Like most industries, the Paper Trade has its own curious vernacular that, on occasion, can mystify the uninitiated. The purpose of this glossary is to attempt to convert industry terms into plain English. If you should come across a term not included below, please let us know and we will endeavour to include it in the next update.
A sizes are the trimmed sizes as specified by the International Organisation for Standardisation (ISO). Using A0 (841x1189mm=1 square metre) as the benchmark, each subsequent size is obtained by dividing the long edge in half. E.g. A3 is 420x297mm so A4 must be 210x297mm, and so on. Using the A-series, two “untrimmed” series have been designated; RA and SRA. For the RA series an external perimeter is added to the A size, so as A2 is 420x594mm then RA2 is 430x610mm. The SRA series has a “Maltese cross” added within so that A4 once trimmed out of the parent sheet a “theoretical” cross will remain-this is so that the A4 can accommodate a bled image; SRA2 is 450x640mm (see link: Paper Sizes Chart)
This denotes the propensity to which the surface of a paper will absorb ink, varnish, adhesives and of course water/moisture. The lower the absorbency, the greater the ink lift etc.
This derives from the period where papers were generally SIZED (see below) with acidic compounds. Acidity within paper, limits its longevity and therefore certain publications necessarily required ACID FREE paper and special non-acidic substances were used for this purpose. However, in modern papermaking acidic compounds are rarely used for sizing and so this is far less of an issue. Nonetheless, long life papers are still required for specific applications and an ACID FREE specification originating from the Virginia State Library is widely used. This means that coatings and fillers (see below) are treated to ensure their PH value is non-acidic, and therefore the paper’s internal and surface PH’s must have values of 7, or greater.
This term is generally used nowadays to describe uncoated papers with a coarse woven surface.
Refers to a paper’s capability to remain ‘sound’ after decades in storage (See ACID FREE above).
Paper comprises cellulose fibre derived from commercially generated woodpulp. However, the woodpulp “mat” alone would not provide all the properties required of the eventual sheet of paper. Consequently, minerals are added during papermaking to enhance opacity, shade, evenness of look-through and to fill in the gaps between the bonded fibres. These minerals are described collectively as the ASH CONTENT, and the term derives from the % weight of the ash residue when a piece of paper is incinerated in a laboratory. The ash will be the non-cellulose matter.
The ISO introduced this series for where A sizes are too small. B sizes were envisaged for the poster market originally, but derivatives have leaked into general printing. The problem has been that the rules have not followed so more than one version of a B size often exists (e.g. B1 can be 700x1000mm, 710x1020mm and most commonly 720x1020mm).
Describes the process of printing the reverse of a sheet during a SEPARATE pass through a printing press (see also Perfecting below).
The generic term for paper destined for a further process prior to printing. E.g. base paper for coating, base paper for envelope production, base paper for self-adhesives, and so on.
This refers to the most commonly used method to meter the coating weight of material applied to a base paper. The “blade” literally scrapes off the excess coating to yield the weight/thickness required. The coating mix is a combination of china clay and/or chalk with binders, such as latex, added.
Woodpulp has to be cleaned of the impurities that affect the colour and longevity of the paper it is destined to comprise. Bleaching is the term used to describe the “chemical cooking” of the wood chips. Historically, chlorine gas was used but in recent years pulp producers have eliminated the use of elemental chlorine and employed safer chlorine derivatives or alternatives such as hydrogen peroxide.
In ‘fine’ papermaking, the term BOARD is usually used to describe uncoated paper of 200 microns and above, and coated paper of 200 gsm and above.
This is a collective term is used to describe the waste paper and rejected paper generated within a paper mill.
Technically, this term relates to how thick a paper is in relation to its weight. High bulking papers have a disproportionately higher thickness to grammage ratio than standard papers. The phrase BULK RATIO is in common use and is calculated by dividing the thickness (in microns) by the weight (in gsm). E.g. an 80gsm paper with a thickness of 106microns has a bulk ratio of “1.325:1”. See also; VOLUMETRIC.
CALENDERED PAPER & BOARD
Calendering is the term used to describe the process of smoothing and/or polishing the surfaces of the paper to create a smoother finish and, particularly in the case of coated papers, impart a gloss or silk/matt finish. These differences in finish depend upon the material used to create the surface of the rollers and the amount of pressure and heat used. Highly polished rollers under extreme pressure and heat will generate a glossy surface on the coated paper. Reducing the pressure and heat and using less polished surface materials for the rollers will generate a “matt” surface. The process employs a stack of cylindrical rollers which, when combined with pressure and heat, will smooth the surface of the reel of paper as it passes through. Smoothing in this way also reduces thickness and necessarily increases density. Although normally associated in the finishing of coated papers, calenders have been used in recent years to achieve the smooth finish necessary for uncoated papers destined for use as high-resolution substrates with colour ink-jet and laser printers. Calender stacks have therefore, enjoyed a new lease of life in making super smooth offset and laser/ink-jet papers possible. Small calender stacks can form a part of the paper machine and are normally located after the second section of drying cylinders. More usually though, a larger version is used as an “off machine” process (normally referred to as supercalendering).
The term used to describe the thickness of a sheet in microns.
Self-copying paper whereby the image is generated by placing a sheet with a “coated reverse side” (sheet A) on top of a sheet with a “coated upper side” (sheet B) and applying pressure (handwriting or impact printing). The pressure causes the colourless dye capsules in coating “A” to burst and the contents immediately react with the colour formers in coating “B” to form a permanent image.
A particular coating technique that yields a very high gloss surface. The coating mix is metered onto the base paper and when still wet is “cast” against a large super-heated highly glossed cylinder. The coating dries and “mirrors” the surface of the cylinder.
A mineral commonly used as a “filler” additive during papermaking. It acts to enhance opacity and shade and also fills-in any small holes which can appear between the cellulose fibres as they bond during the forming and drying processes of papermaking.
See FIBRE below.
A high quality bond paper, which has a chemically treated surface which when tampered with using a solvent or bleach causes a permanent stain to materialise.
Originally the most common form of “filler” additive during papermaking. Although still widely used, it is increasingly being replaced by chalk and chalk derivatives. China clay is also a primary constituent of the mixes used to produce coated paper.
Not to be confused with the building material, this version of the term refers to a board made from waste paper for packaging applications. It is often lined with white paper and frequently coated to provide a good printing surface, hence WHITE LINED CHIPBOARD.
COATED PAPER & BOARD
This is the generic term to describe the application of a variety of materials to the surface of the paper. The most common form is the application of a clay-based slurry to create conventional coated paper for four-colour offset printing. There is little difference in the coating mixture used for gloss and silk/matt papers because the final surface properties are achieved through CALENDERING. The different surfaces depend upon the combination of the variables available to the papermaker during CALENDERING (see above). Coating mixtures typically contain: clay; chalk; latex binders.
Coating also encompasses the manufacture of: CARBONLESS PAPERS; THERMAL PAPERS; SELF ADHESIVE LAMINATES; CAST COATED PAPERS; GUMMED PAPERS; SILICON RELEASE PAPERS (for imitation leather fabrics, or as a backing sheet for self-adhesive papers) etc.
Coating can be carried out ‘on machine’ whereby the coating unit forms a part of the paper machine, or ‘off machine’ where the coating is a separate self-contained process. There are three primary coating methods: BLADE COATED (this is the most widely used-see above), air-knife and Meyer bar. The latter two are highly specialised: please contact DHM should you require further details.
This describes paper that has become warped, either due to exposure to an atmosphere of a significantly different temperature and humidity to that of the paper, or due to variations in the cross and/or machine direction temperature profiles of the drying cylinders in the paper machine, or even a process fault when the paper is forming on the wire at the wet end of the paper machine. The latter two cases are obviously manufacturing faults, and are particularly rare. The most common cause is exposure of the paper to an environment with which it does cannot achieve equilibrium-the paper will either dry out, or take up moisture in an effort to achieve equilibrium and will distort (cockle) in the process.
The centre core on which a reel of paper is wound. It is usually made of strawboard.
CRACKING (on the fold)
Usually a problem associated with folding coated boards where the outer seam created by the fold ‘cracks’ or splits. This could be due to a fault in the board but more often is associated with the use of the incorrect matrix and/or rule size. (See link: Cutting and Creasing).
Creases in the sheet have two primary causes. In unprinted paper the problem invariably occurred at the mill. In these circumstances inspection of the stack should reveal creased sheets (say) every third/fourth/fifth/sixth sheet checked. This frequency relates to how many reels were cut on the sheeter simultaneously (i.e. three reels up, four reels up etc.). Creases that occur DURING printing are usually due to the sheet having either wavy or tight edges caused when the paper’s edges have been exposed to an environment where the relative humidity is higher than the paper’s (wavy edges) or dryer than the paper’s (tight edges). To avoid this always wrap paper in moisture proof material between processes. (See link: Wavy or Tight Edges).
Describes the cross grain of a sheet which by definition must be at 90 degrees to the grain direction/paper machine direction.
Describes a sheet or stack of paper that does not lie flat. Curl can occur around the grain where the long edges of a long grain sheet tend towards each other, or across the grain where the short edges tend together. Providing the sheet is in equilibrium as regards relative humidity, curl across the grain is usually the result of too much tension in the reel of paper prior to sheeting. Whereas curl around the grain is normally associated with dimensional instability caused during the drying of the paper on the paper machine.
A roll covered in a phosphor bronze fabric located towards the end of the WET END of a paper machine to assist is creating the surface character of the TOP SIDE (as opposed to the WIRE SIDE) of the sheet. WATERMARKING (see below) is introduced by placing the reverse of the required image onto (or into) the surface of the dandy roll.
The general term used to describe paper fibre or paper coating that contaminates the printing blanket creating tiny spots in the non-image area of the printed sheet. Debris can originate from poorly cut edges (either from the mill sheeter or when the sheet is guillotined down to a smaller size) or from ruptures in the paper surface created when the inked impression is made (due to either a sub-standard surface or too high an ink tack).
The industry term describing the usable width of a paper machine. Good deckle means a high proportion of the paper machine’s width is employed to produce saleable paper. Bad deckle indicates poor machine width utilisation and invariably attracts a premium price to compensate for the inherent inefficiency.
Usually associated with boards. Single ply (single wire) boards can delaminate at the impression unit where either the internal strength of the board is inadequate or where too much pressure has been applied at the NIP (see below) in the unit. Duplex (i.e. a board made by laminating two sheets together) and twin wire boards are more prone to delamination because of the nature of their construction: the causes however are the same as for single ply boards.
A specially formulated ink which renders the CF surface impervious to the carbonless capsules in the CB coating
A biologically harmful compound sometimes found in the effluent of pulp and paper mills where elemental chlorine compounds have been employed to clean paper fibre. The widespread use of elemental chlorine free and totally chlorine free bleaching agents has eradicated the dioxin problem as regards pulp & paper producers.
The extent to which the size of the image dot on the originating medium enlarges when it is printed onto paper. As a rule of thumb the rougher the paper’s surface the larger will be the dot gain.
The processes to dry paper on machine are many and varied. The first stage is the use of a wire. This is a continuous nylon (formerly metal hence the term wire) mesh that allows water to drain from the slurry of paper fibre and minerals that are the constituents of the eventual finished sheet. The next stage is the press section where the web of damp paper is pressed into absorbent felts (see below). From here the hard and expensive work begins with banks of drying cylinders. These steel cylinders have highly polished surfaces and the hollow core is filled with super heated steam. The damp paper web is wound through and against these cylinders causing the paper to dry much in the same way as a damp shirt dries when ironed.
DUCT STABLE INKS
A common formulation of offset printing inks, which are chemically treated to resist oxidation thus allowing them to be left in the press, ducts overnight. Such inks by their nature rely on absorption into the surface of the paper to dry. It is best to avoid the use of such inks on hard surface papers such as ivory boards and super smooth text & cover papers because such surfaces have poor ink absorption properties.
Board produced as a result of laminating two lighter weight sheets together.
This term describes pulps & paper making processes employing Elemental Chlorine Free (ECF) compounds to clean and bleach the paper fibre during manufacture (see also TCF).
Poor electrostatic discharge (i.e. static) is most common cause of sheets of paper appearing to stick together at the press feed. This is most common in the winter months where the ambient humidity is relatively low and heaters are in operation in the pressroom. The heaters act to reduce the humidity even more, making the air around the paper exceptionally dry. All paper carries an electrostatic charge and normally this is ‘naturally’ discharged to the surrounding air. Moisture in the air is an essential component of this process so when the pressroom humidity is exceptionally low the electrostatic energy will not discharge easily. Discharging is even more difficult where the temperature of the paper is significantly below that of the pressroom. The best solution to avoid static is to introduce humidity control to the pressroom but where this is not possible, anti-static devices must be used once the paper is at room temperature.
The process used to impart a pattern into the surface of finished paper. A steel cylinder carrying the pattern required is located against either a cylinder carrying an identical pattern (for two sided embossing) or a plain cylinder (for single sided embossing). The two cylinders are then pushed together to form a nip through which the reel of paper or board to be embossed will be unwound.
Although by nature oil based inks and water repel each other, water can contaminate ink through the phenomenon of emulsification. This occurs when (just like oil & vinegar) an emulsion of water suspended in ink is formed and can occur when there is too great a film of water on the printing plate. Excess water can pass up to the ink ducts via the inking rollers and mix with the ink. This is commonly known as emulsification and it can considerably impair the printed result and often manifests itself as a mottle effect in the printed image.
See SELF-SEPARATING GLUE below
FELT & FELT MARKS
Paper machines use an array of several different techniques to remove water from the slurry of paper fibres and minerals used to make finished paper. One common drying method involves the use of specially designed absorbent fabrics (known in the trade as felts). These fabrics are made into a continuous loop and the web of very damp paper is then pressed against them in the section (known as the press section) immediately before drying cylinders are employed (see drying above). Because the paper web is so damp and pliable it will take on the pattern of the surface of these felts: this allows paper makers to introduce subtle patterns (felt marks) to the surface of the paper at this stage of the process.
FIBRE (Paper Fibre)
The primary constituent of modern paper is cellulose fibre. This is a natural organic fibre found in cotton, grasses and trees. Originally paper was made from recycled rags because cotton was the primary clothing fabric. The advent of man-made fibres coupled with the escalating cost of recycling rags, meant papermakers were forced to switch their attention to timber as the principal source of cellulose fibre. Large-scale cultivation of forests emerged in the early to mid 20th century. Nowadays not only does the replanting rate exceed the rate of felling by over two to one, but tree stocks in the Western hemisphere are significantly greater than they were a century ago. A position as necessary for ecological reasons as for economic ones. There are two groups of fibres. Long fibres that come from softwoods (e.g. pine) and are typically around 1.5mm in length and short fibres (about 0.5mm long) which come from hardwoods (e.g. eucalyptus). Mixing these two fibre types enables the papermaker to alter the properties of the eventual sheet e.g. Strength, tear, opacity, bulk (see above), folding properties etc.
Filler is the term used to describe the minerals mixed with cellulose fibre (see fibre above) to create the eventual sheet. Fillers are generally derivatives of clay and chalk and perform several functions. Firstly they fill in the gaps that would otherwise exist in a mat of dried cellulose fibres. Secondly they assist in controlling properties such as opacity and shade
Finish refers to the what has been done to the sheet to achieve its eventual finish e.g. gloss, matt, silk, antique, machine coated, smooth, embossed etc.
The generic term for multi layered packaging boards often with a white liner. The constituent layers can be of differing furnishes (see below).
The corollary of cut sizes where cut sizes arguably begin with A3 and folio sizes are RA3 and above.
This describes the ‘look-through’ of the finished sheet when held up to the light. Formations vary depending upon the speed of the paper machine, the mix of fibres used, and the nature of the wet end configuration (see wet end below). E.g. a cloudy formation in an uncoated sheet can create a mottled appearance when a solid is printed upon it. Such sheets though tend to have good strength and folding properties. Good formation (i.e. a clean look through) is particularly important where watermarking is involved.
FOUNT (Fountain solution)
The solution used in offset printing to ‘wet’ the non-image area of the printing plate. This is distilled, treated or even tap water with a variety of chemical additives.
Most modern paper machines are descendants of the design first developed by the Fourdrinier brothers in Hertfordshire in 1803. This process involves forming a continuous web of paper on a horizontal ‘bed’ before drying via felts and drying cylinders. Prior to the Fourdrinier machine paper was made via a laborious process using moulds immersed in vats of fibre slurries and then air drying using festoons or similar apparatus.
The term to describe the recipe used to make a particular paper variety. The ingredients can include short fibre, long fibre, recycled fibre, mill broke, clay, titanium dioxide, and chalk (calcium carbonate) and its various derivatives. Other additives relate to internal & external sizing (see sizing below).
See COATED PAPER & BOARD above.
Paper machines by design must impart a grain into the finished sheet. The grain direction is the direction the paper travels through the machine and occurs because the paper fibres naturally orientate themselves in their direction of travel. Therefore long grain means that the long edge of the sheet parallel to the grain, whereas short grain means that the short edge of the sheet is parallel to the grain. The grain direction can be critical when processing/converting the paper. Firstly offset printing assumes the grain of the paper is run parallel to the axis of the printing press. Secondly when folding finished paper against the grain there is a risk that the paper could crack along the fold. Knowing the grain direction means the printer and the finisher can adjust their processes accordingly.
The metric value of a weight of paper expressed as gsm or g/m2 (grammes per square metre).
This process applies the ink directly to the paper rather than offsetting via a blanket (see offset litho). Gravure printing is based upon a printing cylinder with recessed cells within its surface. The depth and dimensions of these cells determine the character of the printed image. The ink rollers apply ink to the cylinder and this then fills the surface cells. A doctor blade then remove surplus ink before the rotation of the cylinder brings it into contact with the paper which then pulls the ink from the cells. This process is reel based and usually heat set (see below) and is ideal for long runs such as magazines and packaging. The cost of producing gravure cylinders precludes this process from short run work.
Pulp fibre generated by applying an abrasive tool to debarked timber. This is also known as mechanical pulp because no chemical treatment is involved and the wood impurities remain with the fibre. Such pulps are considerably cheaper to produce than the more widely used chemically treated varieties and are ideal for short life applications such as newspapers. Groundwood (mechanical) pulps are typified by the fact that the paper made from them yellows in direct sunlight. This is due to a reaction between the sunlight and a wood ‘impurity’ lignin. (See also: Fibre)
HARDWOOD ‘PITH’ CELLS
These are tiny rectangular cellulose cells common to hardwoods such as eucalyptus. They can slip through the pulp preparation process and emerge in the final pulp stock. They bond poorly to cellulose fibres and can lead to pick outs from the surface of the eventual paper when it is printed.
See FIBRE above and PULP below.
The term to describe the use of a heat source (often infra red) to accelerate the drying of ink on press. Most commonly used with web presses hence ‘heat set web offset’.
A common alternative to elemental chlorine based bleaches. It is used to remove impurities such as LIGNIN (see GROUNDWOOD PULP above) when the wood fibre is cooked to produce chemical pulp (see BLEACH above).
A doughnut shaped image in the printed matter caused when a particle of debris sticks to the surface of a blanket or the printing plate (more common). The debris can emanate from the paper or can be a contaminant from the ink or even the immediate area surrounding the press (including from above where debris collects on light fittings etc and are eventually released through vibrations etc).
A term which came into common use in the late 1970’s and referred to dried ink on one surface of a matt coated paper rubbing onto an adjacent ‘clean’ paper surface due to weight in the stack and/or friction when the stack is moved and processed. Improvements in the chemical composition of coating formulations largely eradicated this problem in the 1980’s. The term though remains in general use because of the advent of stay-fresh/duct-stable inks. Such inks are formulated to resist oxidation so that they can be left in the press overnight without skinning. The corollary is that these inks rely almost entirely on absorption into the paper to dry. Because papers with a closed hard surface (such as Ivory board) necessarily have poor absorption properties, the printed image tends to ‘rub’ onto the reverse of the sheet above. Low stack heights, the use of set off spray, the use of sealers (and in extreme circumstances the use of interleaving paper) will help to minimise ink rub where stay-fresh inks have to be used.
ISO refers to The International Organisation for Standardisation. A non-governmental organisation (NGO) formed in 1947, ISO is a worldwide federation of national bodies representing over 140 countries. Two ISO standards are widely applied to the paper & related industries. First is the ISO 9000 series which deals with quality management systems verified by an approved independent third party: accreditation occurs under ISO 9001:2000. The second major relevant standard is ISO 14001:1996, which requires that an organisation’s Environmental Management Systems (EMS) provides the framework to both identify AND address the significant environmental aspects and related impacts of its activities, products and services. ISO 14001 requires compliance with all relevant legislation and a commitment to continual improvement of the organisation’s EMS.
Refers to very high strength papers used for wrapping. Available unbleached (brown kraft) and bleached (white kraft). The word Kraft is derived from the German for ‘strong’.
Paper or board with a ‘coarse ribbed’ finish are known as LAID papers/boards. The finish is achieved via a dandy roll (see above) with uniform, narrow, pronounced wires running at 90 degrees to the machine direction (see below) held together by a cylindrical wire frame running in the machine direction. The narrow wire pattern creates the laid lines whereas the wire frame imparts what are known as the chain lines.
To achieve a weight beyond the capability of any given paper machine the mill/merchant or indeed printer can elect to laminate (with an adhesive) two or more sheets together to yield the required weight or thickness. This process enables different papers (by virtue of quality, colour, thickness, finish etc) to be fixed together.
A term used to describe a thin mineral coating (as opposed to normal ‘size’ -see below) applied to both sides of the paper at the size press (see below) of the paper machine.
The term used to refer to paper dyes or printing inks that resist fading in prolonged exposure to natural light. Paper dyes were traditionally made up of pigments applied in powder form at the pulp preparation stage (see below) of the paper making process. These dyestuffs were notorious for their poor light-fastness. The advent of liquid dyes enabled light fast properties to be introduced.
LIGHTWEIGHT COATED (LWC)
The accepted definition is a coated paper of less than 72gsm comprising a ‘mechanical’ (see below) base paper with a mineral coating usually applied ‘on machine’. LWC papers are almost exclusively supplied in reels only and printed ‘heat set’ (see above).
A component of wood which binds the fibres together. Lignin is an impurity from the pulp & paper makers’ perspective and has to be chemically removed during the pulping process to yield what is commonly referred to as woodfree pulp (i.e. free of wood impurities). Mechanical pulps by definition will still contain lignin (see GROUNDWOOD PULP & FIBRE above and PULP below).
Lint is the term to describe loose fibres released from the surface of the sheet during printing. Lint will accumulate on the printing blanket and cause print interference.
Cotton linters are the tiny hairs that appear on the cotton bowl at the heart of the cotton plant. Although these cotton linters are unsuitable for fabric manufacture they are a form of cellulose and are therefore ideal for making cotton content papers.
See Fillers above.
See Grain Direction above.
See Formation above.
See Lightweight Coated above.
See Deckle above.
See Grain Direction above.
Refers to the nature of the paper’s surface at the point it leaves the paper machine. This includes calendering but only where the calender stack is ON the paper machine.
MACHINE GLAZED (MG)
Please refer to CAST COATED above. The same process can be applied to other papers such as manillas to yield a ‘glazed’ surface to one side of the paper. MG papers are typically rough on the non-glazed side.
MAGNETIC CHARACTER INK RECOGNITION (MICR)
MICR is a computerised reading process using magnetic inks. The process requires particularly pure paper to avoid the potential for misreading.
The industry wide term to describe the making ready of the printing press prior to running the saleable job. This includes fitting plates, fitting and packing blankets, running up colour and then adjusting the press until a printed image can be passed. At this point the ‘job’ can start.
Manilla is most commonly associated with low cost envelopes. Originally manilla was made from manila hemp but nowadays it is a combination of softwood kraft and recycled fibre.
See Coated Papers & Boards above.
METALLISED PAPERS & BOARDS
The result of fixing an ultra thin layer of metal (most commonly aluminium) to the surface of a paper or board. Special inks are required for offset printing upon such surfaces.
Describes a printed image whereby the alignment of coincident images/colours is not correct. The fault can be attributed to a pre-press error, or incorrectly fitted printing plates/blankets or a paper problem.
MECHANICAL PULP & PAPER
Please see GROUNDWOOD PULP & FIBRE above. Mechanical papers are primarily based upon groundwood (aka. mechanical) pulps.
The primary constituents of all papers are cellulose fibre, minerals such as clays & chalk and water. The moisture content is expressed as a percentage of the papers absolute weight at a relative humidity of 55ERH @ 23C. Moisture content is a property critical to the paper’s suitability for further processing. E.g. Offset papers typically have a moisture content of between 6-8%, whereas laser papers usually have a moisture content below 5%.
A cloudy or blotchy character within half tones and solids. Please refer to COMPLAINTS in the Technical Information section elsewhere in this site.
Although paper is most commonly made on FOURDRINIER style paper machines the original mould made process is still in operation especially for the production of banknote papers (see FOURDRINIER above for a brief description).
NBSK (Northern Bleached Softwood Kraft)
For many years the European and North American market price for NBSK pulp has been used as a leading indicator as regards pulp price trends. NBSK is a softwood pulp originating from Scandinavia & North America. However, due to the increasing proportion of hardwood (short fibre) pulps used nowadays, the prices for both NBSK and Eucalyptus pulp (see above) are closely monitored.
Until the 1970’s the internal sizing of paper was based upon an acidic compound known as rosin and alum. Acidity is measured on the litmus scale whereby values below 7 are increasingly acidic and values of 8 and above are alkaline with 7 representing neutral. Acidity necessarily impairs the longevity of paper it also has a corrosive effect upon the pipe work around paper mills. These factors accelerated the adoption of so-called neutral sizing agents. Today the vast majority of papers have neutral sizing and thereby considerably longer shelf lives than their forebears.
The term used to describe the paper used for newspapers. It is a mixture of mechanical pulp (see above) and recycled fibres. See also: Fibre.
Describes the point two rollers meet either on a printing press or a calender stack (see Calender above) in a paper mill. Increasing the ‘nip’ refers to increasing the pressure between the coincident rollers.
OBA (Optical Brightening Agent)
Chemicals added to paper to improve/alter the perception of whiteness. An analogy would be the ‘blue whiteners’ used in detergents. Typically such chemicals possess the property to fluoresce under ultra violet light sources (most notably office strip lighting).
OCR (Optical Character Recognition)
The optical reader version of MICR (see above). To ensure the optical reading instrument performs correctly OCR papers must be clean, blemish free and free of OBA’s (see above). Usually OCR papers have to be made without the use of mill broke (see broke above) to ensure cleanliness.
OFF MACHINE COATED PAPER
See Coated Paper & Board above.
The most common form of sheet fed printing and relies upon the principal that oil (ink) and water (fount) will repel each other. Offset is described as a planographic process in that the printing plate is completely flat (rather than having raised and/or recessed areas to differentiate the printed image). The plate has a light sensitive polymer coating. When the plate is developed (either via direct imaging from a computer; or when exposing to a ‘photographic negative’ in controlled light) the polymer coating in the image area disappears. The plate is fastened to a cylinder. The adjacent cylinder carries the printing blanket. Ink and fount solution (see above) are SEPARATELY fed via rollers to the printing plate. Ink is attracted to the image areas but repelled from the non-image areas, which still have the ‘unexposed’ water attracting polymer layer that has by now taken on fount solution (principally water). As the plate cylinder rotates it impresses upon the blanket cylinder offsetting a negative image upon it. As the blanket cylinder continues to rotate it impresses the negative image onto paper passing over the impression cylinder (the third cylinder in the set of three that form the printing unit). The result: a positive image emerges on the printed sheet.
This term can mean different things to different people. Generally people refer to opacity in terms of a paper’s capacity to obscure the printed matter on its reverse side OR the printed matter on a sheet it is placed upon. The paper maker considers opacity to be a key property and usually enumerates its value as a percentage of the light reflected back through the sheet when it is placed upon a ‘black’ block. Poor opacity means the black block will absorb light and therefore a low percentage of reflected light will be recorded. A highly opaque sheet will obviously prevent much of the light reaching the black block causing most of the light to be reflected back giving a high percentage reading.
The term describing a representative sample taken from a specific making of paper, or an example of a printed job.
Originally the writing substrate made from animal skin, ‘parchment’ nowadays is used to describe either Vegetable Parchment (see below) or fine papers made to appear parchment-like by virtue of a particularly cloudy/mottled formation such as G F Smith’s Parchmarque (see Formation above).
A paper containing up to a maximum of 50% mechanical (groundwood) pulp. The remainder is usually chemical pulp (see Pulp).
Describes paper with a remoistenable adhesive (gum) on the reverse. The gum is coated onto the sheet and metered using a bar with a ribbed grain. The effect is to ‘particle-ize’ the gum such that it dries to a matt finish making it difficult to see with the naked eye.
This describes two sheets of paper that have been pasted together to create a board.
The process of printing on both sides of a sheet during a SINGLE PASS through a press (see also Backing Up above).
The logarithmic scale associated with litmus paper. Readings of 1-7 are acidic whereas readings above 7 are alkaline therefore 7 (or strictly speaking 7.07) is considered neutral.
Describes a paper fault whereby small particles pick out of the surface due either to a poor internal bond within the paper or too great an ink tack (or indeed a combination of the two). Small pick outs leave tell tale cavities in the originating sheet and generally adhere to the printing blanket causing print interference. In extreme cases pick outs, where large enough can adhere to the printing plate (see Complaints Section).
A paper problem where ‘pipes’ are visible in the grain direction of the sheet: genuine piping can only occur on the paper machine. Please also note that where a ‘piping’ effect is confined to an area close to a long edge of the sheet, the cause is likely to have been where the paper machine reel has impacted a hard object (metal work or masonry) during transport within the mill or in transit as reels off site.
An MG (see above) paper quality for exterior posters. The MG side gives a good printing surface whilst the rough reverse is ideal for adhesives to key to.
Whilst the primary colours BLUE, GREEN and RED are the constituents of white light, the derivative process colours (cyan, magenta & yellow) combine to give BLACK. These process colours widely referred to as CMYK (i.e. cyan, magenta, yellow & black). In ink form they are semi transparent allowing the printer a vast array of perceived colours in the printed image. Although C, M & Y combine to give black, black is also used in the 4 process-colour set for added emphasis for illustrations and of course for text work.
A printed image generated by a dedicated pre-press machine (e.g. high resolution colour ink jet or colour laser printer or in days gone by via the chromalyn process) or in extreme cases on the printing press itself (wet proof). The proof is used to illustrate colour reproduction and image accuracy. It is common for a client to approve and sign such proofs prior to the job going to press.
See Fibre above. There are two groups of pulp. Groundwood pulp (see above) and chemical pulp. Chemical pulp refers to the process of chemically cooking woodchips to free them of impurities such as lignin (see above). The character of chemical pulp means that the cellulose fibres are freed enabling the paper maker to treat and refine them in order to achieve the properties required of the eventual paper. The bonding of chemical pulps is superior to that possible with Groundwood pulps.
This is the process whereby dried sheets of pulp are converted into a porridge-like water based slurry (around 1% solids) before being REFINED (see below) prior to release onto the paper machine. Pulp preparation includes: cleaning, refining, introduction of additives such as FILLERS (see above), internal SIZING agents (see below) and dyestuffs etc.
A widely used term to describe woodfree uncoated boards usually classified by thickness, and commencing at 200 microns (around 160gsm). Even thicker pasted woodfree boards are usually classified as pulpboards.
A term often used to describe papers with a cotton fibre content. Rag is derived from the period where cotton rags were a principal source of papermaking fibre.
In the days of imperial measures a ream referred to 480 sheets of paper. A ream was a score of quires where a score is of course 20, and a quire is 24 sheets of paper. Over time and with the advent of the metric system, a ream now refers to a quantity of 500 sheets.
For decades the UK paper industry has sourced over 50% of the fibre it needs from recovered paper (as much for economic reasons as environmental ones). Cellulose fibre however can only be effectively recycled about 5 or 6 times before it becomes inert. It is essential therefore that ‘virgin’ fibre continues to enter the chain to ensure the average recycled fibre tends towards 1 rather than 6 recycles. The increased sophistication with which recycled paper is segregated has enabled the average quality of recycled papers to improve significantly over the past 20 years.
When a paper machine reel (or a reel on a REWINDER) breaks the reel has to be made good with a join (usually via industrial strength self adhesive tape). Such joins should be clearly marked on the reel so that the next user of the reel can act accordingly when the join approaches the ‘business end’ of the process. E.g. the operator running a sheeting machine should ensure that the sheets containing the join are removed prior to the sheet stacking unit at the end of the machine. A reel join can cause blanket and even cylinder damage were it to reach an impression unit on a printing press.
The process whereby cellulose FIBRES (see above) are mechanically cut and bruised to ensure that tiny fibrils are encouraged to protrude from the fibre’s surface. These fibrils assist in ‘locking’ the fibres together as the paper is dried. Originally this was done in large vats known as beaters (because the fibres were literally beaten) but modern papermaking employs machines known as refiners. A refiner contains discs with a variety of patterned surfaces. The discs are pushed together and rotated at speed. The pulp slurry (see PULP PREPARATION above) is forced between these discs under pressure causing the fibres to fibrillate. Fibres with little refining are an ideal basis for blotting paper, whereas fibres that are very heavily refined yield the properties necessary for tracing paper. A variety of characteristics can therefore be created between these two extremes.
The term to describe the alignment of the constituents of a printed image. See MISREGISTER above.
RELATIVE HUMIDITY (RH)
At a constant air temperature and constant air pressure, a humidity of 100% is the point at which the air is saturated and can hold no further moisture. At this point you have precipitation of water (effectively it rains!). So against this level you can then measure the relative humidity of any pocket of air at a given temperature by expressing the reading as a percentage of saturation. In papermaking and printing terms the ideal condition is a relative humidity of 55% at 23 degrees centigrade. As paper is a live product paper mills necessarily have to control humidity, and for offset grades will generally work to a target RH of 55 within a range of 40 to 60 at 23 degrees centigrade. For laser grades where the paper must have a low moisture content to restrict the sheet’s propensity to distort in the fuser unit (temperatures of 180-200 centigrade are common at fuser units) of the laser printer or copier, the target RH of the paper will be between 25-30.
The machine that trims and re-reels the ‘raw’ reel as soon as it leaves the paper machine. Rewinders can either produce finished reels for web printing/conversion or alternatively reels suitable for cutting to folio or cut sizes on sheeting machines.
SELF ADHESIVE PAPER
Paper with a thin coating of pressure sensitive self-adhesive material on the reverse. The adhesive is protected by applying a silicon coated release paper to it, thus forming a ‘temporary’ laminate consisting of the face paper, the adhesive coating and the silicon coated backing paper. The backing paper can be solid or scored (for ease of removal). Adhesives generally fall into two categories namely, ‘permanent’ whereby the adhesive fix to a suitable surface will be essentially permanent: and ‘removable’ indicating that the adhesive is weak enough to allow a label carrying it to be removed relatively easily.
Refers to carbonless paper whereby both the colourless dye and colour former capsules are contained within the same coating on the surface of the self-copy paper (see CARBONLESS PAPER above).
SELF SEPARATING GLUE (aka Fanapart Glue)
A specially formulated adhesive that is differentially absorbed when applied to the edge of a collated stack of carbonless sets. The glue absorbs more easily into the coated back (CB) and coated front (CF) coatings than it does to the uncoated surfaces on the top of the A sheet (CB) and the reverse of the C sheet (CF). Consequently when the stack is fanned after the glue has dried, the carbonless sets ‘miraculously’ separate.
The term used to describe the incidence of ink partially transferring to the surface of the sheet above i.e. part of the image has ‘set off’ to the back of the sheet above. This could be due to a number of factors (and indeed a combination of them). For example: the stack is too high causing the weight of the stack itself to encourage set off (reduce stack heights); the ink is drying too slowly (add drying compounds to the ink and/or employ set off spray); the ink is not absorbing into the paper (change paper or ink). The widespread use of so-called stay fresh inks (see DUCT STABLE INKS above) has increased the occurrence of this problem.
SET OFF SPRAY
A starch based compound in particle form, set off spray is a convenient way of introducing a barrier between two printed sheets so as to reduce the potential for ink to transfer to the sheet above. Many modern 4-colour presses are fitted with set-off spray applicator units just prior to the delivery point.
See WATERMARKS below.
A machine that cuts reels of paper into FOLIO sizes (see above). Disc knives cut with the grain in a scissors-like action and rotating drum knives cut across the grain.
See GRAIN DIRECTION above.
The degree to which a printed image on the reverse side of a sheet is apparent when looking at the top side of the same sheet.
See COATED PAPER & BOARD above.
These are synthetic coloured fibres which were originally added a security feature during the manufacture of paper. In recent times they have also been employed as a ‘design’ feature.
Uncoated paper has to be treated internally to prevent ink from being able to strike through from one surface to the other. Internal sizing used to be an acidic compound combining rosin and alum. Nowadays PH neutral sizing chemicals are used in order to enhance the longevity of the finished sheet (see NEUTRAL SIZING above). Surface sizing of the paper is also essential to make the surface receptive to ink (without surface sizing uncoated papers would behave like blotting paper). Surface sizes are usually starches derived from milk or cereal crops.
Modern papermaking machines invariable employ a size press around two thirds of the way along the drying cylinder section. The size press applies a coating of SIZE (see above) to both surfaces of the sheet.
The process of converting a jumbo reel from the paper machine into more manageable reel widths and lengths.
A key characteristic controlled by the papermaker is the smoothness of the eventual sheet. Most commonly measured by the Bendtsen method smoothness is often controlled by using CALENDERS (see above).
See FIBRE above.
An electrostatic charge can occur to the extent that the sheets appear to ‘stick’ together making them difficult to feed into a printing press. It is rare, but nonetheless possible for this to occur during manufacture. More commonly static appears when cold paper is opened in a warm and very dry pressroom (typical of winter months). Where the moisture level in the air is very low (usually below 20 RH) any static contained in the paper stack will have difficulty in discharging itself into the air surrounding it. If the pressroom is at around 50 RH @ 21C then the paper will normally discharge any static present into the surrounding air (providing it is at or near pressroom temperature of course).
The term stock is used by papermakers to refer to the suspension of the refined mix of fibres in water. At the point stock is released to the paper machine it is only around 1% solids.
A term often used to describe GREYBOARD – the material most commonly found as the backing material for ruled pads. GREYBOARD is produced from recycled fibres nearing the end of their useful life (see RECYCLED PAPER above).
Refers to the weight of a given sheet of paper normally expressed as grammes per square metre i.e. grammage (g/m2 or gsm).
See CALENDERED above.
Papers created from man-made fibres or from non-woven petrochemical derivatives. These are typically expensive and have limitations as regards conversion (e.g. folding, creasing etc).
Refers to Totally Chlorine Free pulp i.e. pulp free of all chlorine compounds (see also ECF).
The term to describe a synthetic version of embossed images created via die stamping. Thermography involves coating a nylon powder onto a wet freshly printed image and then passing the sheet under a powerful heating element which causes the nylon to ‘blister’ making the image rise and appear as though it were embossed.
The appearance of a tint in what should be the non-image area of the printed sheet. This is usually associated with ink having contaminated the rollers applying fount solution to the printing plate (see OFFSET above).
A black powder compound which when heated to around 180c (in the laser/copier fuser unit) and then pressed against the surface of paper will adhere to it. Toner comprises a black pigment in powder form and tiny plastic particles: fusing occurs when the pigment is fixed to the paper’s surface when the plastic particles melt.
A sheet of paper (or more usually board) created by fixing together the webs from two different paper machine WET ENDS (see below). This enables the paper maker to create a sheet with identical surfaces.
A conventional watermark is introduced at the so-called WET END of the paper machine. The two types of watermark are LINE MARK and SHADOW (embossed) MARK. The line mark is an image formed by fixing raised electrotype to the phosphor bronze fabric covering the dandy roll. In this case a phosphor bronze sleeve can be used to fix over the body of the dandy roll (sleeves are obviously far less expensive than the full bodied dandy roll). Shadow (or embossed) marks are recessed into the phosphor bronze surface of a full-bodied dandy roll. A line mark when impressed into the surface of the paper being formed at the WET END will create a light image when the eventual sheet is held up to the light; whereas the shadow mark as you might expect produces a dark image.
A widely used paper industry term essentially describing a continuous stream of paper. As such it is used to describe both reels of paper and the paper ‘web’ as it is being created on the paper machine.
This term describes the early part of the paper machine. The wet end comprises the head box, the wire, the suction boxes, the dandy roll (see above) and the press section. The head box is usually a closed container in which the paper fibre, minerals, additives and water are finally combined before release onto the continuous wire mesh that begins the process of forming the paper. The mesh is principally a device to drain the paper; the suction boxes then remove more water prior to the first press section at which point the web of paper is just strong enough to carry its own weight although it is still around 60% water at this stage. The DANDY ROLL is usually located in the same area as the suction boxes and helps impart a finish on the top side of the sheet. The dandy roll is also used to impart the watermark.
The propensity a sheet of paper has to retain its strength whilst absorbing moisture. This is important when it comes to food packaging & labelling as well as for certain ‘heavy duty’ map and chart papers.
The wire side refers to the fabric imprint left on the surface of the sheet by the continuous wire mesh during its journey through the WET END of the paper machine. The corollary of this is the top side whose character (not withstanding any subsequent calendering or embossing) is determined by the surface of the dandy roll.
A very misunderstood term. It was originally coined in the 1960’s to differentiate between mechanical pulp (which contains wood impurities such as lignin) and chemical pulp (which by its nature has had the wood impurities chemically removed). Strictly speaking therefore WOODFREE means ‘free of wood impurities’.
A WOVEN sheet means that the dandy roll had no background pattern such as a laid line and/or chain line. It also infers that the sheet has not been subsequently supercalendered or embossed.