Page:The museum, (Jackson, Marget Talbot, 1917).djvu/141

Rh particles are lost through friction while the panel is being reduced. The next process is to reduce the panel by planing it down on the back. This process is a very delicate one as, unless it is very carefully done, when the wood has become thin the gesso on which the painting has been applied will crack and separate. The usual custom is to shave the wood down until there is hardly more than a paper thickness of wood at the back of the gesso. Upon this is applied the strong mastic and the canvas is laid down as in the case of re-backing a painting on canvas. When this has been done, there is always a little thick edge which may be seen when the painting is out of the frame which will indicate its history. Frequently, however, paintings on panel, especially in the northern countries like Germany, England and the United States, contract and expand with the changes in the weather. This often means that the face of the picture will crack open. When there is any danger of this, but the condition is not sufficiently serious to call for rebacking, a process known in English as cradling has been invented. This is called in other countries by the French name parquetting. In this process the wooden panel is somewhat thinned by planing on the back. It is then treated with a varnish which is supposed to render it impervious