IPI’s Guide to: Preservation of Digitally-Printed Images Electrophotography Dye Sublimation Inkjet Introduction Should it be called The purpose of this guide is to provide basic information digital? on the preservation of digitally-printed images in scholarly and cultural collections. While there are Though digital many printing technologies for output from computers, technology was this guide focuses on the three most popular forms used in its creation, of image hardcopy: inkjet, dye sublimation, and the printed object itself is not digital. electrophotography. It includes information to help It is recommended understand these technologies, strategies to develop that the specific print identification skills, a schema for media naming, process (inkjet, as well as recommendations for storage conditions, dye sublimation, selecting enclosures, proper handling, and display. electrophotography,) Although there are a wide variety of collection object be used instead types created using digital-printing techniques, to describe the including images, manuscripts, books, ephemera, materiality of the etc., this document focuses solely on digitally printed object. images. The intended audience for this guide is collection care personnel in institutions; however, it Is it a print or a may also be useful to photographers, artists, and the photo? general public. There has been some debate as to Understanding the Technologies whether an inkjet Three technologies are used to print the majority print from a digital of digital images: inkjet, dye sublimation, and camera file should be electrophotography. None of these technologies was called a photograph originally developed for computer output. Each process or a print. Since the has an analog predecessor, which was later modified to inkjet paper is not print digital images. light sensitive, inkjet printing is technically not photographic in Inkjet nature. The preferred The first printing device to utilize drops of ink emitted term is print. from a nozzle was developed by Lord Kelvin in 1867 to record telegraph transmissions. It was not until over a century later that inkjet printing devices were adapted for computers and became commercially available. In 1984, Hewlett Packard (HP) introduced the first desktop inkjet printers. From then, rapid developments in technology led to improvements in print quality, the inclusion of color, and the printing of images. Other major advances included color desktop inkjet in 1987, wide-format printing in 1993, full-color pigment inkjet in 2000, and inkjet digital presses in 2007. 1 All inkjet printers use drops of ink emitted from a nozzle to create colored dot patterns on paper (or What’s in a Name? other printing supports). There are two main types of inkjet printers – continuous and drop-on-demand. The term inkjet comes from the fact that ink Continuous inkjet printers were the first to be used is jetted from a nozzle for high-end, fine-art inkjet images. The technology’s to the paper. The ink original purpose was to create printer’s proofs in must cross an air space commercial printing facilities. Continuous inkjet printers before striking the emit a constant stream of charged ink droplets. The paper. The print head drops can be electronically deflected into a recycling never directly contacts system or allowed to pass and hit the paper, which is the paper surface. attached to a metal cylinder rotating under the print head. A variety of papers can be used as long as they are flexible enough to fit around the printing drum. Currently continuous inkjet is used only in inkjet digital presses. Drop-on-demand printers eject drops of ink only as required. There is no need for a deflection and recycling system. There are two different ways in which ink can be forced from the printing nozzle. The first, called thermal inkjet printing (also known as bubble jet printing), utilizes a heating element inside the print head to temporarily vaporize some of the ink and create a bubble. The formation of the bubble forces a drop of ink out of the nozzle. In the second type, a ceramic piezo electric tile flexes into the ink reservoir in the print head and physically forces a drop from the nozzle. All desktop inkjet printers and most wide format printers use drop-on-demand technology. Inkjet inks can be water-based (aqueous), solvent-based, UV-curable, latex, or wax. These carriers of the colorant are the ink’s vehicle. The majority of inkjet-printed objects in cultural collections were created using aqueous inks. The solvent, UV-curable, and latex inks are primarily used for industrial and commercial applications and only rarely for image or fine-art printing. Wax inks are used primarily for text, as opposed to images, as image quality is low compared to aqueous inkjet. The ink colorants may be either dye or pigment. Dyes are organic compounds that are soluble in the vehicle. Pigments are inorganic, organic, or a combination of both, but they are always insoluble in the ink liquid. Printers use either all dyes, all pigments, or a mixture of the two - where the colors are dyes and the black is pigment. In general, pigment colorants have tended to be more resistant to environmental deterioration (moisture, pollution, light), while dyes have been more durable during handling. Inkjet ink sets can be larger than those used in other printing methods. 2 Continuous Inkjet piezo element Continuous inkjet printers pump emit a constant stream of charged ink drops. The stream of charged drops can be electronically deflected into a recycling system or allowed to pass and hit the paper (or the reverse where the deflected drops hit the paper and the undeflected drops enter the recycling system). charge electrode image signal deflector ink droplet recycling gutter paper Drop-on-demand Inkjet Drop-on-demand printers only eject drops of ink as required. There is no need for a deflection or recycling system. image signal 3 While the most basic inkjet printers only use cyan, magenta, yellow, and black colorants, some high-end What is fine-art inkjet printers include inks of varying tone (such as paper? greys, light cyan, and light magenta) as well as other hues (such as orange, red, green, blue, or violet) to The term fine-art improve the range of colors that can be produced. paper may seem too vague to some, A variety of papers are used to print inkjet images. especially those that Because plain paper absorbs ink in ways that diminish have cared for works color and line quality, paper manufacturers have of art on paper made created special image receiver layers (IRL) for inkjet with other media papers. These very thin layers are intended to keep the such as watercolors ink at the paper’s surface and make the colors brighter or drawings. There and the lines sharper. These coatings were first applied are many supports to fine-art papers such as watercolor paper. The made specifically coatings were typically mineral particles in a starch or for inkjet printing polymer binder. Ink was absorbed into spaces between that rely on papers the particles hence the name porous-coated paper. like these; however, Early on, the mineral particles were too large to form a special image a smooth glossy surface, so the papers were always receiver layer has matte or took on the texture of the underlying paper’s usually been added surface. during manufacture to improve image As inkjet printing of images grew more popular, quality. These manufacturers began attempting to make papers that papers are very mimicked traditional photographic prints such as resin- different from the coated (RC) chromogenic and fiber-based, silver- thin, inexpensive gelatin papers. The RC papers are coated on both sides office papers used with a thin polyethylene layer. The layer on the back in inkjet printers to is clear to show company logos printed on the paper, create documents. while the layer on the front is pigmented white to make For the purpose the image appear very bright. There are two sub-types of distinguishing of inkjet RC papers – polymer-coated and porous- these very different coated. types of papers, it is recommended Polymer-coated inkjet RC paper swells and absorbs that “fine art” and aqueous inks as they are ejected onto the paper by a “plain” paper be used printer. These papers are typically used only with dye respectively. inks because many pigment particles are too large to be absorbed into the coating. There are several advantages to these papers. One is that they can be manufactured to a higher gloss than the porous-coated papers, which is preferred by many photographers. 4 Also, because the ink is absorbed fully into the polymer layer, the prints are more resistant to abrasion during What is coated handling or fading caused by airborne pollutants. These paper? papers have, however, become less popular because they can take several minutes to several hours to fully The word coating dry. Handling polymer-coated prints while they are is used in different ways across the still wet can lead to smudging of the inks. They are also paper, printing, sensitive to high humidity bleed. and photographic industries, and As stated above, porous coatings were first used on there is no standard fine-art papers. What differentiates porous-coated definition. Coating photo paper from earlier fine-art papers is the RC sometimes refers support to which the mineral coating has been applied, to sizing layers as well as the size of the particles used in the coating. In applied to the paper addition to having a plastic-laminated paper support, during manufacture the particles used in the ink receiver layer are smaller to improve the and more tightly compacted than in most fine-art paper’s smoothness, papers significantly increasing the glossiness of the opacity, density, paper’s surface.