Term Explanation
Acid Free Paper made without acid in the pulp. Having acidic ingredients in the paper causes it to degrade more quickly.
Acid Migration The transfer of acid from an acidic material to a less acidic paper. This occurs when neutral or alkaline papers are exposed to atmospheric pollutants or acidic papers. Acid can also migrate from adhesives, boards, protective tissues, acidic art supplies etc.
Archival A paper with long-standing qualities, it is acid free, lignin free and with good colour retention. The paper will be made with Cotton or Woodfree pulps, which are known for their long life, and Buffered to increase their alkalinity. Archival papers are also known as conservation grade (Woodfree papers), and museum grade (Cotton papers).
Caliper The thickness of paper.
Calcium Carbonate Is an alkaline chemical compound, used in papermaking to give the paper an alkaline pH and ‘Buffering’. Buffering helps the paper survive in acidic surroundings (for example atmospheric pollution in a city), helping it retain its archival properties.
Calendered 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 United Kingdom.
Chemical Pulp Lignin is removed from wood pulp by treating it with chemicals to separate out the cellulose fibres and dissolve the Lignin. Chemical pulp is archival (also called Woodfree).
Cockling Uneven expansion when water is applied to paper.
Cotton Cotton (Cotton Linters) offer the purest form of cellulose available for papermakers to make the finest archival papers. Cotton is a crop that is grown annually, predominately for the textile industry. Papermakers use cotton linters, which are a by-product of the textile industry. Cotton is often called ‘Rag’. This is an old term that originates from when papermakers recycled rags from old clothes to make paper. This source of cellulose is no longer suitable to papermakers, since most textiles contain synthetic fibres.
Cotton Linters The fibres that stick to the cottonseed after ginning. This is used as the raw material in 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’s made across the width of the paper machine.
Cylinder Mould Machine / Mould Made This type of paper machine makes paper on a revolving wire covered cylinder. This is almost a mechanised handmade process the fibres lay randomly across the sheet, making it much more dimensionally stable than traditional fourdrinier made papers. Cylinder Mould Machine’s are quite special and rare. Mould Made papers are known for their heavier weights, superb watermarks and superior surface stability.
Deckle Edges These are the beautiful ragged / frayed edges seen on handmade and Mould Made papers. On Mould Made papers, there are two natural deckle edges and two handmade torn edges.
Dimensional Stability 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.
Felts Woollen felts are used to create the beautiful surface textures on St Cuthberts Mill’s artist papers.
Felt Side The upper side of the sheet (also known as the Top Side on fourdrinier made papers). The Felt Side is usually considered to be the superior side to paint upon, and the surface texture is visually more pleasing to the eye. It is called the Felt Side because it’s in contact with woollen carrier felts, as opposed to the Mould Side, which is in contact with the mesh of the cylinder mould.
Fourdrinier Machine A papermaking machine that forms the paper in a continuous sheet on a wire belt. It was named after the Fourdrinier brothers who financed the first operational machine. Most paper is Fourdrinier Made (also called Machine Made).
Grain Direction / Machine Direction The direction the paper is travelling along the paper machine. With Fourdrinier made papers over 50% of the fibres align themselves with the their lengths parallel to this direction. Mould Made papers have more randomly distributed fibres, hence their greater stability.
Grams per Square Metre The weight of a single piece of paper measuring one square metre. This short hand is g/m² or gsm.
Imperial This is the name for an old paper size that is still used for watercolour paper. It is 560 x 760mm (22” x 30”).
Lignin A naturally occurring chemical compound that is found in wood that binds the fibres together. Paper made with pulp containing Lignin, with time, goes yellow and isn’t archival (a good example is newspaper). Archival grade pulps have the Lignin chemically removed, and are known as Woodfree.
Mechanical Pulp Pulp made by mechanically grinding wood. This type of pulp includes Lignin and other impurities. It is not archival, and is used for low-grade papers like newspaper.
Mould Made Mould made paper refers to paper made on a Cylinder Mould Machine. This is the closest you can get to a mechanised handmade 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.
NOT A term used in the UK to describe the surface texture of a watercolour paper. It means not Hot Pressed (HP). The term Cold Pressed, or CP, is used to describe the same surface texture. The surface has a medium Tooth.
Optical Brightening Agents (OBA) These are used to artificially enhance the colour of paper. OBAs absorb light in the ultraviolet and violet region and re-emit light in the blue region, making materials look less yellow by increasing the overall amount of blue light reflected. This blue tone causes an increased whitening’ effect. OBA’s are not archival, and the extra whiteness they achieve only lasts a short period of time. They are very commonly found in papers with a high whiteness like stationery and printing papers, and are also used in washing powder to give a ‘whitening’ effect to washed clothes.
pH Is a measure of acidity or alkalinity. Archival paper will be neutral to alkaline (usually in the pH range 7.5 to 9.5).
River Axe This river discharges from the cave mouth of the famous Wookey Hole caves. The water is very clean, having naturally filtered through the limestone of the Mendip Hills. It is perfect for alkaline papermaking.
Salle The traditional name for the finishing department of a paper mill. From the French word for room.
Sizing Internal Sizing – A chemical is added to paper to give it water resistance. This stops watercolour paint or ink from being absorbed into the sheet.

Surface Sizing – The application of a product (such as gelatine) to paper by submersing it in a bath (or tub) of liquid. It enhances the holdout of the sheet, or increases the surface strength. Gelatine surface sized paper are not vegan or vegetarian (also called ‘gelatine sizing’, ‘tub sizing’ or ‘external sizing’).
Stuff The papermaking name of the wet slushed pulp/cotton before it is formed into a sheet.
Tooth The texture of a sheet of paper is described as Tooth.
Waterleaf A paper with no sizing. It is very absorbent eg blotting paper.
Watermark 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. Watermarks are a famously used as a security measure on banknotes.
Woodfree This is the term for pulp (made from wood) with the Lignin removed. It is also called Chemical Pulp and is archival.