Sparkling Pictures Big as Life: Preserving Slide Film

Slides are imbued with nostalgia; people born before 2000 will likely have fond memories of sitting in a friend’s or relative’s living room watching slide after slide, projected onto a white screen or wall, depicting family holidays, birthday parties, and other significant events.

“For sparkling pictures big as life…Kodak 35mm colour slides… Your living room is your theatre” was one of Kodak’s commercial slogans in the 1950s. While slide film is rarely used by photographers anymore, we likely all have some in our family archives — tucked away in boxes inside wardrobes or cupboards gathering dust.

Advertisement in LIFE Magazine, page 90-91, 5 May 1958

Few methods to display or view slides are available today, the projectors are out of production and are harder to come by, which increases the urgency to digitise these items so they can be shared with generations to come. Unfortunately, slides are prone to degradation, the troubled history of permanence in colour photography (an extreme example being the rapidly fading Kodacolor prints from the 1940s and early 1950s) is a continual reminder that these items will not last forever.

Invented in 1935, slide film (sometimes called reversal film) is different to negative film because it is a positive image on a transparent base. Slides are individually mounted transparencies and were not typically used to produce prints — instead, they were created for use in a slide projector so they could be viewed by a larger audience.

Its name comes from the way the mounted transparency “slides” from one image to the next when placed in the projector’s carousel or magazine. Most slide film is 35mm and is framed in a 2×2 inch plastic or cardboard mount – sometimes short inscriptions are written on the frames indicating what or who the photo shows. A slide is a very high-resolution positive photograph, it can be magnified by a factor of 100 and still maintain a detailed and crisp projected image. The colours produced by slide film are rich and accurate — they often come close to the real colours and tones present during exposure.

Slide film was popular for around 35 years; from mid-1960 until the mid-1990s. At the time, when there were limited technologies available, slides were an easy and relatively cheap way to create high-quality images that could be shared with groups of people.

Safekeeping your Slides

Photographic material is fragile and can be destroyed or damaged by poor storage, the inherent properties of the film itself, and frequent or improper handling.

Slide film is prone to degradation because of the delicate nature of the three different components it is made from, including:

  1. The support which is usually plastic film (nitrate, acetate, or polyester).
  2. The binder or emulsion is made of gelatin that holds the final image material to the support.
  3. The final image material is made of colour dyes suspended in the emulsion or binder layer.
Slides from a family slide collection, c.1960.

Storage recommendations

Deterioration of photographic material, like slide film, can be exacerbated by poor storage conditions; incorrect or fluctuating humidity levels, high temperatures, inappropriate storage enclosures, and prolonged light exposure.

Relative humidity (RH) is a measure of how saturated the air is with moisture — all photographic material is sensitive to low, high, and fluctuating RH. When RH is high, the gelatin binder of the slide can become soft and sticky which makes it very vulnerable to handling damage and deterioration like mould. Low RH will cause the binder to shrink and crack while the support curls.

Most slide films will deteriorate rapidly in temperatures over 18°C and they require cool or even freezing conditions for extended-term storage. Generally, the colder the storage the better, especially if RH is also low (30-50%).

One common consequence of incorrect humidity levels is vinegar syndrome – a condition that can affect all film with a cellulose acetate base. If your slides have vinegar syndrome, a strong vinegar odour is a tell-tale symptom, especially in the later stages of decomposition. The smell comes from the chemical breakdown of the acetate into acetic acid which is accelerated in humid conditions.

film degradation_NZMS
An example of what happens to film in the later stages of vinegar syndrome.

Vinegar syndrome shrinks the film base which causes the gelatin emulsion to separate from it, lifting up in folds. The film also becomes very brittle, losing its suppleness, with jagged or “spoked” edges. Unfortunately, once the decay starts, it cannot be reversed. The affected films will infect other “healthy” films, so they should be separated immediately to prevent this from happening.

Slides should be stored in acid-free cardboard slide boxes, ideally those that can pass the Photographic Activity Test (PAT). PAT is a test developed by the Image Permeance Institute that explores the possibility of chemical interactions between photographs and other materials after prolonged contact.

Plastic sleeves are not a good option, especially if relative humidity increases because the emulsion will stick to the plastic and become damaged. It also increases the risk of mould growth by trapping moisture.

Left: a slide with colour fade that produced a red cast. Right: The same image after post-capture processing to reduce the colour shift.

Slides should be kept in the dark — they will experience dye fading or colour shifting with even a small amount of light exposure from visible or UV light. Different film stocks are affected differently; Kodachrome has excellent colour stability in dark storage, but it can fade significantly within only 10 minutes of projection! Fujichrome film dyes are more stable under light exposure but fade at a quicker rate than Kodachrome in dark storage.

The ideal storage conditions for slide film are:

  • A reasonably dry environment (30-40% humidity).
  • Cool temperatures (4°C or below).
  • Dark, with as little exposure to light as possible.
  • Acid-free cardboard box enclosure.

Handling and cleaning advice

Nitrile gloves should be worn while handling any photographic material. Gloves ensure you do not mark the film with fingerprints and stop the transfer of organic material. In the long term, the oils from your skin can cause deterioration such as mould and even colour shifting from chemical reactions.

Before scanning film, it is important to remove all dust and other debris from the slide’s surface. Not only can dust damage the highly sensitive photographic emulsion, but it can also hide important details of the image. Dust is especially noticeable on scanned 35mm film because the photographic material is so small (24mmx35mm) and even one dust particle can cover a large section of the slide. Usually, an air-blowing tool, called a blower, or compressed air will easily remove dust.

If the slides have been exposed to humid conditions, the dust might have stuck to the surface of the film. This type of dust is more difficult to remove and you will need to use a soft brush, one intended for use on delicate surfaces, to gently brush it away. Once all dust has been removed your slides will be ready for digitisation and you can be assured you will get the best result possible.

Flatbed and drum scanning technologies used to be the primary method for digitising film such as slides, however the introduction of digital cameras means that these methods are now outdated. Although the three scanning technologies currently coexist, servicing or technicians able to maintain options such as drum scanning are scarce. The main difference between the three technologies is the scanning time per slide. It is much quicker to use a digital camera, and it still produces a digitised image of equal (if not better) quality.

At NZMS we digitise slides using a high-resolution digital camera attached to a stand, over a lightbox and a slide cradle. This allows us to capture a high-quality image and ensure the digitised file accurately represents the original photograph.  Once captured, our technicians can neutralise the white balance, apply colour corrections so the photograph doesn’t have a particular hue or colour cast, and fix any fading from film deterioration.

Left: a scanned slide before any post-capture adjustments. Right: The same slide after colour correction.

Why Digitise?

Cultural heritage institutions like museums, archives, and libraries often have extensive photographic collections, which can include slide film.

Digitisation of this material not only ensures preservation but also contributes to a common goal of increasing public engagement. Digitisation allows for the creation of online digital collections that can be remotely accessed — which is particularly relevant in the aftermath of COVID-19 lockdowns.


A slide photograph of children attending a birthday party c.1966.

However, it is not only institutions that benefit from digitisation, individuals with family slide collections will find it useful as well. Digitisation guarantees preservation; digital images do not deteriorate. They cannot fade, grow mould, discolour, be scratched or torn, and they are not adversely affected by acids or materials present in storage papers or plastics. Digital images can be reproduced or copied to various mediums without compromising the original while maintaining image quality. This means you can easily distribute copies to family and friends.

Digitising slide film collections is important for the protection of cultural heritage, both for institutions and those interested in preserving their family’s history. Slides are not permanent items, they will not last forever, and are constantly fading or deteriorating — the disappearance of this material can only be halted through digitisation.

NZMS is committed to ensuring photographic slides are around for future generations and our technicians are trained to handle delicate material with the utmost care. Get in contact with us for a quote!  

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