Scanning glass plates

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Glass was the main carrier for the negative emulsion until the 1920s. What many people don’t know is that glass was also used for 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.

6x13cm glass stereoview. A printed diapositive image.
6×13 cm glass stereoview. A printed diapositive image.

In this article an Epson V700 scanner is used to scan 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 very good scanner. To understand more about scanners and resolution, this excellent article on Imaging Resource is recommended.

The scanning software used is Ed Hamrick’s VueScan. The extensive options and clear user interface is a good alternative to the standard Epson software that comes with the scanner.

Scan preparations

Before start scanning, I recommend these precautions:

  • Wear cotton gloves when handling the glass slides. They not only protect 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 detect, but there’s a trick. The deterioration 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.

The blue glow of silver oxide at the bottom left, this is the emulsion side
The blue glow of silver oxide, this is the emulsion side

Scan holders

In general, 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 can create 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 can have exotic sizes, like 45x107mm and 6x13cm stereoscopic slides. Creativity is needed to place the slides in the holder.

Luckily, the 6x13cm slides fit exactly in the medium format film holder. To place the 45x107mm slides in this holder, a little plastic strip is used to make it fit. Again, a little creativity is needed.

45x107mm slide in the scan holder which rests on the right side on a plastic strip
45x107mm slide in the scan holder which rests on the right side on a plastic strip

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. You’ll have to find out what’s the best height for your scanner by doing some tests and compare the results.

Epson V700 scan holder height
Height adjuster with arrow facing the circle (3mm)

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 negativeB&W positiveColor positive
ModeTransparencyTransparencyTransparency
MediaB/W negativeSlide filmSlide film
Bits per pixel16 bit Gray16 bit Gray48 bit RGB (16 bits per RGB channel)
Output color spaceGrayGrayAdobe RGB

What’s the best scan resolution?

The right scan resolution depends on what you want to do with the scan. A photo which is scanned with a scan resolution of 300 dpi will result in a print with a print resolution of 300 dpi when the printer prints the actual size of the original photo. This is the starting point, as 300 dpi is considered the base resolution for a good quality print. If you want to publish your scans on a website, a lower resolution is usually sufficient. A higher scan resolution is useful if you want to enlarge the original, but it doesn’t necessary improve the quality.

Things are different when scanning glass negatives. A negative is the source material to be developed. A high resolution is important to squeeze as much information as possible in order to make a good positive image through post processing. A resolution of 2400 to 4800 dpi is recommended here as starting point.

Scanning without holders

If your glass slides don’t fit in any holder, there’s still a way to scan the glass slides by placing them on the scanner’s glass plate but you should be aware of its limitations and it isn’t supported by each scanner. The Epson V700 has a Dual Lens System with two lenses for scanning reflective media and transmissive media. By choosing Transparency 8 x 10 in VueScan, the lens for transmissive media is used and the glass slides can be placed on the scanner’s glass plate. It results in sharp scans, but the scan resolution is limited to 4800 dpi and you have to beware of Newton’s rings. Try what works best for you.

VueScan mode setting

Good to know

  • 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. 48 bit is preferred when the extra file size is not a problem. It will not affect the scan speed.
  • An output color space of sRGB would be suffice for the toned color positives, but the wider Adobe RGB space is recommended. You can always choose a more narrow color space later.
  • Don’t apply all kind of filter settings like grain reduction, sharpening, contrast enhancements or restoring fading during scanning. Post processing with Adobe PhotoShop or Lightroom gives better control over the adjustments.

After you’ve scanned your image you might want to do some restoration work to repair damage.

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