Silvergum – gum over silver gelatin

Denise Ross explains how to print gum bichromates over handcrafted silver gelatin paper.

Not too many years ago, the word was going around that photography was dead.

‘All that can be said, has been said.’ It turns out that was a bit premature. Today, photography has never been more dynamic and creative. We are able to draw on three centuries of photographic history, mix things up, and together, to make a vision of our own.

I have always been interested in the historic photographic processes, and I don’t know of one I haven’t at least tried, but my ‘real’ work was always with film and black and white paper – ubiquitous, cheap, dependably available and of course, silver gelatin.

Fast forward to today, and unbelievably film and B/W paper are well on their way to becoming the newest ‘traditional’ processes. A couple of years ago, I got the notion to commit my work to recreating the old silver gelatin emulsions before that knowledge was lost. I thought I would have to give up working with color for the sake of the cause. Well, it seems Color and I are inseparable and multi-color gum over the handcrafted paper I was making seemed like a natural.

Until now, gum has not gone well with silver gelatin (at least so far has I’ve been able to discover). Commercial papers based on baryta stock are too slick for the gum to adhere. Gum with platinum or cyanotype have been the main expressions of the gum-over idea.

But, handmade emulsion on Fabriano Artistico HP watercolor paper (and I’m sure there are other great papers) is a perfect substrate for gum. The emulsion is as hard as nails so there is little risk of staining and the original print acts as a registration base and as a ‘K’ layer that gives the final print a beautiful depth.

Gum printing fits with my idea of a perfect craft. It is possible to get started and continue forever with the most basic of tools and space. I did all my first explorations with one good brush and a photoflood bulb.

Making the silver gelatin print is a bit more complicated, but not near as much as you might imagine. As with all things art, there is a progression of the levels of difficulty. On my website, thelightfarm.com, I have tried to offer a number of approaches, from the simplest requirements for tools, technique and space to more advanced methods.

In a nutshell, if you have worked with any salted paper, you almost certainly have what it takes to make a basic black and white print. The ‘Kitchen Lab Emulsions’, ‘Paper and Coating’, and ‘Contact Printing Paper’ chapters should have all the information you need to get started.

The Process: An Overview

There are so many excellent sources of information on the basics and beyond of gum printing, there’s no need to go into most of that here. I use a 15% solution of potassium dichromate, commercial gum arabic from Daniel Smith Art Supplies, six-inch varnish brushes, and a simple UV (black light bulb) printing box.

On principle, and because I value a certain amount of repeatability, I try to get my negatives ‘right’. But, I have no reluctance using selective pigment removal techniques. This is art, after all! I have a selection of small brushes, a plastic syringe of water, and a spray bottle of water handy and ready to use. If it seems like I’m making the same mechanical adjustments each time, I make a new registration negative. The burn tool is a dandy for darkening an area. A darker patch on the negative means there will be less pigment density in the final print.

My gum prints start in a digital camera. The image to the left was made with a Pentax K10. The only adjustments I made to the factory default settings were to set the contrast as low as possible and to kick the saturation up a notch. I shoot in RAW and work in Photoshop CS2 at 720 dpi.

Crop and re-size to your liking and then save that file, labeled ‘Prime’. This image is ‘RainbowSeep-prime.psd’. This allows all of your subsequent layers to align so that if you want to change a layer, you always have the original file as a reference.

I print all of my layers on Pictorico OHP, on an Epson R2400, with matte black ink on the ‘Best Photo’ setting and gamma 2.2.

Working with a digital file in RAW means you have almost limitless control and potential for creativity. The first image on the left is a straight negative, made by desaturating and inverting. If I made the black and white print from this negative the red rocks and yellow algae would print out very dark and the gum colors would to a large extent be lost.

The middle negative is made by using the hue/saturation slider and selective color tool. I increase the red, magenta, and yellow saturation and then run a +100% black in all three colors, and then invert and sharpen. The red rocks and yellow algae end up very light in the final print.

One point to love about gum-overs is the ease of negative registration. The first layer aligns with the black and white K layer. After each subsequent step, dry and flatten the print in a dry mount press. (A clothes iron over the print inside a couple of sheets of 2-ply mat board or similar, followed by flattening under a weight works just as well). On a light table, register the appropriate negative with the image and tape it at the top with film tape. When you go to coat, swing the negative up. When the coating is dry, swing the negative back down – still in perfect registration.

Negative registration is greatly improved by the addition of digital negatives to the process. With each step, the paper shrinks just a bit. I’ve calibrated that shrinkage (and it’s very consistent) for Fabriano paper and my work flow.

Starting with a 3.5 inch high print in mind, I make the original print negative 0.5 mm longer and 0.3 mm wider. There is considerable shrinkage during the first step. With each color layer there is just a bit more shrinkage, getting less with each pass. You would think it wouldn’t be enough to matter, but perfect registration makes an enormous difference in the finished print, and since with digital negatives custom sizing is one-step easy, why not? A little trial and error and good notes will quickly establish the calibration that works best with a particular set of materials and workflow.

An excellent way to figure out both registration number and color balance is with a GretagMacbeth ColorChecker Color Rendition Chart.

Scan the chart and make a negative. I then go through and darken to black one half of each square. This gives you two sets of information – pure color and color on density. The right hand image is the resulting black and white print.

With a little trial and error you can see how each individual color layer is affecting the final result. You are working on balancing both color and the density curves.

I’m closing in on the details for this set of pigments. It’s still not ‘perfect’. I would like to see more separation in the blues, a better true red and a deeper yellow. Also, I’m running to blue in the highlights and to yellow in the shadows. I think the next round or two will solve these issues.

It’s my philosophy that you need to have a pretty good command of materials and techniques. I love when luck happens, but I’d rather not rely on it.

I’ve decided that trying to create adjustment curves for each layer is an exercise in head-banging futility. I advocate a much simpler and straightforward method – a combination of saturation and selective color.

Here is my Yellow layer: From the ‘Prime’ file:

  • [+50% yellow saturation followed by +!00% black in Selective Color/yellow] x2.
  • In Selective color: -100% black in blue and magenta, -75% black in cyan, and -50% black in red.
  • [+50% green saturation followed by +100% black in Selective Color/green] x2.
  • Invert the image and go into Channels/blue.
  • Then, in Curves (blue) take 0 to 25 and 255 to 160. This both increases the density and flattens the curve to prevent over-printing. Any sections of a negative that are too thin can give you a nasty orangish ‘dichromate burn’ during printing.

All the other layers are color-specific variations on this theme.

The Gum Layers

I have been working out the details of 5-layer gum printing. I was unable to achieve the range and subtlety of color I want with just blue, yellow, and magenta, so I have added cyan and red. I use all Daniel Smith watercolors. You’re not limited to one pigment per layer. My current favorite set includes Prussian blue, Verditer blue, Quinacridone Rose, Rhodonite Genuine, Hansa Yellow Medium, Hansa Yellow Light, and Naples Yellow.


One last lovely bit of artistic control: Hand painting. The image below on the left is a five-color silvergum. The one on the right is a three-color silvergum with Daniel Smith’s "Sap Green" hand painted on the leaves between the magenta layer and the yellow layer.


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