First, on your paper mark of the area in which you will be applying the emulsion. Traditionally gum emulsion is applied to be larger than the area of the transparency. This gives the print that iconic painted looking border. Always use pencil as pen ink will bleed during development. I simply put little dots at the corners using my ink-jet negative as a guide. (We will be making those in a later section don't worry)
Emulsions in the Light
Coating a layer of gum Bichromate emulsion onto paper is the most important step involved with gum printing. It is also probably the easiest!
Remember that you are mixing together a light sensitive photographic emulsion. However, unlike many other photographic processes you do not need to run into the dark. Normal indoor lighting is perfectly suitable for all steps of gum printing. Besides, the emulsion is not sensitive to light until it is dry.
Once applied to paper, my emulsions are left to dry in a dimly lit room anywhere from 3-48 hours and I have never noticed any exposure by ambient light. This is because the sensitivity of gum emulsion is so weak, it takes quite a lot of energy to start exposing.
- Mixing tray
- Sponge brush and a mixing tool
- Watercolor paint
- Gum Arabic
- Ammonium Dichromate Solution
- A dry prepared sheet of water color paper
You will need:
How to Make the Emulsion
You need make a choice of color and squeeze a little "worm" of watercolor paint into your tray. I am using Charcoal Gray here and making a worm of pigment about 1/4 inch or so.
Often I use more or less pigment depending on the circumstances. The less pigment you use, the more transparent the layer will be, and the less likely staining will occur. A thin emulsion also exposes evenly by allowing UV light to transmit through it.
Then you will add equal amounts of gum Arabic and Ammonium Dichromate Solution so that the proportions are 50%-50%. Precision does not matter in gum printing so I just eyeball it. I make a little pool for each about the size of my thumb which is just enough to coat one print layer. Getting the right total amount just takes practice and experience.
Now take your mixing brush and stir it all together, keeping most of the emulsion together to keep it from drying. I tilt the entire tray and mix it along one side to keep it in a little pool. Be sure to squeeze your mixing brush in a clean area to get any excess emulsion out of it so you can use that emulsion too!
Next, get your sponge brush and load it up with emulsion. Don't worry about getting it all on the brush, just come back for more as you need it.
Begin coating your paper around the edges of the intended image area, then coat the middle. This helps you see the boundaries better as you are coating. Or, if you prefer clean edges, you can mask off the edges with painters tape. I prefer the iconic lively messier borders myself.
The paper may feel very dry at first. Keep coating and it will begin to feel smooth through the brush. If continues to feel dry, you probably need a little more gum arabic and dichromate in your emulsion to make it wetter.
Continue coating in overlapping vertical and horizontal strokes, getting the emulsion layer to look as even as possible. There will always be light and dark streaks from your brush. After a few passes, the emulsion should appear fairly even. When printing multiple layers, these streaks will blend to create a subtle textural effect I personally find pleasing.
If you brush with too much pressure the paper may begin to damage. Also when the emulsion begins to feel very sticky as you are brushing, it means it is beginning to dry and you should stop.
Finally, set your paper somewhere dark or dim to dry for about an hour or two. A low speed fan to circulate the air helps. Most uneven streaks from your brush will tend to diffuse out onto the paper, although some will always stay.
Important!: the Shelf-life of Gum Bichromate
Dichromate works by oxidizing the gum Arabic. This process is what makes the gum Arabic insoluble to water allowing us to develop an image from it. Oxygen in the air is also oxidizing gum prints just as it does to cut apples left in the open.
This oxidation begins happening once the emulsion is dry. Exposing the emulsion to ultra violet light greatly accelerates this reaction into a matter of minutes. Normally you should try to expose an emulsion within the same day or the next day after mixing it and coating a sheet of paper. I usually try to expose my prints 8-24 hours after they are coated with emulsion. The longer you wait, the darker the print will turn out. After 1 week, the emulsion is probably fully oxidized and will no longer develop.