If you are new to watercolour then it can seem that there is an overwhelming amount of terminology associated with the practice! This page contains some of the more common words that you may come across when you are choosing your watercolour paper, and also some terminology from the paper-making process for the real paper aficionados. Once you get to grips with these terms then selecting your watercolour paper will become second nature!

Acid Free

A paper is acid-free when it is made without the presence of acid in the pulp. Acidic content would mean that the paper would degrade quickly.

Acid Migration

The transfer of acidic material from an acidic sheet to a less acidic paper. This can happen when a paper is exposed to a pollutant or an acidic paper. Acid can also migrate from adhesives, boards, protective coverings or acidic art supplies.


A paper that is acid free, lignin free, is buffered to increase their alkalinity and that has good colour retention. These papers will be made from Cotton or Wood-free Pulp which are known for their long life. Archival grade papers are also known as conservation grade (wood-free papers) and museum grade (cotton papers).


The thickness of the paper

Calcium Carbonate

An alkaline compound used in papermaking to buffer the paper and give an alkaline pH. Buffering helps the paper to withstand acidic surroundings, including atmospheric pollutants, to help it retain its archival qualities.


Paper that has been smoothed between sets of rollers called calender stacks. This process is done at the dry end of the paper machine.

Cold Pressed

A medium surface texture. Also known as NOT in the UK.

Chemical Pulp

A wood pulp used to create archival Wood-free paper. Lignin is removed from the wood pulp by treating it with chemicals to separate out the cellulose fibres and dissolve the lignin.


An uneven expansion of paper when water is applied


Cotton Linters offers the purest form of cellulose available to make the finest archival papers. They are a by-product of the textile industry. It is often called 'Rag', which is an old term from when paper makers recycled rags from old clothes to make their paper. Traditional 'Rag' can no longer be used as modern textiles commonly contain synthetic fibres.

Cotton Linters

The raw material used for cotton papers.

Cross Direction

The opposite of the Grain Direction or Machine Direction. When paper is made, the cross direction is the dimension on the sheet that is made across the width of the paper machine.

Cylinder Mould Machine / Mould Made

The type of paper machine that makes paper on a revolving wire covered cylinder. This is Almost a mechanised handmade process as the fibres lay randomly across the sheet, making it much more dimensionally stable than traditional fourdrinier made papers. Cylinder mould machines are rare, and mould made papers are known for their heavier weights, superb watermarks and superior surface stability.

Deckle Edges

These are the ragged, frayed edges on handmade and mould-made papers, that are sometimes used decoratively by artists. Mould made papers have two natural deckle edges and two handmade torn edges.

Dimensional Stability

Is the ability of a paper to retain its original size and shape when exposed to changes in moisture content and relative humidity.

Embossed Mark

After sheeting a metal stamp with a unique design is pressed into the sheet to prove authenticity.


Woollen felts are used to create beautiful surface textures on watercolour paper.

Felt Side

The upper side of the sheet (also known as Top Side on fourdrinier made papers). The felt side is considered to be the superior side for painting as the texture is usually more aesthetically pleasing. It is called the felt side as it comes into contact with the woollen felts rather than the mesh of the cylinder mould.

Fourdrinier Machine

A papermaking machine that forms paper in a continuous sheet on a wire belt. It was named after the Fourdrinier brothers who financed the first operational paper machine. Most commercial paper is Fourdrinier made (also known as Machine Made).

Grain Direction / Machine Direction

The direction that the paper travels across the machine. With Fourdrinier made paper over 50% of fibres align themselves with their lengths parallel to this direction. Mould made papers have more randomly distributed fibres, resulting in increased stability.

Grams per Square Metre (gsm or g/m²)

The weight of a single sheet of paper measuring one square metre.


The name of an old paper size that is still used for watercolour paper. A full Imperial sheet will measure 30” x 22” (76cm x 56cm)


A naturally occurring chemical compound that is found in wood that binds the fibres together. Paper made with pulp containing lignin will yellow with time and so is not considered archival. Archival grade pulps have lignin removed an are known as Wood-free.

Mechanical Pulp

A pulp made by mechanically grinding wood. This type of pulp includes lignin and other impurities. It is not archival quality and is used for low grade papers like newspaper.

Mould Made

Mould made paper is made on a Cylinder Mould Machine and is the closest you can get to a handmade mechanised process.

Mould Side

The bottom side of the sheet. It has a slightly less random texture, as the wire mesh of the cylinder mould is just visible.


A term used in the UK to describe the texture of Cold Pressed paper. It means 'Not Hot Pressed' and refers to a paper with a moderate texture.

Optical Brightening Agents (OBA)

These are used to artificially enhance the colour of the paper. OBAs absorb light in the ultraviolet and violet region and re-emit light in the blue region, thus making materials look less yellow by increasing overall amount of blue light reflected. The blue tone causes an increased whitening effect. OBAs are not archival as the whiteness achieved will only last a short period of time. Commonly found in office stationery.


A measure of acidity or alkalinity. A neutral paper will measure between 7.5 to 9.5 on the pH scale. A lower measure will mean the paper is more acidic and a higher number will mean it is more alkaline.


The traditional name for the finishing department of a paper mill, from the French word for room.

Sizing – Internal

A chemical is added to the paper pulp to give it water resistance. This stops watercolour paint or ink from being absorbed into the sheet.

Sizing – Surface

The application of gelatine to paper by submerging in a water bath. It increases the surface strength of the paper. Gelatine sized papers are not vegetarian/vegan.


The papermaking name of the wet slushed pulp/cotton before it is formed into a sheet.


The texture of a sheet of paper described as Tooth.


A paper with no sizing that is very absorbent eg. Blotting paper.


An impression formed by a design sewn or soldered on to a mould or dandy. The watermark is visible when you hold the paper to the light. On mould-made papers the watermark is very clear, as the paper is made thinner to create the design.


This is the term for wood pulp with the lignin removed. It is also called chemical pulp and is archival.