For paper to be suitable for use with watercolours it must have a certain degree of absorbability but must also have enough moisture resistance that its surface texture will hold up and the paper fibres will not pull free when exposed to wet watercolours. Such paper must also absorb paint evenly and, in instances when the surface has been dampened, the paints must be able to run into one another evenly and cleanly. The sizing of the paper is the main way that these attributes are regulated. Unsized paper (e.g. blotter paper), for example, is so absorbent that it can hardly even be used for painting and writing at all.
The higher the quality and the thicker the watercolour paper is, the wetter it can be worked with. But even the best watercolour paper will wrinkle and warp if only dampened on one side. The best approach, then, is to wet both sides of your paper with a sponge before you begin to paint on it.
In the following text you will find explanations for certain terms that are always coming up when watercolour and other types of artist paper are being discussed.
Alkaline buffered: In order to make a paper that is permanently acid-free and therefore aging resistant, calcium carbonate (chalk) is added at the sizing stage. Calcium carbonate neutralizes harmful acidic substances in the air and ensures that the paper will remain permanently alkaline.
Aging resistance: Paper and cardboard that are rated as highly resistant to aging conform to the highest service-life class defined under DIN 6738 as well as ISO 9706, which means that their essential attributes will stay intact for at least hundreds of years. The aging process, which can never really be completely stopped, is, however, slowed down by the use of wood-free raw materials like cotton fibres or alkaline macerated pulp with very low lignin content. In addition, aging resistance can be built in during the manufacturing process of paper: the sizing process must be done in a neutral way and calcium carbonate can be added as a buffer against acids.
Real handmade paper: What in English is called handmade paper is in German called Büttenpapier, which actually means paper made in a Bütten, which is a special kind of vat, a large container in which the paper pulp is prepared and then subsequently removed with a screen in the old-fashioned way!
For our purposes, handmade paper will include any paper that has been made in this basic way even if the actual removal of the pulp by the screen was powered by a machine the most important thing is that it is only in this way that the well-known deckle (uneven) edge is organic to the production of a sheet of paper. This characteristic deckle edge comes about through the thinning of the wet pulp on the edge of the skimming screen. Because the final size of a handmade paper is determined by the size of a frame on the screen itself, deckle edges will always be produced on all four sides. Authentic deckle edges cannot be made ex post facto or modified in any way.
In contrast to the old-fashioned process of skimming the pulp out of a vat, making paper with a modern fourdrinier machine consists of pouring or squirting a mixture of water and fibres (the slurry) onto a continuous web (screen). Fourdrinier paper is always produced on rollers in a continuous web and cut into sheets after production. That is the reason why authentic deckle edges cannot be made with this process. In addition, the fibres making up any paper made on a fourdrinier machine are almost all laid out in the running direction of the screen. On the other hand, in the case of paper made with a circular screen, the fibres are distributed evenly in both the length-wise and cross-wise directions much like the distribution seen in a real handmade paper. This, it should be noted, produces a higher degree of dimensional stability and dimensional accuracy in the resulting sheet of paper. These attributes are particularly important for watercolour paper where the painting process involves a great degree of wetness.
Rags: Rags (refers to non-lignified plant fibres from cotton or linen) are, because of their long-chained fibres, the best basic raw material for making paper. Paper made with (cotton) rags or with a percentage of rag content is more aging resistant and sturdy than paper made out of cellulose. In the field of watercolour painting, rag paper has proven itself to be very useful especially when the painting is particularly wet in application.
Stock sizing: Stock sizing means that the sizing agent is added to the stock (pulp mixture or slurry) in the vat before it is formed into sheets, which enables a complete and even sizing of all the fibres. When painting on such paper, the paint will permeate it.
Neutral sizing: In order to make a paper that is aging resistant and acid-free, the sizing agent must be alkaline in configuration; namely, the pH must be under 7.0. By adding calcium carbonate (chalk), the paper will remain alkaline for a long, long time and that is the prerequisite for attaining a high degree of aging resistance.
Surface sizing: In the case of surface sizing, the sizing agent is applied to the dried surface of the sheet of paper after the production process. Surface sizing allows a paint to permeate the paper less quickly and enables you to make more frequent corrections.
Circular screen machine: see: Real Handmade Paper above.
Acid-free paper: see: Neutral sizing above. A more comprehensive exposition on paper can be found under Information about PASSEPARTOUT + MUSEUM MOUNTING BOARD.