Page:The chemistry of paints and painting.djvu/60

26 large mural paintings have been executed either in Gambier Parry's medium or in the paraffin-copal medium. Some of these works have been painted directly on plastered walls, some on canvases which have been afterwards affixed by marouflage to the surfaces prepared to receive them. To the latter category belong nearly all the paintings in the Ambulatory of the Royal Exchange, London. Each of the compartments has been very carefully arranged with a view to secure dryness and freedom from soluble saline matter. In front of the wall itself has been fixed a slate slab slightly inclined forwards at the top and having a ventilated airspace behind it. Upon the slate the finished picture has been attached (or maroufle) by means of a thick paste of white-lead, oil, and copal-varnish, spread not only upon the slate, but simultaneously upon the back of the canvas. It may be affirmed that paintings so secured are free from all risk of injury from the back. In an atmosphere like that of London the surface of the painting must either be protected by glass or be periodically cleansed from deposits of dust, soot, tarry matters, and the other impurities which are described in Chapter XXV. of this handbook. Several fresh materials have been recently employed as painting-grounds. They are either patent or secret preparations, dependent in general for their solidification upon reactions between insoluble earthy and alkaline earthy matters, such as china-clay, asbestos, and compounds of lime and magnesia, with solutions of such salts as magnesium chloride, aluminium sulphate, and alum. There is sometimes a lack of tenacity, and always a lack of toughness in these mixtures, but some artists find them to possess precisely the texture and absorptive character they desire in grounds not only for tempera, but also for oil