Scanning Glass Plate Negatives and Positives
I have a collection of vintage black & white glass plate negatives and color positives. Glass was the main carrier for the negative emulsion until the 1920s. What most people don't know is that glass was also used for creating positive images. This mainly concerned glass slides that could be viewed with a stereoscope or projected with a magic lantern. Scanning glass plate negatives and positives comes with some challenges that will be addressed in this article.
This stereoscopic image of the First World War was produced around 1920 and could be viewed with a stereoscope
Epson V700 and VueScan
I use an Epson V700 scanner for scanning glass plates. This high-quality flatbed scanner has an optical resolution of 4800 dpi for reflective media (photos, documents) and 6400 dpi for transmissive media (film, glass). The optical density is 4.0 Dmax. Epson now has a successor with the V800, but the V700 is still a great scanner that serves me well. To understand more about scanners and resolution, I recommend this excellent article on Imaging Resource.
The scanning software I use is Ed Hamrick's VueScan. I prefer the extensive options and clear user interface over the standard software that came with my scanner.
Scan preparations
To prepare your scan, you need to perform these steps first:
- Wear cotton gloves when placing the glass slides. It not only protects the sensitive emulsion of your slides, but also ensures that you do not leave fingerprints and scratches on the scanner's glass plate.
- Remove dust particles from the slides with a blower, followed by a soft brush (only when needed).
- Place the slides in the scan holder with the emulsion side facing the scanner glass plate to get the best scan results ("emulsion down").
The emulsion side has a matte surface. Sometimes it's difficult to recognise this, but there's a trick. The decline of vintage slides often causes the metallic silver of the emulsion migrating to the surface and it oxidises to silver oxide. It produces a blue or purple glow. It's especially visible in the darkest parts because that's where the most silver is. It can help you to the detect the emulsion side.
this is the emulsion side
Scan holders
It's not a good idea to place your glass slides directly on the scanner glass plate. You will not get sharp results and glass-on-glass creates Newton's rings. You preferably place the slides in the scan holder, but this can be challenging.
A flatbed scanner usually comes with a number of scan holders for 35mm film and medium format film. The problem is that vintage glass slides have exotic sizes, like 45x107mm and 6x13cm. You'll have to be a bit creative to place the slides in the holder.
Luckily, the 6x13cm plates fit exactly in the medium format film holder. I also put the 45x107mm slides in this holder, but for this format I've added a little plastic strip to make it fit. Again, a little creativity is needed.
Holders height
The Epson V700 doesn't support auto focus. You will have to adjust the height of the holder to achieve perfect focus and get sharp scans. The holder has height adjusters that can be placed in different positions to change the height of the holder. With no height adjusters installed, the gap between the holder and scanner plate is 2,5mm. When installed with the arrow facing the plus, the height is 3,5mm and with the arrow facing the circle (default value), the height is 3mm.
I get the best results with a height of 3mm, but you'll have to find out what's the best height for your scanner by doing some tests and compare the results.
VueScan settings
After your preparations and with your slide in the holder you can switch to VueScan. Here's an overview of the most important VueScan settings. You'll find comparable settings if you use other scanning software:
| B&W negative | B&W positive | Color positive | |
| Mode | Transparency | Transparency | Transparency |
| Media | B/W negative | Slide film | Slide film |
| Bits per pixel | 16 bit Gray | 16 bit Gray | 48 bit RGB (16 bits per RGB channel) |
| Output color space | Gray | Gray | Adobe RGB |
Some remarks:
- Make sure that if you have made a 48 bit scan (16 bits per RGB channel) that you save it as a 16 bit file, like 16 bit TIFF. If you choose 8 bit TIFF or JPG, you will lose the extra bits of scanned information because it doesn't "fit" in these file types.
- The color positives are mainly black and white images with a color tone. In fact, a color depth of 24 bit (8 bits per RGB channel) should be enough, especially if you don't plan to do a lot of post processing on the scanned file. I choose 48 bit anyway as it does not affect the scan speed and the extra file size is not a problem for me.
- An output color space of sRGB would be suffice for the toned color positives, but I choose the wider Adobe RGB space. I can always choose a more narrow color space, like sRGB, later.
- I don't apply all kind of filter settings like grain reduction, sharpening, contrast enhancements or restoring fading during scanning. I do my post processing with Adobe PhotoShop or Lightroom which gives better control of my adjustments.
What's the best scan resolution?
The right scan resolution depends on what you want to do with the scan. A scanned (paper) photo with a scan resolution of 300 dpi will result in a good print with the same size when printing with a print resolution of 300 dpi. This is the starting point. A higher resolution is useful if you want to enlarge the original, but it doesn't necessary improve the quality. I scan my 45x107mm and 6x13cmm positives with a resolution of 1200 dpi so I'm able to make a descent print if I want to.
It's different for scanning glass negatives. A negative is the source material to be developed. A high resolution is important to squeeze as much information as possible from the negative in order to make a good positive image through post processing. I usually scan my negatives with a resolution of 1200 to 4000 dpi, depending on the size of the negative.
After scanning your glass plates, the next step may be to restore your scans digitally. I share my workflow and best practices in Restore old Photographs.