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A traditional painting is constructed, in order from bottom to top, of the support, size, ground, paint, and coating. Because all of these layers can affect the appearance of the painting, paintings require special care to keep them looking their best for the longest time possible.
Supports can be divided into two groups: rigid and flexible. Wooden panels are one of the earliest and most common rigid supports for paintings. A variety of trees have been used, mainly dependent upon availability. Other rigid painting supports such as laminated panels and metal have also been used.
Supports made of textiles are, compared to wood, flexible and lightweight. The fabric is usually linen or cotton. Traditionally, fabric supports are stretched over stretchers and attached at the edges with tacks or staples. The stretcher is fitted with keys (flat triangular wedges, traditionally wooden) in the corners to adjust the tension of the fabric and to prevent bulges and creases.
Untreated supports, flexible and rigid, are usually too absorbent to permit the uniform application of paint. Therefore, intermediate layers, called size and a ground, are first applied. Size is a thin solution, often a dilute glue, that is brushed directly onto the support. The solution permeates the support surface without coating it. It provides a good mechanical bond between the support and the subsequent coatings.
The ground provides a smooth surface for the paint and serves as a "sponge" to absorb excess binding media of the paint. Most supports are unevenly absorbent. It is difficult to predict the results of a paint film laid on an unevenly absorbent surface. A ground will ensure a particular kind of paint will perform with reasonable predictability. A ground is also a structural element, a key for the paint films - a toothy coating for the paint to grip - and a definite layer between the support and the paint.
The paint on top of the ground can be a very thin single layer or multiple layers. There are several types of paints: oil, acrylic, alkyd, water color, casein and tempera. Each of these are made up of natural or synthetic pigments suspended in a particular binder.
Natural pigments are those which are mineral, vegetable or animal in origin. Synthetic pigments are those made by processes of chemical synthesis from chemical elements or compounds. They may be inorganic (compounds containing metals) or organic (complex compounds of carbon).
A binder is an adhesive liquid that distinguishes one paint from another. The binder dries in a continuous layer and locks the coloring agent into it, more or less. The binder also has a visual effect on the pigment: it brings our the chromatic character of the pigment, giving it different optical quality than it had in its dry state. This is significant, for it makes an ultramarine blue in oil look different from the same ultramarine blue in, for example, water color.
On top of the paint, there is generally a coating. The coating, synthetic or natural resin, is used to provide saturation and to protect the paint underneath from dirt, abrasion and moisture.
Natural varnish resins are hard or semihard tree saps, insect secretions, or fossil deposits formed by decaying vegetation. The names of the resins may be derived from their places of origin or the port that ships them. As a coating, they provide an unmatched saturation of the pigments. Their main drawback is that they yellow considerably with age.
Synthetic resins are acrylic or ketone compounds dissolved in an appropriate organic solvent. They form a tough transparent film that may not yellow as rapidly with age.
The Lifetime of a Painting
From the moment a painting is made, it begins to age. Depending upon the quality, combination and nature of the materials which have gone into its construction as well as its environment, a painting may age well or not. A new painting in good condition will begin to deteriorate as a result of time alone. Cracks appear as the paint films dry and shrink.
As the painting continues to age, both the paint film and priming lose flexibility and become brittle. Expansion and contraction of the fabric support, due to absorption and release of moisture from the air, adds further strain which leads to crack formation. Cracks of a very similar nature can also be caused by mechanical means, such as pressure, large or small, on either the front or back of the painting.
Some pigments are naturally sensitive to ultraviolet light, acids or bases. Exposure of these sensitive pigments to any of these conditions will cause them to fade.
The lifetime of a painting depends a great deal upon its care and handling.
How to Keep Your Painting Looking Its Best
Beyond conscientious care and careful handling of your paintings, paintings conservators can aid in preserving your paintings for the greatest time. Paintings conservators are professionally trained individuals with years of experience. You should solicit the advice or services of a conservator any time that you have questions or concerns regarding a painting. They can help you with the general upkeep of your paintings (dust and grime removal), as well as handle any unfortunate mishaps (paintings that fall from their hooks, fire, flood, insect damage, etc.). Conservators are committed to the preservation of the original artwork and the artist's original intent.