Shades of grey – A look at the tones of a salted paper print

Last night I felt like doing a salt print. Why not do some measurements on it? Just having fun with the photospectrometer, really! And sometimes, just playing can give some insights. Perhaps a surprise, here and there?

A salt print or salted paper print is the most basic form of a silver halide print. It’s literally just silver chloride on paper. I’ve always enjoyed making salt prints, as they are so simple to make – and yet, they do require a little care if I want to get it right.

My workflow for salt prints is as follows:

  • Select a suitable paper. What makes a paper suitable? Well, try it out, and if it works well, it’s suitable!
  • Take a 1.6% sodium chloride (NaCl) solution. Brush onto the paper with a decent quality spalter brush. Dry with hairdryer at high heat.
  • Mix drops of 11% silver nitrate solution with a few drops of 10% citric acid. A ratio of 10 drops of silver nitrate to 2 or so drops of citric acid works well. The citric acid prevents fogging. Blast with hairdryer again.
  • Expose in contact printing frame to UV light. I currently use a 365+395nm combined wavelength light source, but any kind of UV works OK for salted paper. An exposure time of 10-15 minutes tends to work well for a decently powerful light source. Silver chloride is kind of slow.
  • Rinse print for a few minutes in tap water acidified with a pinch of citric acid. The initial rinse water will be milky. A second or third bath should come out crystal clear.
  • Fix in Fuji Hunt C41 fixer diluted 1+10 for 4 minutes. I don’t subscribe to the whole myth (IMO) of “alt. process prints should be fixed in a weak sodium thiosulfate fixer or otherwise delicate highlights will degrade”. My experience is that if a weak fixer is used, staining of the prints will occur in due course. Conversely, if a stronger fixer is used but sufficient exposure is given, there’s no degradation of highlights (nor of shadows/dmax) and prints come out fine. I suspect many people are solving problems with weak negatives by using a weak fixer and thus end up compromising print quality. To each their own.
  • Wash thoroughly; I generally put the print face down in a tray filled with water and let the tap trickle water as the tray gently overflows. This effectively creates a constant but slow current of water across the print surface. I wash for 30-60 minutes. Insufficient washing will result in stained prints. Hang to dry after wash.

I usually gold tone my salted paper prints, at least if they’re intended to be a ‘final’ print. I tone using a thiourea-gold toner as described by Sandy King on this webpage about Van Dyke prints. About 20ml of toner is sufficient to tone a 4×5″ print to completion. This uses 1ml of 1% gold chloride, and although gold chloride is heinously expensive, it’s such a small amount that it doesn’t hurt too much. And the prints improve dramatically in terms of density and hue, in my opinion. I tone after fixing, as the whites generally stain very easily if the prints are toned before fixing. Sometimes, I wash and let the prints dry so I can evaluate them before toning, but often, I tone immediately after fixing (and a brief wash) and then wash thoroughly.

All considered, making salt prints is a simple process; it just takes a little patience.

Last night’s print is this one:

This was made on Fabriano Tiepolo paper. I bought a scrap of this to test it, but upon receipt I found the texture a little too course to my taste. It’s a fine paper, but I was already using Schut Laurier, which I prefer. In the scan, ignore the course rendition of some of the tones (e.g. step #17 on the Stouffer); this is just the scanner getting into a catfight with the paper’s texture, and losing miserably. The actual print is beautifully smooth in tonality, which is one of the reasons I like salted paper prints so much:

Oh, a quick word about the image itself. It’s a pretty random negative I picked; it’s an accidental double exposure I made at the end of summer when I was collecting images for an alternative process print exchange on Photrio. I ended up not using this image for the exchange, but looking back, it’s probably the best I made on that day. Happy little accident.

Since I included a Stouffer step wedge in the print, I can tell a few things from it. Most importantly, this gives an impression of the requirements on the negative for a good salt print. Looking at the print, the lightest visible tone in good light is around step 18, so 19 is paper white. Steps 1 and 2 are blended into dmax, and only step 3 starts to differentiate visibly. This means that the range of steps needed to picture the full density range in the print is 19-2 = 17. The Stouffer 2115T used here has 0.15logD steps, so this means a negative that must print all the way from dmax to paper white needs to have a tonal scale of 17 x 0.15 = 2.55logD. That’s pretty long!

In fact, the long tonal scale is the cause for the most problems I’ve seen (and experienced) with salted paper prints. I’ve seen (and made) many prints with negatives that have insufficient contrast. This results in flat prints, and people often resort to underexposing prints, leading to poor dmax and an overall anemic look to the print. Alternatively, all manner of workarounds are sought in processing, such as insufficient washing and fixing, which invariably results in problems later on (esp. staining). In my experience, starting with a suitable negative and giving the print sufficient exposure prevents most of the problems.

Furthermore, I suspect (know) that processing problems in many salt prints stay hidden for quite some time due to the lack of masking of the non-image borders. Personally, I always mask the borders with rubylith material. I do this partly because in most images, I don’t want the overly heavy/distracting ‘natural’ border of a brushed print. But another important reason is to spot processing problems. In a properly processed print, the borders will remain clear, also after weeks, months and years of exposure of the print to environmental influences. But poorly fixed and/or washed prints will show yellowing of the masked areas, sometimes within minutes in the final wash bath. I consider masking of the borders an essential acid test of a salt print process.

There’s one more trick that I often apply to salt prints. It’s ‘heat toning’. It’s not really toning in the chemical sense. It turns out that if you heat press a salt print, its hue tends to get a bit cooler. Contrast also seems to improve. So I often take the finished and dried print, and run the clothes iron over it at its highest setting. That’s all there is to it! The effect is not permanent; if the print is allowed to get moist, it will revert to its original state. But as long as it’s kept bone dry, it’ll remain in this ‘heat toned’ state.

Left: the print as it appeared this morning after air drying. Right: the same print after ‘heat toning’ with a clothes iron.

I’m not sure exactly what the heat toning does. I’ve read online (can’t remember where, exactly) that it’s supposed to let the silver grains migrate to the surface of the paper so they create a thinner layer that absorbs light better than the more open structure the image has originally. I’m a bit doubtful about this explanation, but I can see something happening along the lines of the fibrous structure of the paper contracting if it’s subject to high temperature, which will make the image mesh physically denser. Something like that. Yeah.

Let’s have a closer look at the differences:

Yes, it’s a very subtle effect. It’s in fact much more apparent in reality; there’s a distinct difference between the print before it’s being ironed, and immediately after it. The transformation seems like a minor form of magic!

Optically, I would describe the effect as I did above: the image gets more contrasty and seems to have better dmax, and the tone gets cooler. In the illustration above, you can see (if you look closely) the more purple image hue in the top right half of the image, while the bottom left half remains more yellow/warm brown. And this is where the photospectrometer comes in, because this should allow a more objective measurement to illustrate what’s happening. Surely, this change of density, contrast and color should be measurable?

Let’s look at the H/D curves first. I’ve measured the reflective density of the step wedge on both the print as it was originally, and after ‘heat toning’:

Lo and behold…very little difference! This actually surprised me, because subjectively, I had always experienced the density of a heat toned print to be higher than that of an untreated print. This is only true to the extent that some of the shadow densities do indeed increase, but the actual maximum density remains exactly the same. At around 1.25 logD, there’s no discernible difference anymore, and both versions of the print max out at ca. 1.35 logD.

This maximum density, coincidentally, is pretty much on par with matte-surface sliver gelatin papers. For instance, Ilfobrom Galerie matte has a dmax of about 1.4 logD. I’ve regularly heard people express doubts about the density of salted paper prints, but this measurement proves that this 19th century printmaking process is no slouch, dmax-wise.

What also stands out in the plot above is the S-shape of the curve. There’s a distinct toe region as well as a shoulder. I should really do some more of these on Van Dyke, Cyanotype and carbon transfer to show the differences. My impression (but I should check this) is that Van Dyke and classic cyanotype have even more distinct S-curves, while carbon (I know this for sure) shows virtually no toe and extends in pretty much a straight line. Interesting is also the apparent reduction in density after dmax is reached. It’s really an archetypical textbook curve in pretty much every aspect.

Alright, so there’s a slight gain in most shadow densities due to this heat toning approach. What about the color shift? Is that measurable as well? The first thing I did was have a look at the Lab values, and specifically the a* and b* values, which express hue. This is what they look like, plotted for the entire step wedge scale and both the untreated and post-heat toning print:

I found this plot to be pretty interesting too, since it shows a clear split-toned behavior of the print, which I had not even picked up consciously by looking at it. The untreated print starts off slightly yellow in the highlights (the paper base is slightly yellow, apparently), then gets redder as density increases. But at step 9, the direction changes and the hues actually become less yellow and less red – they trend towards neutral.

The effect of heat toning is very visible in this plot; the highlights start at the same point (the paper after all hasn’t changed), but as density builds, the tone shifts towards neutral/blue on the vertical axis right away. There is the same shift towards red as in the untreated print, however, and starting at step 9, the hues of the heat-toned print interestingly trend towards the same end point as in the untreated print.

The spectral reflectance plots for patch #7 shows the difference between the untreated and the heat-toned print as well:

Here, it becomes clear that there’s a slight difference in reflectance of blue light, with the heat-toned print indeed reflecting slightly more blue light. Moreover, the heat-toned print absorbs green, yellow and red light slightly more so than the untreated print. This supports the overall ‘cooler’ image tone.

Let’s have another look at spectral density. This is just for the heat-toned print, but it shows the spectral density for each of the steps of the step wedge. As a result, this should give an impression of what changes to the color occur as the density of the print increases.

Marked in yellow, orange and purple are steps #18 (a very bright highlight), #13 (a midtone) and #7 (a shadow), respectively. Starting with the shadow, there is a slight rise to the curve towards the left, blue side of the spectrum, and a strong rise to the right, the red side. This is the characteristic purple brown of a gold-toned salt print. The midtone shows mostly a lot of red reflection, so this is more of an orange-brown. The highlight exhibits decreasing density as the light becomes more blue, and as a result, the image tone here trends towards yellow. Mind you, the paper base itself plays a big role in the brightest highlights. While this particular paper is quite neutral, an even cooler-hued paper would make the highlights go cooler as well.

At this point, I could continue mining these data and in fact, I already had some additional charts cooked up. In particular, I was curious about removing the influence of the paper base from the readings so I could understand that actual hue the silver and gold image itself imparts. But frankly, things would start to get somewhat ridiculous here. Trying to understand the color of a print without the paper seems kind of silly, and certainly rather abstract. Alright then, if you’re wondering: this is the normalized color of patches #18, #13 and #7 with the offset of the paper base removed from the data:

This plot suggests that the shadow densities are dominated especially by red, that the highlight densities are still quite yellow, relatively speaking. But really – the practical meaning of this chart is pretty vague, and I guess it’s a good idea to leave it at this in terms of the Excel exercise.

What I take away from this is that salted paper is indeed a long-scaled process (no surprise there) and its H/D curve has all the aspects that you generally encounter in the textbooks: a nicely defined toe and shoulder, a fairly straight middle section and even a reduction of density beyond the shoulder of the curve. Also, this process of heat toning actually does not increase dmax, although I always assumed it did. It does boost shadow densities, and in doing so, the contrast in the lower midtones increases at the expense of contrast in the deeper shadows. The effect on image hue is very real, however, and can be recognized both in CIELAB readings as well as spectral reflectance.

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