TheLightFarm - Silver Gum

Silvergum

"Gum Bichromate Process ...The method may be summarised as follows: — Ground pigments are mixed with a thin mucilage of gum-arabic, and a little soluble bichromate, this mixture being applied to the paper as a thin wash. When dry, the paper is exposed under a negative, and the picture is developed by soaking in water (cold, warm, or hot) until the unexposed portions of the pigmented gum are sufficiently washed away."

The Dictionary of Photography and Reference Book for Amateur and Professional Photographers, 9th Edition. E.J.Wall, 1912.

Index to this page

Basic 3-color
5-color Silvergum
Gallery

It seems only natural to return to the beginnings of emulsion history and combine that with an art form that is essentially unchanged from its own beginnings, and then add digital color separations to create something brand new. I'm calling it Silvergum; three-color gum over silver gelatin emulsion.

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. I've moved on to three brushes! and a UV printer from (Jon) Edwards Engineered Products.

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. One point that may be different from other gum printing is the ease of negative registration. After each 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 (blue, red, or yellow) with the image and tape it at the top with film tape. For coating, swing the negative up and out of the way while you brush on the pigment and then dry it. (I use a hairdryer on low. I live in a very humid climate and the darn things would never dry without some help.) When the layer is dry, swing the negative back down - it will still be in perfect registration - and place the negative/paper sandwich in a contact printing frame and expose to your light source.

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 b&w 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.

Digital negatives from Pentax K10 digital file.

B&W silver gelatin 'K' layer.

'Warm ♥' emulsion on Fabriano HP Extra White watercolor paper.

This emulsion was made with a low tech recipe (Hershey's Tornado). For more info, please go here.

Blue layer: 0.4 g Prussian Blue and 0.5 g Verditer Blue (Daniel Smith watercolors) in 20 ml gum arabic solution (Daniel Smith brand) and 25 ml 15% potassium dichromate solution.

A Blue layer tip: When the pigment load on the brush is right for coating, I quickly give the bristles a few spritzes of Everclear and immediately brush the paper. This seems to improve the smoothness of the layer.

Red layer: 0.7 g Quinacridone Rose and 0.2g Rhodonite Genuine in 20:25.

I run 1/4-inch wide polyester film tape (3M #850) around the edges of each color separation negative. This preserves the clean black emulsion as a border around the final print.

Yellow layer: 1.1 g Hansa Yellow Medium in 20:25.

If, at this stage, the print still needs color adjustments or additional density, more layers can be printed, but new negatives are advised because the print won't be the same size it was on the first layer.

There's hardly an art form that has more potential for creative expression than gum bichromates. I've settled for now on a pigment set I like, but there are dozens of three-color combinations possible and countless single colors.

You aren't constrained to color separation negatives. Sensitized pigment can be brushed on freehand, as long as it's applied in a thin, even layer. And, of course, a single color can be used over the silver gelatin print.

Negatives can be made in a number of ways ; from scanned film transparencies or negatives or from digital camera files. And, there is always the pre-digital method of in-camera separations.

'Yggdrasill'

A digital negative set made from a scanned handcolored b&w print (dyes over Ilford Multigrade glossy).

The original enlarged print is from a 5"x7" b&w negative.

Only the blue and yellow layers are from color separation negatives. The tree trunk and lower branches were hand-painted red with a fine brush as a final step.

Tip: Don't needle-spot the black and white print before applying the colored gum layers. The pigment will sink into the bare paper and stain. The color layers will cover all but the biggest peppers, but if need be, they can be carefully spotted out after the last color layer goes on. I generally use any flawed prints as test prints.

5-color Silvergum

'Rainbow Seep'

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.

My gum prints start in a digital camera. This image 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.

Upper left: I like to outline each separation negative with silver film tape, leaving a tiny clear space around the image. This preserves the clean, black emulsion and makes the final print look framed.

Upper right: Print with the color separation negative registered, taped, and swung up and out of the way for coating.

Lower: Perfect registration after five layers of gum.

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.

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 (disable 'Constrain Proportions'). 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

1	2	3
4	5	6
BLUE: Prussian blue, Verditer blue and Q Rose. Yellow channel. CYAN: Prussian blue, Verditer blue and Hansa Yellow Light. Yellow channel. MAGENTA: Q Rose and Rhodonite. Green channel. RED: Rhodonite and Naples Yellow. Green channel. YELLOW: Hansa Yellow Medium, Hansa Light and Naples Yellow. Blue channel. This is the final print that heads this section.
		6. No matter how hard one strives for repeatability, each print is still a handmade individual, not a reproduction. I started with two black and white prints of Rainbow Seep. They were identical until the fourth, Red, layer. For no discernable reason, the red on one printed darker. So, on the final Yellow layer, I pulled the printing time by a minute. The result is a distinctly different interpretation than #5.

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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