"Engraving (die stamping) is far more popular in the States than in Europe. One of the reasons is that engravers and the Engraved Stationery Manufacturers' Association have made an all-out effort to tell their story to the design community," says Dick Kerans

Not too many years ago, engraving was used for executive stationery and attorney letterheads and that's about all. Today, it's a different story. Virtually all prestigious US corporations (some of which include: IBM, Chrysler Corp and Marriot Hotels), who want to convey a prestigious image through their correspondence use engraving. Paper can convey prestige and so does an engraved message.

In order to expect that designers would buy and specify engraving, more than 200 US engravers felt the necessity not only to tell the engraving story, but to point out its advantages and, at the same time, explain the dos and don'ts of designing for the process.

Engraved stationery is the ultimate in quality. Letterpress and lithography can produce spectacular results, but there are things that you can do with engraving that simply can't be achieved any other way. It adds style, grace, elegance and commands attention. It is generally used with premium, quality cotton fibre content papers and with textured papers. As for the cost of engraving on cotton content papers, in the quantity runs normally involved in a stationery job, for example, the end comes out about the same as any process.

In appearance, however, an engraved job is decidedly different. This is what designers like about it. They can tell just by the look and the feel. The letters and artwork stand out. If you run your fingers over an engraved letterhead or business card, you can feel the ridges of the inks or embossing. This is an inherent result of the process.

Engraved stationery is produced on an engraving press, with one sheet stamped at a time. As the sheet is fed into the press it is positioned above a backing die and below an engraved plate. The images to be reproduced are formed in the metal of the plate as an incision or well. The base metal of the plate is usually steel or copper, although, brass and zinc are also used. Because the imprinting area is carefully etched into the plate, the amount of ink applied to the plate carefully controlled and metered by the size and depth of the engraved areas. As the process comes down and the plate comes into contact with the paper, the ink is transferred.

Engraving is an art. It also makes broad use of the tools of science and technology. In the past, all engraved plates were cut out by hand in the base metal. This is still done today for special effects, but by photo-mechanical instead. The fact that engraving can exactly reproduce typography and artwork has led to a growing use of this reproduction technique.

Modern engravers have a wide variety of typefaces and styles in patterns that can be directly transferred to the metal in any size, with reductions down as far as the visual limit of the human eye. The engraving of 2pt. type with an absolute clarity in reproduction is done all the time. Letter spacing and the condensing of typography is also possible by creating an engraving plate that fits the type to the space specified. There are no graphic limits. The designer sets them and can use any typeface imaginable, or create a new one if necessary. The most distinctive characteristic of engraved stationery is the fact that the ink remains on top of the paper. The result is three-dimensional. The amount of ink placed on the paper varies with the size of the image. The wider the image, the greater the depth of the well and the greater amount o transferred ink. Engraving places an unusually large amount of ink on paper and therefore, gives a more intense display of colour than any other reproduction process. In engraving, the true colours stand out. You can use any colour system, such as PMS or your own matched inks. Light inks will show true on dark papers. Metallic papers, such as gold and silver, retain their metallic glow. You can even cover black paper with white ink. The inks transfer as solids and thereby, provide altitude of design that can be achieved in no other way, with absolute control over the results. In terms of the graphic qualities, the designer has far more control over the end product when it is engraved, than with any other process. You know exactly what your art will look like and colour control is absolute. You can adjust the typography to fill the requirements of space and content and you'll never face the problem of a job that was over-inked or 'got away' on the press.

The quality starts in the designer's mind and holds true right down to the sheet that comes out of the engraving press. Blind embossing of typography and art elements offers another unique design opportunity. Similarly, the process is the same, except the ink is eliminated. Once again, a recessed plate is used with the make-ready on the press, forcing the paper up under pressure into the depths of the die. The resulting image is formed in the paper as a three-dimensional element, visualized by the eye as a result of the light that shadows cast by the rises in the paper. Embossing is at its best when it stands alone and is not inked. This is because colours tend to eliminate the contrasting shadows that make the embossing stand out and the dimensional effect is therefore, often lost. All companies can also do blind embossing, as the same process is used.

Engraving Do's
Engraving provides a number of unique possibilities that aren't possible with any other form of reproduction. They add the distinction and appearance that quickly identifies engraved stationery and makes it makes it stand out as artful, dignified and elegant. Here are a few of the major dos to help create the right design:

• Very Small type

No form of reproduction can reproduce type or art elements as small as engraving can. The result is perfectly clear and easy to read. Copy can be stretched or reduced to the virtual visual limit, yet each letter and fine line is faithfully reproduced.

• Faithful reproduction

Precise engraved plates reproduce artwork exactly as created, be it lettering, typography or art elements. The engraving can be done by hand or by photomechanical means to get exactly what is needed.

• Mixed techniques

The use of multi-media to create stationery can be very interesting and exciting. The combination of screens for a large design area plus engraved type and embossing, can make a design unique.

• Blind embossing

Three-dimensional blind embossing is very rich and subtle. You not only see it, you feel it. It looks best if done without ink on a premium grade of paper. Embossing is done on an engraving press, so it can all be done in-house by the engraver.

• The look of quality

Engraved stationery is distinctive and denotes professionalism and stature. It is regarded as the absolute in quality. On premium cotton content papers it has a look and feel all of its own and cannot be duplicated.

Engraving Don'ts

While engraving has a number of unique graphic advantages, there are a number of restrictions on the work. These should be recognized in order to avoid the pitfalls of the technical limits of the craft.

• Restrict the engraving area

There is a practical limit to the area that can be engraved in a single impression of the engraver's press. This can be a factor in design and cost. Engraving both the top and bottom of a sheet requires two passes through the press. Most engravers have presses that have a useable limit of 5" x 9". The engraving design should be kept within an area this size, if possible.

• Avoid large solid areas

Because of the heavy layer of ink placed on paper by the engraving process, large panels of colour should be avoided. The raised ink tends to flatten or becomes splotchy. If large panels of solid colour are used in the artwork or logos, it is better to reproduce it using lithography or letterpress, adding the type and line art by engraving or blind embossing the solid area.

• Don't over design

Engraving has an appearance and graphic feel all of its own. The fine paper, plus the engraving and embossing all blend together to create a graphic entity unto itself. The 'design' goes beyond the graphic elements, with the whole sheet becoming a form of design. A simple letterhead that is engraved can look outstanding, whereas a letterhead that is printed may not look distinctive at all. Use the advantages of the engraving process, but don't overuse them.

• Don't mix colours

Engraving can't blend colours together, as is achieved with duo-tones or 4-colour process printing. In engraving, the colour you specify is the colour you get, so if you want coloured halftones, use printing. However, if 'correct' colours one at a time are the order of the day, engraving is the way to go.

In Europe, engraving is a well kept secret. It shouldn't be. There are many outstanding craftsmen out there, ready and willing to work with designers and to accept their challenges.

The British Engraved Stationery Association stands ready to help designers who have little experience with the process.

Based on text by Markus Khun, lecturer with the University of Cambridge Computer Laboratory

The ISO A4 paper size is today commonly used throughout the world. The following explains the ISO 216 paper size system and the principles.

The ISO Paper Size Concept
With the ISO paper size system, all sheet sizes have a width to height ratio of the square root of two (1:1.4142). By placing two sheets of A series paper next to each other, or any cutting one in half, parallel to its shorter side, the resulting sheet will again have the same width to height ratio.

ISO paper sizes are based on the metric system. The square-root-of-two ratio does not allow the height and width of pages to be given simple whole metric sizes. The sizes, therefore are rounded to two whole millimetre lengths. As the weight of paper in the metric system is specified in grams per square meter (gsm), a simple method of calculating the mass or weight of a publication is possible where the size and number of pages is known.

ISO 216 defines the A series of paper sizes as follows:

The height divided by the width of all formats is the square root of two (1.4142), hence, an A3 sheet is approximately 141% larger than an A4 sheet.

The base format A0 (841mm x 1189mm) has an area of one square meter.

A1 is A0 cut into two equal halves, where the A1 sheet long dimension is the same as the short dimension of A0 and the A1 short dimension is half the A0 long dimension. Thus, A0 measure 841mm x 1189mm and A1 measures 594mm x 841mm, where 594mm is half the length of 1189mm and rounded down to the nearest millimetre.

All smaller A series formats are defined in the same way by cutting the next larger format parallel to its shorter side into two equal halves, so A2 measures 420mm x 594mm, where 420mm is half of 841mm, rounded down.

For applications where the ISO A series does not provide an adequate range of sizes, the B series was created to provide a greater choice. The C series of formats has been defined for envelopes.

The width and height of a B series format is the geometric mean between the corresponding A format and the next larger A format. For instance, B1 is the geometric mean falling between A1 and A0. In other words, the factor that scales A1 to B1 also scales B1 to A0. So where A0 has a length of 1189mm, B1 will have a length of 1000mm and A1 will have a length of 841mm, each is approximately an 84.1% reduction of the larger size, or a 118.9% enlargement of the smaller size. This percentage ration between sizes applies equally to both the height and width dimensions of each sheet size. These percentages are demonstrated below when referring to photocopying machines.

Similarly, the sizes of the C series are the geometric mean between the A and B series formats with the same number. For example, an A4 letter fits into C4 envelope, which in turn fits into a B4 envelope. Folding the A4 letter to A5 format, enables it to fit into a C5 envelope.

The following table shows the width and height of all ISO A and B paper formats, as well as the ISO C envelope formats. The dimensions are in millimetres:

A Sizes (mm)	B Sizes (mm)	C Sizes (mm)
4A0 1682 x 2378
2A0 1189 x 1682
A0 841 x 1189	B0 1000 x 1414	CO 917 x 1297
A1 564 x 841	B1 707 x 1000	C1 648 x 917
A2 420 x 594	B2 500 x 707	C2 458 x 648
A3 297 x 420	B3 353 x 500	C3 324 x 458
A4 210 x 297	B4 250 x 353	C4 229 x 324
A5 148 x 210	B5 176 x 250	C5 162 x 229
A6 105 x 148	B6 125 x 176	C6 114 x 162
A7 74 x 105	B7 88 x 125	C7 81 x 114
A8 52 x 74	B8 62 x 88	C8 57 x 81
A9 37 x 52	B9 44 x 62	C9 40 x 57
A10 26 x 37	B10 31 x 44	C10 28 x 40

The allowed tolerances are approximately 1.5mm for dimensions up to 150mm, approximately 2mm for dimensions above 150mm up to 600mm and approximately 3mm for dimensions above 600mm. Some national equivalents of ISO 216 specify tighter tolerances, for example, DIN 476 requires approximately 1mm, 1.5mm and 2mm respectively, for the same ranges of dimensions.

Application examples
The ISO standard paper size system covers a wide range of formats, but not all of them are widely used in practice. Among all formats, A4 is clearly the most convenient for daily office use. Some main applications of the most popular formats can be summarized as:

A0, A1		technical drawings, posters
A2, A3		drawings, diagrams, large tables
A4		letters, magazines, forms, catalogues, laser printer and photocopy machines
A5		note pads
A6		postcards
B5, A5, B6, A6		books
C4, C5, C6		envelopes for A4 letters: infolded (C4), folded once (C5), folded twice (C6)
B4, A3		newspapers, supported by most copying machines, in addition to A4

One of the main advantages of the ISO standard paper sizes becomes apparent when using photocopy machines:

Example 1:
When copying an article from an A4 publication, you can copy two A4 pages on a single sheet of A4 paper. By exposing two A4 pages (double page spread) and setting the reduction factor to 71%, or be selecting the pre-set A3 to A4reduction facility, both A4 pages will be reproduced side by side on a single A4 sheet. Each of the original A4 pages will have been reduced in size to A5. No large blank margins appear, no text disappears and there is no need to experiment to find the appropriate magnification or reduction factor. The same principles work for books in B5 or A5 format.

Copying machines designed for ISO paper sizes usually provide special keys or pre-sets for the following frequently used magnification or reduction factors:

71% A3 > A4
84% B4 > A4
119% A4 > B4	(also B5 > A4)
141% A3	(also A5 > A4)

Example 2:
When preparing a letter, knowing the weight of the paper helps when calculating the cost of mailings. This can be very conveniently calculated with the ISO series A paper sizes. Standard stationery paper weighs 80gsm. An A0 page has an area of 1sqm and the next smaller A series page has half of this area. Therefore, the A4 format has an area of 1/16 sqm and weighs 5 grams per page. Allowing an estimate of 20 grams for a C4 envelope, 16 A4 pages can be included before the initial 100 gram limit is reached.

The ISO formats have are used for surprisingly many things besides office paper: the German citizen ID card has format A7 and the European Union passport uses the B7 format, library microfinches use the A6 format.

Aspect ratios
There are occasions when paper sizes other than the standard A series are required, such as labels, tickets etc. These can be achieved by cutting standard series sizes into 3, 4 or 8 equal parts, parallel to the shorter side, in order that the ratio between the longer and shorter side is greater than the square root of two. Some example long formats in millimetres are:

1/3 A4 99	x 210
? A4 74x	x 210
1/8 A4 37	x 210
? A3 105	x 297
1/3 A5 70	x 148

The 1/3 A4 format (99 x 210mm) is also commonly used for reduced letterheads or compliment slips, or for short notes that contain not much more than a single sentence message and fit without folding, into a DL envelope.

Envelope formats
For postal purposes, ISO 269 and DIN 678 define the following envelope formats:

Format Size (mm) Content Format

C6 114 x162 A4	folded twice = A6
DL 110 x 220 A4	folded twice = 1/3 A4
C6/C5 114 x 229 A4	folded twice = 1/3 A4
C5 162 x 229 A4	folded once = A5
C4 229 x 324 A4
C3 324 x 458 A3
B6 125 x 176 C6	envelope
B5 176 x 250 C	envelope
B4 250 x 353 C4	envelope
E4 280 x 400 B4	envelope

The DL format is the most widely used business letter format. It size falls somewhat out of the system and equipment manufacturers have complained that it is slightly too small for the reliable automatic enveloping, therefore DIN 678 introduced the C6/C5 format as an alternative for DL.

There is no current ISO standard for envelopes with an address window, there is however, a corresponding DIN standard. DIN 680 specifies that a transparent address window should measure 90mm x 45mm with its left side 20mm from the left edge of the envelope. For C6, DL and C6/C5 envelopes, the bottom of the window should be 15mm from the bottom edge of the envelope. For C4 envelopes, the top of the window should be either 27 or 45mm from the top edge of the envelope.

Untrimmed paper formats
All A and B series formats described so far are trimmed paper end sizes, i.e. these are the dimensions of the paper delivered to the end user. Other ISO standards define the RA and SRA for untrimmed, raw paper. These formats are only slightly larger than the corresponding A formats. Sheets in these formats are cut to the final size format after printing and binding. The ISO RAO format has an area of 1.05 sqm and square foot of 2 ratios and the half-area rule, but the dimensions of the base format have been rounded to the full centimeter. The common untrimmed paper formats that printers order from the paper manufacturers or merchants are:

RA Series Formats SRA Series Formats

RAO 860 x 1220		SRAO 900 x 1280
RA1 610 x 860		SRA1 640 x 900
RA2 430 x 610		SRA2 450 x 640
RA3 305 x 430		SRA3 320 x 450
RA4 215 x 305		SRA4 225 x 320

Punched holes for filing
ISO 838 specifies that for filing purposes, two holes of 6+/-0.5mm diameter can be punched into the sheets. The centres of the two holes are 80+/-0.5mm apart and have a distance onf 12+/-1mm to the nearest edge of the sheet. The holes are located symmetrically in relation to the axis of the sheet/document. Any format that is at least as large as A7 can be filed using this system. Not specified in ISO 838, but also widely used, is an upwardly compatible 4-hole system. The two middle holes correspond to ISO 838 and the two additional holes are positioned 80mm above and below these to give greater stability. Sheets with four punched holes corresponding to the ISO 838 standard can be filed in ISO 838 2-hole binders.

Folder and file sizes
ISO 623 specifies the sizes of folders and files intended to receive either A4 sheets of simple folders (without spine) that are not designed for any particular filing system of cabinet. The sizes specified are those of the overall rectangular surface when folders or files are folded, excluding any margin or tabs. Simple folders, without a spine or a binding mechanism are 220 x 315mm large. Folders and files with a very small spine (less than 25mm) with or without a binding mechanism are 240 x 320mm large. Files with a wide spine (exceeding 25mm) are 250 x 320mm (without a mechanism) or 290 x 320mm if they include a binding mechanism. All are maximum dimensions. Standardizing folder and file sizes allows the optimization of shelf design.

Overhead projectors
ISO 7943-1 specifies two standard sizes of overhead projector picture areas: Type A is 250 x 250mm (round corners with a radius less than 60mm) and Type B is 285 x 285mm (round corners with a radius less than 40mm, or cut off diagonally no more than 40mm). Therefore, when using A4 transparencies, leave at least 30mm top and bottom margin.

Identification cards
ISO 7810 specifies identification cards and defines the following three standard formats: ID-1 = 85.60 x 53.98mm (= 3.370 x 2.125 in), ID-2 = 105 x 74mm (=A7) and ID-3 = 125 x 88mm (=B7). ID-1 is the common format for banking cards (0.76mm thick) and is also widely used for business cards and driver's licenses. The standard passport format is B7 (= ID-3).

History of the ISO paper formats
The practical and aesthetic advantages of the square root of two aspect ratio for paper sizes were probably first noted by the physics professor George Christoph Lichtenburg (University of Gottingten, Germany, 1742-1799) in a letter he wrote in 1786 to Johann Beckmann. After introducing the meter measurement, the French government published in 1794 the "Loi sue le timbre" (no. 2136), a law on the taxation of paper defining several formats corresponding exactly to modern ISO paper sizes: "Grand Registre" = ISO A2, "grand papier" = ISO B3, "moyen papier" = ISO A3, "petit papier" = ISO B4, "demi feuille" = ISO B5 and "effets de commerce" = ISO ? B5.

The French format was never widely adopted and soon disappeared. The modern A, B and C series paper formats, based on exactly the same design principles as the French system, were reinvented over a hundred years later in Germany by Dr. Walter Porstmann. They were adopted as the German standard DIN in 476 in 1922 to replace the huge variety of other paper sizes in use until then and to make paper stocking and reproduction methods cheaper and more efficient.
The DIN paper formats were soon also introduced in many other countries.

ISO paper format standards are available from:

International Organization for Standardization
Case postale 56
1, rue de Varembe
CH-1211 Geneve 20
Switzerland
www.iso.org

Based on the full text International Standard Paper Sizes by Markus Kuhn.

Problems that can bite back, faced by designers, printers, buyers and specifiers. Ink techniques, paper storage, recycling, office printing, digital printing and the paper industry today. In this short information series, we offer advice and solutions on the topics.

Precautions you can take to avoid ink rub

Even though designers, printers, buyers and specifiers are more knowledgeable today than they have ever been, there is always a place for a little extra advice and support. Here, we take a look at the eternal problem of ink rub on silk and matt coated stocks.

As designers become ever more imaginative (and mistakes increasingly unaffordable) a little foresight can avoid ink rub spoiling a beautiful print job.

As you probably know, it is the coating of calcium carbonate that prevents glare and makes these papers suitable for large areas of text. This coating is in fact, made of very uneven and sharp edged microscopic particles. Pressure and scuffing can break off these particles making them act as an invisible irritant to ink and cause ink rub to occur on unprinted areas. In addition, the pH and conductivity of machine fount solutions must be considered, otherwise, a chemical reaction will occur, resulting in the paper coating being deposited on the offset blanket.

There are a number of precautions you can take to avoid ink rub. These include:

Performing seals
It is essential to use inks specially formulated for silk/matt coated stocks. Specifying the proposed paper and supplying samples of the actual stock to your supplier can help at the outset.

Ideally, all silks and matts should be sealed with an oil-based varnish or water-based sealant. Breathable emulsion varnishes work extremely well. Under-varnishing does not minimize ink-rub, but overall varnishing or varnishing of unprinted areas in contact with print helps enormously.

You will need to consider whether work should be film laminated or varnished after initial sealing. Although sealants containing wax helps reduce print rub, the UV varnishing and field lamination processes work best on inks with reduced wax. UV varnish will not readily key to oil-based varnishes, but water-based sealants can be used satisfactorily.

An in-line varnishing unit will also reduce ink rub problems, as well as improve turn-around time.

Here are some recommendations you may not yet know:

• Ideally, pages should be designed so that large areas of print face each other, to reduce to likelihood of ink rub ? especially if no seal or varnishes are to be used. Solid and heavy tone areas will also be more effective if designed back to back to minimize show-through.
• Standard Reflex Blue is particularly prone to ink rub and should be avoided. Substitutes are available.
• Keep handling to a minimum and ensure that paper transport on the machine is adjusted precisely. If marking does occur, a silicon spray can be used in the final stages on the areas creating ink rub.
• Avoid heavy print areas which bleed off the page, as these can cause ink rub when trimmed.

Improving your print performance

Apart from the more obvious effects of sunlight and dust ? which can be prevented with good stack covers ? print performance is dependent on the effects of temperature and humidity on paper.

This is a particular problem in cold weather. When cold material is exposed in a warm environment the surrounding air quickly condenses and the excess moisture is absorbed by the material. This causes wavy edges. It is therefore, essential to equalize temperatures before the moisture-proof covers are removed.

Here is a chart giving recommended recovery times to allow incoming material to reach as near a room temperature as possible. Recommended recovery times for a 750kg pallet of material (times based on temperature difference between paper and surrounding atmosphere):

Temperature Recovery Time (Hrs)
10c 24
15c 40
20c 75
25c 90

The relative humidity (RH) of both the material and the press room is also critical. High humidity can cause wavy edges. For example, if the outside atmosphere is damp, with relative humidity of 75% and the relative humidity of the paper is 45%, the fibres will expand as moisture is absorbed from the atmosphere.

The opposite effect, tight edges, results when the humidity of the room is too low and paper contracts. This is a common problem in winter, particularly in freezing temperatures, when factory heating is increased. Relative humidity drops significantly (to about 20%) and edge shrinkage will occur as the material (at about 45% RH) gives off moisture to the atmosphere, causing fibres to contract.

To protect your material from the effects of relative humidity variation, you need to ensure that all material remains moisture- proof wrapped until conversion is protected at each stage of production and/or conversion. It may also be worth considering investing in humidity control machinery, which is relatively inexpensive.

Your paper need not be a hostage to fortune. If you take the precautions described, you should avoid such problems.

Here are some recommendations you may not already know about:

• Ensure that all material remains moisture proof protected at each stage of production and/or conversion.
• Invest in machinery to control humidity
• In a small room, increase relative humidity locally with buckets of water, wet rags or sawdust.

Office printing tips

The choice of paper used in the office depends on more than weight, colour finish and technical compatibility. Personal taste is obviously important and most requirements can now be satisfied from the wide range of papers available.

Which papers perform best is determined by the types of printers you use. All printers require paper that will feed well, run freely and not jam. So the maximum requirement is for paper that is dimensionally stable (i.e. it won't expand or contract), dust free, accurately cut to size, with an appropriate rigidity and smoothness to ensure good imaging.

Many papers are suitable for use in a variety of printers, but it is necessary to check compatibility first. Even the most expensive printer will deliver poor results if the wrong paper is used.

Photocopiers and lasers
Photocopier and laser printers all run at very high temperatures, so there must be strict control of the moisture content to prevent curling, misfeeding and dirty images.

Whether a laser printer uses heat pressure or freezes the ink onto the page by gas fusion, paper for either process needs a particularly smooth surface to guarantee a good image reproduction.

Ink-jet
Ink-jet printers fire tiny droplets of ink onto the paper and so require a correctly sized surface which allows the ink to be readily absorbed and to dry quickly without feathering. Too soft and the ink will show through. Too hard and it will smudge.

Dot Matrix
Dot matrix printers are well suited to printing multi-part forms. They may be less sophisticated, using impact technology and inked ribbons, but the paper requirements are no less stringent. Heavier weight paper is necessary to avoid unsightly impressions on the back of the paper. A smooth surface helps give a good image with minimal smudging.

Here are some recommendations:

• Always check that the paper you are using complies with the manufacturer's specifications for your printers. Office printers are generally set up to take 80g/m2 paper (e.g. copier paper), although most should work with weights up to 160g/m2.
• Conduct trials to test compatibility
• Keep paper protected, somewhere dry and at the recommended temperature (not too hot or too cold) to ensure it remains flat.
• Use inks or toners that are specified for your particular printers.

Digital printing ? which paper is best?
Digital printing is the most rapidly developing technology in printing today. It may be leading edge stuff, but it still uses paper. Here we clarify some of the processes and what they mean for paper selection.

All digital printing involves converting computer generated data ? usually carried via ISDN or disk ? into an image. Some machines employ laser scanners first to transfer the image to a plate and then to offset and impression cylinders. Other types convert the data into a bitmap image (via a Raster Image Processor) and reproduce the image directly onto the substrate.

It is not clear cut which is best ? they are generally designed for different purposes. All offer increasingly exciting opportunities for economic short run colour printing, with some offering the facility to print variable data in full colour on each sheet. The potential is fantastic, allowing designers to design specifically for short runs and printers to meet client requirements more economically than ever before.

To understand what digital technology means for paper selection, we need to look at the different technologies a little more closely. Put simply, most digital printers are either electrostatic ? fusing toner onto the substrate (like photocopiers or laser printers), or they use ink in a variety of different ways e.g. ink-jet (continuous stream, phase change or drop-on demand), dye sublimation to thermal wax transfer. The Indigo machine is electrostatic in part, but also uses special inks and litho technology.

The various electrostatic processes require paper that has a lower moisture content

• To prevent curling from the intense heat in the printer and
• To improve toner fusion

Paper used for most of the ink-jet processes needs to have a softer surface to allow the ink to penetrate and a harder body to prevent feathering of the print. A specialist coating may also be needed to ensure fast drying which makes for smudge-free, high definition, high quality printing.

We can only give some broad guidelines here, but the key points are that:

• Different processes are suitable for different purposes and
• Each process demands particular performance characteristics from the paper used.

Here are some recommendations:

Before printing

• Data must always be supplied in specified file formats
• Always supply a marked hard copy, together with any original artwork or logos
• Treat large, flat background tints with care. Lighter colours are best and over a large area, it is preferable to add a pattern to vary the tone and pitch of the colour. Avoid cheap software!

Electrographic (electrostatic or die-electric)
Widely employed in the wide-format market, paper is selectively charged and a multi-pass process then attracts toner to build up a colour image.

Ink-jet
Drop on demand: liquid ink is heated and tiny droplets are then forced onto the paper. Coated papers are best.

• Phase change: solid wax inks melt for printing and solidify on contact with the paper. Faster delivery and brighter colours that drop on demand. Plain papers or pigmented films are suitable.
• Continuous stream: a constant stream of small ink droplets form a continuous tone effect. Comparatively slow and expensive, generally used in large formats for proofing.

Dye sublimation
Generally used for visual and positioning proofing. Employs a special paper to receive coloured dyes which are vaporised by a print head.

Thermal wax transfer
Wax coated ribbons transfer pigment to the substrate by a heating process. Some machines demand special papers.