Glass plate negatives are one of the most fragile mediums that archivists and holders of special collections work with, and digitising glass plates requires a special level of care and attention.
Due to their extreme fragility, digitisation is often the only way to enable access to the content of heritage glass plate negative collections without putting them at risk. As a result of this, we are frequently approached by archives and libraries, as well as private organisations, to scan glass plate collections.
As part of our ongoing “How to Digitise…” blog series, below we share our best practice tips to safely digitise glass plate negatives, whilst producing the best quality image outputs…
Digitising Glass Plate Negatives – Top tips
Use a DSLR and Lightbox for Image Capture
Whilst glass plate negatives can be digitised using a flatbed scanner (pictured below) and achieve good outputs, we believe that a DSLR camera and lightbox set up (pictured above) produces the best image results. Although this is, of course, dependent on the model and spec of the scanner/camera being used.
It is important, if you do choose to use a flatbed scanner to digitise glass plate negatives, to ensure it is calibrated to the correct settings for the medium. This is often called “Transparency” mode, but varies between different models of scanner. Details of the relevant settings needed can usually be found in the user guide for your particular scanner or on the manufacturer’s website.
Handle with Gloves
When handling glass plate negatives during the digitisation process (and in general), operators should wear nitrile gloves at all times, in line with National Archives guidance. In addition to helping prevent contaminants (such as oils/acids from skin) being transferred to the plates, wearing gloves also protects the operator’s hands from their potentially sharp edges.
Clean the Plates before scanning
There may be tiny specs of dust and dirt that have collected on the surface of the glass plates, which can adversely affect the quality and clarity of the images produced when digitising them.
To remove the majority of this dirt, the plates can be lightly dusted with a hand held bulb duster (air blower) before digitisation. However it is important to consider the condition of the glass plate before dusting as, if the emulsion is deteriorating or flaking, dusting can damage the image.
Scan Emulsion side up? Or down?
If using a DSLR/lightbox setup to digitise your glass plates, we recommend placing them with the emulsion (matte) side facing upwards. This both ensures the clearest capture as the side bearing the image is closer to the camera lens, and reduces the risk of damaging the image on the plate as there is no direct contact with it.
If using a flatbed setup, we recommend placing the glass plates onto the scanner with the emulsion (matte) side face-down to obtain the clearest image when using a scanner with a variable focus scanning head. Scanning with the non-emulsion (shiny) side of the plate facing down can result in a slightly distorted image and increases the likelihood of Newton’s rings interference (see below).
Furthermore if you are using a fixed focus flatbed scanner to digitise your negatives, it is important to check the focus plane of your particular model and the recommendation of the manufacturer on how to place glass negatives on the scanner bed to achieve the best image result.
Regardless of which setup you use, the resulting image produced can sometimes be the “wrong” way around. This can be easily inverted using graphical software during post-processing or even automatically at the point of scanning with some flatbed machines. It is important, however, to ensure that the software you use causes no loss in quality when inverting the image.
Protect the Scanner Bed
If using a flatbed scanner, the glass of the scanner bed and the glass plates themselves can be scratched easily when placing or removing the plates, as well as by tiny particles of dirt/grit that may remain on the plates after cleaning. This can obviously negatively affect image quality.
To help prevent this happening, if using a scanner with a variable focus head, we recommend placing a transparent mylar or acetate sheet over the scanner bed. This, together with taking care when placing plates onto the scanner, will protect both the scanner bed and the glass plates themselves from being damaged.
This technique can still be applied using a scanner with a fixed focus head, but may result in some minor loss of image quality due to the glass plate being further away from the scanner head than normal.
Minimise pressure on the Glass Plates
“Glass plates are often broken… when the lid of the scanner is closed over them” – JISC guide to digitising difficult objects.
Depending on the thickness of the glass plates being scanned and the depth of the bed of the scanner being used, there may be some pressure placed on the glass plates by the hood of the scanner during the digitisation. This can be potentially damaging to the plates.
The risk of plates potentially being damaged in this way can be mitigated by raising the hood of the scanner slightly (approx. 1-5mm is ideal). Some scanners allow the height of the hood when closed to be adjusted, but this can also be achieved by placing felt, foam, or cardboard strips, around the edge of the scanner to cushion and elevate the lid slightly.
Not having the hood in contact with the glass plate also decreases the potential risk of the plate being inadvertently lifted up and dropped (due to glass-on-glass suction) by the hood when the scanner is opened.
Watch out for “Newton’s rings”
This only really applies to digitising glass plates with a flatbed scanner – the glass-on-glass contact between the plate and the scanner bed can create a pattern of distortion on the images produced, known as Newton’s rings. Scanning emulsion (matte) side down, as we recommend above, goes some way to mitigating the risk of these distortions. Preventing the glass plate coming into direct contact with the scanner bed can also help to avoid them – this can be achieved via a transparent mylar or acetate sheet (as described above).
Resolution, Format & Pixel Depth
As standard we recommend scanning glass plate negatives at a minimum resolution of 600ppi, depending on the size of the original plate, to uncompressed TIFF format as a master file. Compressed lower resolution surrogate JPEG files can then be produced from these master TIFFs.
In its guidelines, JISC recommends digitising glass plate negatives at the highest pixel depth available to capture as much tonal information as possible. However this can be impractical, due to large file sizes and longer scanning times (which, if outsourcing scanning, can result in increased costs). We find that, for the majority of the organisations we work with, 16-bit pixel depth is an ample amount for their glass plate digitisation projects.
Full Colour vs Greyscale Image Capture
Due to the nature of glass plate negatives, all of the images in the collections that we handle are in greyscale. Over time, as the emulsion coating on glass plates begins to deteriorate, the image on the plate can fade and discolour – turning yellow/brown. This alters the appearance of the plate and presents an important question when digitising…
To scan the plate in full colour, capturing yellowing and all, in order to preserve the truest representation of the archival object at that point in time.
To scan the plate to greyscale (as it was originally produced) in order to partially restore its former appearance and to take advantage of the greater image clarity this affords (due to the increased contrast).
As always, the decision to capture in colour or greyscale should be made on the basis of the aims for the digitisation project. If the intent is simply to preserve the content of the glass plate collection and allow access to it digitally, then greyscale will likely provide the clearest and most useful images. However if the purpose is to create a master archival image of the plate, then full colour may offer the truest representation of the artefact.
From Negative to Positive
The last stage of digitising glass plate negatives is converting the master TIFF files generated from negatives to positives. This can be done automatically during the scanning with some models of flatbed scanner or can be carried out as part of post-processing using a suitable graphics program (such as Adobe Photoshop).
Find out more about Glass Plate Negative digitisation