What Is Underpainting? The Layer Beneath That Decides the Final Colour

The layer serves several purposes at once and the technical ones matter as much as the compositional. It resolves the arrangement of light and dark before colour complicates the decision, which lets a painter fix the structure of an image while changes are cheap. It supplies a mid-tone ground so that subsequent work can move in both directions rather than only darker, which is far more efficient than working from white. Technically it seals and tones the surface, reduces the absorbency of the ground and provides a key for later layers. And it remains visible through what follows, since oil paint is translucent to varying degrees and a colour applied thinly over a dark underlayer reads differently from the same colour over a light one. That optical effect is the reason the technique persisted, since the depth achieved by layering translucent paint over a structured underlayer cannot be matched by mixing the same colours and applying them directly.

Several named methods were used and each produces a distinct surface:

• •Grisaille, a full monochrome grey rendering of the image, finished as a painting in its own right before colour is glazed over it
• •Verdaccio, a green-grey underlayer used beneath flesh tones, which neutralises and cools the pinks applied over it
• •Imprimatura, a thin transparent tone over the whole ground, establishing a mid-value to work from
• •Dead colouring, a broad blocking in of approximate colours and values before refinement
• •Bistre and brown lay-ins, used for rapid tonal establishment with a single earth pigment
• •Coloured grounds, where the entire surface is toned before any drawing, common from the sixteenth century onward

These layers are a major source of information for conservators and historians, since they record decisions that the finished surface conceals. Infrared imaging penetrates upper paint layers and reveals underdrawing and monochrome underpainting, which shows where the composition changed, where figures were moved or removed and how carefully the work was planned. Those changes are informative about workshop practice, about attribution, since a copy generally lacks the false starts of an original, and about the artist's method. X-ray imaging reveals dense pigments in lower layers and has exposed entirely different paintings beneath finished ones, since panels and canvases were reused. Cross-sections taken from edges show the full layer sequence and establish the technique used. A substantial part of what is known about how old master paintings were made comes from looking beneath the surface rather than at it.

The optical reasoning behind the technique rewards being spelled out. Paint films are mixtures of pigment particles in a binder, and how much light passes through depends on the pigment, with some being effectively opaque and others transmitting a great deal. A translucent paint applied over a darker layer lets light pass through the film, reflect off the layer beneath and pass back out through the colour, which produces a depth of tone that no opaque mixture of the same pigments achieves, because the light has been coloured twice. That is what glazing means, and it explains why a red glaze over a dark underlayer looks like glowing depth while the same red mixed with black looks like mud. Working the other way, a lighter opaque colour dragged thinly over a darker one leaves the lower layer showing through the gaps, which produces a broken luminous surface. Both depend entirely on the layer underneath being there.

The technique declined as painting practice changed and has not disappeared. Working directly onto the canvas in a single session, applying opaque paint in its final state rather than building up layers, became widespread in the nineteenth century, encouraged by portable tubed paint, by painting outdoors and by an interest in immediacy over finish. That approach makes underpainting largely irrelevant, since opaque paint applied thickly conceals whatever is beneath it. The consequences were visible, since paintings made that way have a different optical character, flatter and more direct, which was the intention. Traditional layering persisted in academic training and in restoration practice, and has been revived deliberately by painters interested in the depth it produces. Contemporary practice includes both, and digital painting has reintroduced the logic of underpainting through layered working, where establishing values before colour is standard advice for the same reasons it always was.

A monochrome or toned layer fixes composition and tonal structure before colour complicates the decision, and it keeps affecting the surface because paint is translucent. Grisaille, verdaccio and toned grounds each change the colours applied over them. Infrared and X-ray imaging read these layers, revealing changed compositions, workshop practice and entirely different paintings underneath.