Surfaces on which you can paint directly are called grounds. First, however, a firm base is required, such as paper, cardboard, pasteboard, artificial boards of various kinds, wood, or cloth. Masonry and stone, even glass and metal, can also be used. These bases are called "supports." With the exception of paper, they are unsuitable as direct painting surfaces. Some would affect the paint chemically and alter it, others are impermanent in color or darken in contact with binders, or do not give sufficient hold for the paint. These supports must be given a ground.
Grounds function mainly as isolators between the support and the paint; they can also act as a sort of underpainting, when they are tinted, for instance, and give the picture a foundation color. In all cases the ground must be appropriate to the type of painting technique used.
Paper is the simplest painting ground. It takes all glue and pastel colors without any priming, but it must be protected with size against the penetration of binders containing oil or wax. Good
quality paper, free of cellulose and as white as possible, is one of the most durable painting grounds if it is properly cared for. Its quality depends on the
materials and methods of its manufacture.
Paper as we know it now was preceded historically in China by sheets made from bamboo pulp and in Egypt by papyrus. At about the beginning of the second century A.D., the first paper production began in China from cloth rags, which are still today the basis of all the best varieties of paper. Europe did not produce paper until about 1200, in France. Until then the only known writing surface was parchment, which can also be used for painting. Parchment is the undressed, smoothed skin of sheep, goats, donkeys, or calves, with the hair removed.
Most paper manufactured today is no longer made from rags but from wood, which is reduced to the finest dust and mixed to a paste with water, glue, and fillers (kaolin). This mixture is finely sieved and laid on broad felt moving belts in the paper machine. The water is squeezed out and after pressing and smoothing, the thin sheet of paper is wound at the end of the machine into great rolls. Any wood content lessens the quality of paper, as can be seen particularly in newspaper, made entirely from wood pulp, which quickly yellows and goes brittle.
Today there are very few papers made entirely of rag. One example, however, is good, quality, handmade watercolor paper. It is made one sheet at a time and is rather thinner and irregular at the edges, which can be observed, together with the watermark, when fhe sheet is held against the light. Watermarks in the paper are not necessarily a guarantee of quality, however. They are made by raised patterns or lettering woven into the sieve. In these places the pul p is rendered thinner and more transparent; the same process can be used in machine-made paper.
A famous brand of paper, made especially for watercolor, is the English Whatman paper, made from pure linen rags. It is sold both in sheets and in painting blocks. Similar good papers are made in other countries as well.
Every paper is sized to prevent the ink or color from running; unsized papers react like blotting paper. The type of size used affects the quality of the paper; good papers have animal glue size, and inferior sorts are sized with resin. The material of the size can be tested by dropping ether on the paper; if it is resinous the ether leaves a brown mark around the edge of the drop.
Most good papers can be used on both sides. In doubtful cases the top of the usable side shows the watermark or stamp the right way around. The surface texture has nothing to do with the quality of paper; more important for painting is the thickness, for if it is too thin, paper does not take the paint well.
The strength of paper is measured by its weight per ream. Painting papers weigh between 72 and 140 lbs. per ream. 210 is a thick card. Still heavier weights produce pasteboard, which is hardly ever made today solely from pure, white rag pulp, and therefore must always be given a proper ground.