SEPIA Guidelines: Preservation issues in digitizing historical photographs

Preservation issues in digitizing historical photographs

Digitization of photographs has quickly become common practice in a great many institutions. It provides an excellent opportunity to disseminate historical images widely and greatly increases access to collections. However, during the digitization process and in choosing equipment process some practical aspects should be observed to prevent damage to the originals and to ensure that preservation requirements are met.

Introduction
About capturing devices
General considerations
Light and heat
Basic advice in choosing imaging devices
Preparation of materials and imaging process
The workspace
Things to watch out for
Further reading
About this document

Introduction

Since the introduction of photography a wide array of techniques has been used to produce images by the action of light. Daguerreotypes, salted paper prints, pannotypes, albumen prints, ambrotypes, and ferrotypes are only a few of the best-known processes. Although produced by different methods, all historical photographs have one thing in common: they are fragile objects and must be treated with great care. Physical and chemical damage can easily occur through handling and (improper) storage.

For general handling procedures and advice on storage, check the SEPIA web publication To have and to hold for basic advice and literature, and the general SEPIA web page for Links and Literature (by keywords ‘handling’ or ‘storage’).

The present text discusses the process of digitization of historical photographs from the perspective of preservation and also briefly lists some preservation issues relating to digital images. This outline aims to point to a few of the numerous preservation issues in digitization of photographs and to offer practical advice.

The wide variety of photographic processes is an important factor in establishing procedures for digitization. Identification of materials is difficult and there are many uncertainties about the long-term effects of heat and light for each type of material. It should also be borne in mind that adverse effects can often not be observed immediately but may still occur over time. For any digitization project, assessment of the risks involved for different types of originals is therefore always a necessary first step.

For more exhaustive treatment of the topics mentioned here, other publications – many of them available on the web – should be studied. In addition, a photographic conservator may offer valuable advice, as well as colleagues in institutions that have experience with digitizing photographs. Careful planning and extensive consultation of available information are essential to avoid problems at a later stage.

About capturing devices

Flatbed scanners are the most frequently used device for digitizing historical photographs. Flatbed scanners can be found in a wide price range and in most cases can safely be used to digitize photographic materials, both transparent and reflexive, with fairly good results.

Overhead scanners are specialized flatbed scanners which light the object from an overhead arm above the object. The advantage from a preservation point of view is that the object does not come into direct contact with either the glassbed or the lid of the scanner. Overhead scanners are specialist, professional devices that are mostly used for scanning bound books.

Drum scanners are mainly used in pre-press and graphics studios for large-format, high-quality commercial photographs; they can produce images of superb quality. However, they are also very expensive, and there are serious preservation issues associated with their use for historical materials because of the use of liquids in mounting originals (which is done to achieve optimal quality).

(Micro)film scanners and slide scanners have been developed specifically for transparancies, particularly roll film, which can be processed relatively quickly with very good results. As they cannot be used for all materials, they may be a less attractive choice for a historical collection containing a variety of formats. A preservation argument discouraging the use of film scanners is the risk of too much pressure on older roll film. In general, the use of any device where the film is moved through the scanner poses risks in terms of preservation. Film scanners are also on average still more expensive than flatbed scanners.

Digital cameras (or cameras with digital backs) are gaining ground as technology develops and prices go down. When a digital camera is used, the object is lighted with lamps as in conventional studio photography, and the light and heat generated by the lamps may be a preservation concern. An advantage of digital cameras is that materials do not come into direct contact with the equipment, which makes them a good choice for digitization of mounted materials or materials with relieved surfaces.

As flatbed scanners are at the moment the scanning device most frequently used, these guidelines focus on the specific preservation problems associated with their use. Some advice relates to digital cameras as an alternative.

For a thorough discussion of quality issues and performance of various types of scanners/cameras, several good guidelines are available on the web.

General considerations

Analyse and select the material you wish to scan. Be aware of the problems attached to certain types of material and the difficulty of identifying photographic processes correctly. A photographic conservator can advise you. The decision to use a scanner or a camera should be guided by the material to be digitized.

In general, a scanner will not cause physical stress to an original. However, if the photograph is in poor condition, or if it concerns materials of large format or with relieved surfaces, a digital camera should be used instead of a flatbed scanner. Cameras have the advantage of not coming into direct contact with the material.

An alternative approach is to first make a photographic copy with a traditional camera which is then used as an intermediate for scanning. This is also advisable when there is a risk that the original will have to be re-scanned several times. The photographic intermediate can, if stored properly, also serve as a long-term reference copy. For instance, with the use of 70 mm rollfilm negatives of good quality can be made that can be processed rapidly by a film scanner.

Consider carefully whether it is worthwhile to invest in expensive equipment. The more sophisticated the equipment, the more expertise is required to achieve optimal results. It may be necessary to hire professionals or invest heavily in training staff to get the most out of an advanced imaging device. Outsourcing can be a more economical option for the imaging process itself (as opposed to selection and preparation, or providing descriptions and metadata). It may also result in greater flexibility if a vendor can offer a variety of technical solutions. Sometimes it is possible to let a vendor do the imaging on-site or to rent equipment.

Light and heat

Scanners

Light
The light sources in scanners are usually either halogen bulbs or cold cathode tubes (fluorescent tubes). Research shows that, because of the short exposure time and the relatively low amount of UV light, scanning does not usually pose a threat to most photographic materials. Also, as the light moves over the material, at each moment during the actual scanning only a small section of the material is lighted.

Heat
The scanner’s mechanism and light source will produce additional heat, resulting in a drop in relative humidity (RH). The temperature on the glass bed may increase to around 40° C, and detrimental effects may be observed if the heat acts as a desiccant on the photograph and causes physical stress. Effects of heat and temperature stress that may typically be observed at this temperature include:

changes in the wax on waxed paper negatives
curling of emulsions/filmbases
layers starting to separate
shrinking of film base

Exposure time
Remember that by setting a higher scanning resolution, the exposure time to the light of the scanner can become considerably longer. Be aware that older scanners may make consecutive scans to produce a colour image. Modern scanners will usually make a pre-scan and even a calibration scan prior to the final scan. This increases the exposure to light and heat.

To be on the safe side, minimize the time an item is placed on the glass bed of the scanner.

Digital cameras

With a digital camera lamps are used to light the object, and the light and the heat generated by the lamps (rather than by the imaging device) need to be monitored. The light can be more intensive and the exposure time will be longer as the whole object is lighted for a certain period of time (whereas with flatbed scanners the light moves, with only a minimal part of the surface being exposed at every specific moment). Therefore, when lighting objects for imaging with digital cameras, strobes or cold cathode tube fixtures should be used. These should not be too close to the object, and light should be blocked off from the object itself until the actual moment of capturing. The heat of the lamps may dry out materials and cause temperature stress. The time that the object is kept in the lighted area should therefore be limited to a minimum.

Whether a digital camera or a scanner is used, in the working area heat will inevitably build up in the course of a working day; ambient conditions should therefore be monitored at all times.

Basic advice in choosing imaging devices

Most scanners meet basic requirements from the point of view of preservation. Therefore, in planning to buy a scanner, it is usually justified to focus on aspects of performance and go for the one that best meets your demands for quality, speed, depth of field, etc.
If you need to digitize materials with special requirements (see ‘Things to watch out for’) the choice of equipment becomes crucial also in terms of preservation.
Given the cost of specialized high-quality equipment and the skills required for operating it, for small projects outsourcing may be a viable alternative.
In choosing the equipment, consult the literature on quality and targets and carefully study the specifications of the equipment.
Remember to check whether the software that comes with the device is sufficient for your needs. The importance of good software in the digitization process is often underestimated.
Do not buy your equipment in a shop around the corner. Use a vendor that supplies professional photographers and offers good service.
Preferably buy all equipment from the same vendor so you can rely on them to make sure everything works together.
Make sure to test equipment and software extensively using your own materials before you take any decisions about what to buy. Consider asking colleagues about their experiences.
Do not use an automatic feeder as there is a high risk of materials getting stuck, wrinkled or torn.
The speed of a scanner may be an important consideration not only a terms of efficient workflow, but also in terms of preservation: with a slow scanner material will be exposed to heat on the glass bed for a longer time.
Some scanners have hardware for scanning installed in the lid; because such lids can be heavy, they may cause damage to materials.

Preparation of materials and imaging process

Analyse and select the materials you wish to scan. As the condition of materials is a consideration in selecting for imaging, a photographic conservator should be involved in the selection process.
Bear in mind there are specific problems attached to certain types of photographs. A photographic conservator can offer valuable advice on the types of materials in your collection.
A condition assessment of the collection can fruitfully be integrated into the selection process, either by a sampling method or by checking the individual items.
Sorting by type of material and format facilitates the capturing process. Avoid processing a variety of materials that requires constant changing of the settings of the equipment; this lengthens processing time and makes it difficult to work efficiently.
Items should be unpacked and prepared in an area separated from the scanning station, preferably in another room, to avoid dust in the scanning area and on the scanner in particular.
Prior to capturing an item may seem to be in need of cleaning, small repairs or other work to stabilize it. Items that are curled or folded may have to be flattened in order to be digitized.
However simple the work may seem, do not take any steps without consulting a professional conservator. Even for a skilled and experienced person cleaning and flattening may be a complex task and none of this should be undertaken without professional advice.
When moving materials from storage to the preparation and the digitization area, take care to minimize variations in temperature and humidity. If materials are stored under very different conditions (cold storage), allow for sufficient time for acclimatization. Monitor conditions constantly with thermometer and hygrometer.
Make sure to have sufficient supplies at hand in the preparation room such as gloves, enclosures and envelopes, and trolleys.
Always document the (temporary) location of the materials to be digitized. Check that each item has a unique number before it is digitized.
Envelopes or enclosures should only be removed at the scanning site, to protect the materials and to avoid loss of documentation.
Do not use post-it notes for attaching information to photographic materials, do not write on the back with ink or markers, and do not use self-adhesive labels or paperclips.
If information is written on the backs of photographs or on unexposed borders, consider digitizing these as well to facilitate identification.

The workspace

The scanning station should be designed as a separate workstation for professional use. It should not be part of a workplace also used for other purposes.

The scanning station should be a dust-proof area. Make sure the room can easily be kept clean and there are no possibilities for dust to gather (curtains, furniture, floor coverings). Dust is not only harmful to photographs, it also affects the quality of the scans. The work flow may be slowed down considerably if lots of specks have to be removed from the images.

The glass bed of the scanner must be kept scrupulously clean and dry. Clean the surface with fiber cloth tissue moistened with an alcoholic solution. Dust can be blown away with an aircan or a similar blowing device.

Light levels, temperature and humidity should be monitored to ensure environmental conditions do not pose a risk for original materials. Be aware the heat generated by equipment may, after a number of hours, cause the temperature in a closed room to go up considerably.

No sun should be allowed to enter the room and light levels should be kept low, also to be able to check the image quality on the screen. Light levels of 35-70 lux are recommended (ISO 3664).

There should be a lot of clean table space to lay out materials and to move them about easily. Working in a cramped space makes it harder to observe correct handling procedures and more quickly leads to accidents.

Do not keep more materials in the digitization area than can be processed in a short amount of time. Keep materials covered as much as possible and do not stack photographs on top of each other.

Make sure cleaning staff receive special instruction for maintaining the scanning area and the preparation room.

Things to watch out for

Dust Loose dust can be removed from an item before scanning by careful, soft blowing with canned air or the equivalent. Do not use air pressure tools and watch out for signs of surface flaking and other loose parts.

Relieved surfaces Items with relieved surfaces should not be placed under the lid of a scanner as the weight of the lid may damage them or crack the image. This refers in particular to pannotypes, daguerrotypes, prints mounted on curved board, wrinkled acetate negatives, etc. A better procedure is to place them on the glass bed and cover them with a dark cloth, to prevent stray light and achieve better scanning results.

Waxed paper Waxed calotypes (waxed paper negatives) must be handled with extreme care as the wax may easily crack. They are also quickly affected by the heat coming off the glass bed off the scanner.

Risk of abrasion Some historical processes produced photographs with especially sensitive image surfaces or structures. Of these daguerrotypes and pannotypes are usually mounted in a case under glass. They must not be dismounted but should be digitized with the glass cover. Other images, such as ambrotypes or ferrotypes (tintypes), generally do not have a protective mounting but are equally sensitive to abrasion of the image surface through direct contact.

Photographs that are extremely sensitive to light Salted paper and paper treated with similar processes, as well as badly fixed prints or negatives, will be damaged whenever they are exposed to light: the image will darken. This will also happen when they are exposed to the light of a scanner. Be sure to check that such materials are not among the photographs selected for scanning.

Photographic albums Albums should preferably be digitized with the use of a (digital) camera. That way there is no risk of cracking the spine and damaging the overall structure of the album, and the quality of the digital images will in most cases be better.

In general

For most of the materials mentioned above, using a digital camera will be a better option, provided that precautions are taken to limit exposure to light and heat.

With such fragile materials, the advice of a photographic conservator is indispensable. Also consult a photographic conservator whenever you cannot with certainty identify the materials or are unsure about handling procedures.

Ten Commandments

Principles for digitization of historical photographs

Photographs are an essential part of our cultural heritage. They are to be treasured for what they tell us about our past, for their documentary and artistic value, and as a record of the history of photographic processes.
Digitisation of photographic collections should be encouraged to facilitate access for a large audience. Digitising photographs is all the more urgent because some common types of photograph are quickly deteriorating. Damage caused by handling of fragile originals can be limited by the use of digital copies.
Digitisation is a tool, not an end in itself. Selection of photographic materials for digitisation should be based on a thorough understanding of the nature and potential use of the collection.
A digital project starts well before the scanning of the first picture. Investments made in careful planning to define the aims, priorities, technical requirements, procedures and future use are essential for an efficient workflow and a result that meets expectations.
Digitisation of photographic collections fundamentally differs from digitisation of text or line art. The creation of a digital image requires photographic expertise with ethical judgement. Even with the best equipment, capturing the essence of photographs in a digital format is a sophisticated activity and can never be a routine job like the production of photocopies.
Digital images of photographs constitute active collections that require regular maintenance. Provisions to upgrade digital collections to keep pace with the changing computer infrastructure should be made at the start of a project. This is necessary to avoid digital collections created at considerable cost becoming inaccessible over time.
Digitisation of photographs should not be the sole responsibility of one department. A good digitisation project is conceived as teamwork, combining expertise on imaging, collection management, information technology, conservation, descriptive methods and preservation strategies.
8. In every project for digitisation of photographs the input of specialists in photographic preservation is essential. Their advice is required for the best selection of materials. They should be consulted on how to integrate preservation measures in the workflow, on how to handle fragile materials and on the equipment used to avoid damage to the originals.
9. Preservation specialists should be trained to advise on strategies for management of digital assets that are in line with the overall preservation policy of the institution.
Museums, archives and libraries have a strong interest in the development of international standards on which a strategy for the preservation of digital collections has to be built. Their active involvement is essential to ensure that the long-term view of heritage institutions is represented in groups working on standards.

<The 10 commandments are also available in French and Spanish>