler, B., 'Investigation of Discoloured Smalt', ‘Studies in Conservation’, 15, 1 (1970), pp.37–44.
The phenomenon of 'blanching' in pictures by Claude is not, of course, in any way confined to those in the National Gallery's collection, as was testified by a number of the paintings in the exhibition, 'The Art of Claude Lorrain', which took place at the Hay ward Gallery in 1969. Of these the most striking case was the large 'Landscape with Ascanius Shooting the Stag of Silvia', lent by the Ashmolean Museum, Oxford, which stood out by reason of its all-over pale bluish tonality. Close to, the surface had somewhat the effect of a pastel seen beneath a sheet of glass (though the picture was not, nor is it at present, exhibited under glass), and it was quite clearly seen that the 'blanching' of the paint surface is below and independent of the protective coating of varnish. Apart from the all-over blanched effect there also occurred in the deep blue drapery of foreground figures on the left the local greyish patches associated with 'ultramarine sickness'.