X-radiograph
The X-radiograph records anything of dense material from the back of the panel to the front, which are shown as white or near white areas on the negative. Iron nails, white lead or vermilion paint register white because of their density. Vermilion, for example, is a mercuric sulfide pigment and highly resistant to the penetration of X-rays.

The Madonna and Child was pieced together from three planks of poplar, reinforced with wood dowels placed across the joins before the panels were glued. Linen strips were applied over the joins. The dowel channels are visible in the X-radiograph and the line drawing to the left.