The practice of transferring paintings first appeared in Europe in the second part of the 18th century. The most vulnerable part of any painting is the base it is painted on: a wooden board or canvas touches a wall that can sometimes be moist; as a result this part of a painting is prone to damages more than any other. Wooden-based paintings in southern countries are especially vulnerable to woodworms. Boards of Italian paintings are often so affected by woodworms that their structures resemble a sponge. Art conservators from past centuries did not have modern milder ways of consolidation and often invented more radical methods of conservation such as transferring the paint layer to a new base.
In Europe such methods were rare and mostly experimental and used for large wooden-based paintings. This complicated technique came to Russia in the middle of the 18th century when European conservators were invited to the Hermitage, but in the 19th century it spread to an enormous extend. Many treasured paintings from the Hermitage were transferred shortly after being acquired. The main motivation for transferring was the humid Saint Petersburg climate unconductive for preservation of old fragile wooden boards. 
The technique of transferring was mastered and bettered by the Hermitage conservator A. Mitrokhin in the beginning of the 19th century. The process in brief was the following: the face of a painting would be covered with several sheets of paper or canvas to lock the paint layer and then glued to a stretched canvas. After that it was time for the most dangerous part: the wooden board was filed down to the base layer and sometimes even to the paint layer. In the 19th century the face of the painting was often painted over with Chinese cinnabar mixed on glue.  Fine cinnabar easily entered the crackles and served as a kind of 'detector' during the aligning of the original base layer helping to prevent possible damage. Afterwards the paint layer was consolidated with a thin mesh cloth stretched on an underframe that was glued directly to the back of the original base layer (if it remained intact). After that the mesh cloth was covered with conservation ground based on oil paint. Oil bases were considered the best options for conservation, as they kept their elasticity for much longer time than glue or emulsion-based solutions which would become very fragile soon after drying. The surface of the conservation ground was polished and the painting was glued to a new firm canvas and stretch on an underframe. In the 19th century conservators used white lead for the ground, and starting 1869 they started using zinc oxide with gypsum, chalk, or pigments depending on the conservation objectives.
Interpretation of the x-rays was a considerable challenge. If the conservation ground contains zinc oxide an x-ray image doesn't provide any information about the original painting. Such image could not be used as a comparative material for attribution. Zinc oxide diffracts x-rays thus not allowing to see the paint layer itself, which is the main interest for researchers. The main information provided by such images concerns the transferring itself, density of the mesh cloth, or how many pieces the board consists of. Quite rarely we can see hints of the image (at the areas with the most amount of white). However, x-rays are still useful for conservators, as it shows areas of paint loss.
In 2013-2014 an Italian masterpiece of one of the most talented students of Leonardo da Vinci Giovanni Boltraffio Portrait of a Boy as Saint Sebastian went through an extremely complex conservation treatment at the Applied Arts Conservation Workshop.  Thick layers of dark varnish and later inpaintings heavily distorted the original image. A part of counts Stroganoffs' collection, the painting was believed to be a work of Leonardo himself. The painting was transferred from a board to a canvas at the Hermitage in 1860 by F.Tabuntsov, an expert in this field. On the back of the canvas there is a writing: «Переложилъ съ дерева на холстъ. С.Петербургъ. Реставрароръ Ф. Табунцовъ 1860» ("Transferred from wood to canvas. S.Petersburg. Conservator F.Tabuntsov 1960").
The x-ray images barely show any traces of the original painting: the left eye, nose and a wrist, the areas with the largest amount of white lead. The areas of paint loss are clearly visible as irregularly shaped light spots. Physical and chemical examinations confirmed the usage of white lead with chalk as conservation ground for transferring. Side light photography showed diagonal cracks in paint layer which are visible to a naked eye. On an x-ray image these cracks are shown as light lines. Such lines can often be seen on x-rays of transferred paintings. They most likely appear due to some instrument used to flatten the ground on the back of the painting (perhaps, these are traces of a flat knife or a spatula filled with white lead-based ground). Based on the Pushkin State Museum's experience in examining such paintings these lines often appear in paintings conserved in 1840-1860s.
F.Tabuntsov who was a student of A.Mitrokhin stood in the very beginning of the Hermitage's tradition of transferring paintings. His followers Nikolai, Alexandre and Michail Sidorov perfected the technique. Hermitage conservators of the second part of the 19th century gradually moved from using white lead as a base for conservation ground opting instead for zinc-based combinations. As a result, x-rays of paintings transferred at that time are much more informative as white lead doesn't diffract x-rays quite as much as lead.  It should be noted that in case of the Portrait of a Boy the original ground containing a thin layer of white lead with additional red lead and black pigment remained intact, as was found in the process of conservation. It was visible at the edges where the inpainings were removed, as well as at the areas of paint loss. 
Botticelli's diptych Annunciation with Archangel Gabriel and Virgin Mary were also transferred from wooden boards to canvases. The paintings were bought in Italy by Stroganoff for his collection. In 1922 they were given to Hermitage and in 1928 they got to the Pushkin State Museum. There are no writings on the back side of the canvas. The 19th century conservators always marked transferred paintings, however in later times this practice was omitted in favour of conservation protocols where all the treatments were documented. Unlike in case of Portrait of a Young Boy, in this case the x-rays helped to examine the paint layers in detail and see the original changes made during the author's work on the paintings, which is very important for art historians.
The x-rays show that Botticelli changed the position of the Archangel's legs who initially was standing on the floor, but then the artist painted him floating thus giving his figure more lightness. His clothes were also changed: they used to be more flowing and loose, with circled lines on his light cloak.
On the other hand, Mary's image was almost intact. In both paintings the line of the marble baluster on the background was slightly lifted. The x-rays show lines made with drawing instruments which was typical for Renaissance masters.
Without a doubt in the process of transferring such important for researchers elements as the original base and ground would be lost. A transferred painting loses its wholesomeness. As shown by experience of x-ray research of transferred paintings, even though the results provide challenges for interpretation, they still bear information that can be useful for both art historians and conservators.
 More on the history of painting transferring see А. Б. Алешин. Реставрация станковой масляной живописи в России. Развитие принципов и методов. Л. 1989. С. 23-98.
 Chinese cinnabar (HgS) – is a type of artificial cinnabar made by melting sulfur and mercury in a clay pot. Evidently its first use was recorded in China. In Arabic East it became known in 8th-9th centuries, in Western Europe in 15th century. Was mentioned in Cennino Cennini's Il Libro dell' Arte.
 Conservator N.Kolesnikov
 More on transferring ground see Никогосян М. Н. Экспертиза картин, переведенных на новое основание: Особенности техники перевода в России XIX века. Экспертиза и атрибуция произведений изобразительного искусства: материалы XIV научной конференции. ГТГ. М.,2011. С. 213-219.// Artemieva I.S., Kalinina K.V., Bonaduce I., Thecnological Examination of the Composition of the Transferred Grounds of Italian Paintings (XV-XVI centuties) from the State Hermitage Museum Collection // ICOM CC Scietific Research Working Group Interim Meeting, Pisa, October 2010 : book of abstracts. P.47.// Рогир ванн дер Вейден Св. Лука рисующий мадонну. К завершению реставрации. Исследование материалов и техники живописи картины. Калинина К. Б. С. 47-63. С.-Пб. 2016.
 Similar grounds were used by Leonardo da Vinci himself as well as his followers. Such ground was found while examining La Belle Ferroniere in the Louvre.