When we go to museums and admire all the beautiful painting collections, what we see is only the surface layer of the painting. But have you ever wondered what lies underneath? Let's explore the different layers of a painting and then have a look at which scientific methods we can use to obtain information about the different painting layers.
The layered structure of a painting
First, every painting needs to have a support, a base on which the painting is built. There are several types of materials that can be used for the support. These include softer materials like canvas, paper, and leather. But we can also use harder materials for support, and these can be wood, metal, or glass. Here we’ll focus more on paintings done on canvas.
On top of the support, there’s a layer of glue. There are two different uses of glue in the painting structure. On the one hand, glue can be used in the lining of painting where conservators used to add animal glue to a canvas and then add another canvas on top of it for consolidation of the canvas. Glues can be used to add the ground layer.
The ground layer is added for different purposes. For example, in oil paintings, it is added to protect the support from the oil diffusing into the support materials, to provide a consistent application of the paint layer, or even in creating certain artistic effects. The ground could have chalk or acrylic polymer emulsion as the basic component.
On top of the ground, the artist creates the sketch. Different carbon-rich materials can be used for creating the sketch. These include charcoal, graphite, and bone black.
The sketch is covered by Imprimitura. This is a thin preliminary glaze added to the painting to prime it before adding the paint layers.
The paint layers contain all those beautifully colored paints we admire when we observe the paintings. These paints consist of the powder pigment mixed with a binder of some sort. Depending on whether it’s an acrylic painting, an oil painting or watercolor artists use the binder that works best for their painting medium, and these can be oils, synthetic binders, or gum arabic.
Finally, a layer of varnish is applied on top of the painting. The varnish has different purposes. It can be used as a protective layer of the painting, shielding the paint layers from dirt and dust. A glossy varnish is also helpful in enhancing the colors of the painting.
It’s important to note that, while this is the general structure of a painting and all of the above are possible layers in a painting structure, that doesn’t mean that every painting consists of every single one of these elements. They can vary from painting to painting, depending on the artist’s technique and the medium and materials they used for their artistic expression.
Scientific methods for analysing painting layers
Different scientific methods can be used to access information on different layers in the painting.
X-rays can penetrate all the layers of a painting. Two non-invasive uses of X-rays in studying paintings are X-radiography and X-ray fluorescence (XRF). X-radiography can reveal the different elements of a painting from the canvas all the way to the top paint layer. XRF can reveal the elemental composition of the paint layers, and thus, reveal the types of pigments used by the artists.
The sketch layer can be accessed and beautifully imaged by infrared reflectography.
When it comes to the paint layers, there are many non-invasive scientific techniques that we can use. These include X-ray fluorescence, and infrared (IR) spectroscopy and Raman spectroscopy for studying the chemical composition. Mobile NMR can be used to study the stratigraphy of paintings and the aging of the paint layers, while the aging of the oil component and that of the varnish can be examined under ultraviolet light.