During the cleaning of the now-dismembered San Pier Maggiore altarpiece, attributed to Jacopo di Cione and others, in the National Gallery, London, traces of discolored varnish were found on one of the panels. The principal components of the varnish were identified as linseed oil and a resin, probably of Cupressaceae origin. It is possible that these traces are from the first documented varnishing carried out in 1371. If Italian tempera paintings of this period were generally varnished, views held by restorers and art historians on the original surface appearance of such paintings should perhaps be reassessed. Italian documentary sources dating from the 13th to the 15th centuries were consulted for information on the varnishing of the paintings, to see if this is likely to have been an exceptional occurrence. To assess the effects of varnishing described in a number of the sources, varnishes were prepared using two 15th-century Italian recipes and applied to test panels and effects on color saturation and gloss were noted.