When using a lot of water with thinner watercolour papers (less than 260 lb/ 356gsm in weight), it may cause the paper to cockle, warp or buckle as it dries. This happens because the application of liquid to one side of the sheet causes it to expand slightly more than the other dry side. As this is undesirable the common practice to stop this from happening is to stretch the paper before use.
Soaking and stretching watercolour paper
Using running water from a tap (another option is to use a container of water to evenly soak the paper), immerse the sheet of paper for around 30 seconds to 2 minutes, (Due to the fragile nature of paper take care that once it is wet not to touch the area you are intending to paint on, as finger marks are likely to show up on your final painting)
Place the soaked sheet on a clean, sturdy, waterproof board slightly larger than the paper you are stretching (MDF boards are popular but require sealing first). If you have a board smaller than your paper, obviously trim the paper to size before soaking it.
Use gummed tape, wet it (don't soak it) with a cloth or sponge and stick the paper down to your board placing the tape around all four edges of the paper, smoothing the paper out and removing air bubbles with a damp sponge as you go.
Once in place, blot away any excess water with a clean paper towel and leave it to dry at a slight angle.
Leave the paper to dry, typically overnight.
Once dry the paper will be stretched tightly on the board, so when you work on it paint and water will not cause the paper to move.
Once you have finished your painting, allow it to dry and remove by cutting the paper off the board, typically with a sharp knife & metal rule.
NB While soaking the paper ensure that you don't accidentally contaminate the paper with residue detergents found in areas like baths or sinks. These detergents attack the sizing causing the paper to become very absorbent and so un-usable.
Ways Paper is made
Sizing- Most paper is sized, done during its manufacture to reduce the paper's tendency when dry to absorb liquid, to allow inks and paints to remain and dry on the surface of the paper, rather than be absorbed into it.
Different manufacturers will apply sizing in different ways.
Made Hand Papers are made from textile fibres such as cotton, linen & hemp and is typically surface sized with gelatine, this results in an unique paper with variable thickness and surface finish. Example: Khadi
Most high quality artist papers are made on a cylinder mould machine, providing the appeal of hand-made paper but with a more uniform structure, some keeping details like deckle edges, giving a consistent quality and providing exceptional surface stability. Example Saunders Waterford, Arches Aquarelle, Fabriano Artistico.
Process used for lower priced papers it results in an extremely regular surface and will often be made of more wood fibres and less cotton. It will usually be less suited to being able to remove colour without damaging the surface so it can't be over-painted again.
Materials paper is made from
The two most common materials for Watercolour papers are Cotton and Cellulose Pulp (chemically created from wood).
Cotton is used in the highest quality papers offering very good stability and archival properties. Examples being Arches Aquarelle, Saunders Waterford and Fabriano.
Other papers like Bockingford are 100% wood pulp and offer a satisfactory paper at a more affordable price.
Other artist papers will contain a mixture of the two (e.g. 50% Cotton), providing a compromise between quality and economy.
There are three main surface textures to watercolour papers:
Smooth or Hot Pressed
also referred to as satine, satina, silk and HP. Has the smoothest surface, suitable for high detail work.
NOT (not one or the other) or Cold Pressed
also referred to as Fin, Fina and CP, has a slightly textured surface and is the most used as it is suitable for most types of work.
Rough also referred to as Torchon or Grossa, typically used for landscapes, seascapes etc where a heavily textured paper enhances the final piece of work.
Does paper have a side?
Because of the way it's made most papers will have a 'correct side' for example with mould-made papers the felt side is the one that touches the woollen felts first during manufacturer and the mould side is the side that touches the wire mesh around the cylinder called a mould.
Both sides of the paper can be used for painting, but the 'felt side' is often preferred, some papers will indicate the correct side by the use of the watermark being able to be read the right way around or even by the manufacturer or distributor using a barcode sticker on the 'wrong' side, if not then you need to look very carefully at the paper to try to see the lines left by the fine wire mesh of the mould.
Forms of watercolour paper
Paper comes in sheets, pads and blocks.
Pads are either square bound or spiral bound, typically along one short edge to create a landscape orientation.
Blocks are glued on all four sides removing the need to stretch the paper, and the next sheet in the block is exposed by running a blade or finger nail around the four sides.
Some papers due to their long production history will be only available in 'imperial sizes' (measured in inches) especially in sheet form and blocks which are sized in divisions of the original sheet size and while pads often follow this sizing many are also produced in 'A' sizes.
Weights of paper.
Papers are produced in a range of thicknesses denoted by its weight expressed in either pounds (lb) per ream (500 sheets) or grams per square metre (gsm)
The common weights are 90 lb/190gsm, 140l b/300 gsm, 260 lb/356 gsm, and 300 lb/640 gsm.
Heavier weight papers have the ability to absorb more water and allowing the surface to be scratched or re-worked without causing damage.
Acid Free and Paper shade
Almost without exception watercolour papers are acid free or pH neutral, meaning they will allow work to be stored for many years without deteriorating, often described as Archival Quality.
A lot of traditional papers are a natural slightly off-white shade that provides a natural warmth to paintings, while others will bleach the cotton or wood pulp to produce a 'high white' paper, for example available in the Saunders Waterford range. Bockingford is available in a range of coloured tints.