Panels are often provided with an ancillary support on the reverse in order to attempt to restrain the panel from warping.Generally this has been added by a restorer or conservator but may have been applied in the first construction of the panel.The merits of a cloth support over a rigid one, that it is light and flexible and easily transported and prepared were recognised by the ancients, but it came into wide use from the 15th century when the most common material was linen.Prior to the introduction of mechanically produced close, tight weaves a hand loomed square weave of coarse single strands was used.This is not, however a rule and should not be relied upon, for example many 19th century European paintings are on mahogany panels.and the movement of oak for panels from the Baltic States to Holland is the subject of much research at the moment.Stretchers and strainers are generally made of wood (most commonly pine or ash) and usually with tongue and groove joins, mitred at the corners and bevelled away from the canvas toward the inside.
The way in which the wood members have been cut from the tree are of great importance for the stability of the panel.
Recently new methods of creating a more even tensioning have been developed using metal inserts in the wood which enlarge the joint evenly through each member of the stretcher.
Large paintings require the stretcher itself to be further supported. The canvas is traditionally attached to the stretcher by tacks (copper or iron)and more recently by metal staples.
They may be either a radial cut (most stable) or a tangential cut (prone to warping).
The various members of the panel are most commonly joined by a simple abutment of the planks using animal glue to adhere them, however various other methods of joining panels are used (tongue and groove, ziz-zag, overlapping half way) and these are often an indication of date and place of the panel’s construction.