Seth Troutner got so into making gum, he even made his own gum kit – to help beginners getting started. Here he shows how he makes his prints.
Chose the paper
Gum prints require that you use a substantial paper that will stand up to repeat soaks in water. Prints can be made with only one coat of emulsion, but most of the time you will find that multiple coats are necessary and with each coat the paper has to be soaked in water. If the paper you are using is not sturdy enough you will end up with a soggy mess. I have tried using several papers and have found that 140# pure cotton watercolor art paper works best for my needs. Pure cotton papers can be very expensive, but well worth the cost; if you are just starting out you will probably want to start with a less expensive artist grade watercolor paper that you can buy at any art supply store. The less expensive papers will work just fine for gum prints and are recommended for someone wishing to experiment with the process. As you get more experience you will find that the cotton fiber papers offer better control over the emulsion and how it develops on the paper.
Shrink the paper
Most likely the paper will have to be soaked in water more than one time and with each soak the paper will shrink or distort slightly. This will cause you register problems between coatings and the image will start to blur or get muddy. To avoid this problem you can Pre-Shrink the paper by soaking it in extremely hot water for a short period and allowing it to dry. The hot water will cause the paper to shrink to its nominal size before any emulsion is applied to the sheet. I personally use an old automatic coffee pot that I keep in my workshop to heat up water for shrinking the paper. You may find that some papers will have to be shrunk more than one time and only experience will tell how a specific paper will react.
Sizing the paper
Sizing the paper simply helps seal the surface of the paper so that the chemicals and color pigments will not stain the paper. You can make decent prints without sizing the paper, but you will find that it is very difficult to clear the highlights because of staining deep into the fibers of the paper. I have made several successful gum prints without using any sizing at all, but getting the proper amount of watercolor pigment was critical. Sizing the paper will give you more latitude pertaining to how much pigment you can add to the emulsion before it stains the paper and darkens your print.
There are many different ways to size a sheet of paper, but I don’t have a lot of experience in this area. The most common sizing consists of applying a Knox Gelatin solution to the paper before the emulsion is applied. There are several sites on the internet which explain this process in detail, but unfortunately I personally have no experience with the gelatin process; click here for some useful information on sizing paper. For my prints I like to use Acrylic Spray which I find much easier to apply because it dries in minutes. I purchase Acrylic Matte aerosol spray from my local art store and a single can will size many prints. I will caution that the spray can be a little temperamental due to the fact that if too much is applied to the paper it will cause the emulsion to fall off the sheet while developing; too little and the paper will stain. If you choose to use acrylic spray sizing I would recommend that you do several test prints to figure out how much spray is necessary. I have found that I get best results if I spray the sheet just enough to coat the paper, but not so much that it leaves a shine on the paper. If you can see shiny spots on the paper then emulsion is probably not going to stay in place.
There is no one right way to make a gum print, but there are wrong ways which can cause personal harm. Dichromate chemicals are dangerous and should be handled with care. Each person should make themselves familiar with proper handling procedures. The chemical can be fatal if ingested and will cause eye and skin irritation on contact. Excess amounts of ammonium dichromate should not be flushed to the sewer. It is recommended that you initially wash prints in a tray of water that can be reused over and again. Store this “first rinse” water in an air-tight container and continue to use until the water is dark brown and will no longer properly function. Dispose of as recommended by your local waste management commission. Only the “first rinse” produces high concentrations and secondary rinse water solutions can be disposed of to the sewer.
Mixing the emulsion
Image above right: Accurate measurement of chemistry will allow you to improve the formula to achieve the best print possible.
The emulsion is the most sensitive part of making a gum print. You should prepare the emulsion in a dimly lit room and should be stored in complete darkness. The emulsion is made by mixing water, gum arabic, ammonium or potassium dichromate, and watercolor pigment. There are dozens of formulas on the internet that will give you a good working emulsion. Each formula will require different techniques and each will give a different look to the finished print. The only way you will know what formula you like to work with is by experimentation. I am listing here the most common formula that I use to give a general idea of how the process works. The ratios can easily be adjusted for your personal preference.
First I prepare the gum mixture: 0.5 ounce (by weight) of gum arabic powder is added to 45 ml of hot water. Stir the mixture until it becomes a uniform brown syrup like liquid. This mixture should be a nearly saturated solution so I strain the solution through a piece of cheese cloth to remove any lumps that might be there. Store liquid gum in an air tight container and place in refrigerator for up to a week. If the mixture gets grainy over time add a tiny amount of warm water and stir.
Next I prepare the dichromate solution: Mix 0.5 ounce (by weight) of ammonium dichromate to 50 ml of warm water. Stir until all crystals are dissolved. You may also use potassium dichromate, but the ratio will be different and is not listed here. Store dichromates in light safe air tight container and dispose of liquid mixture after 72 hours (this is my personal experience). If crystals form in the solution you can warm the container slightly by holding in your hand and stir until crystals re-dissolve.
Prepare the Emulsion: I mix equal amounts of liquid gum and liquid dichromate solutions in a small container. I like to use a tablespoon (disposable) to measure out the mixture. My formula uses 2Tbl gum to 2Tbl dichromate in a small bowl and then mix to make the emulsion base. This amount is generally enough to cover a 9×12 piece of paper two times. Do not expose this mixture to bright light.
Add Pigment to the Emulsion: The pigment can be any color of watercolor paint that you choose. Some pigment colors work much better than others and black is the obvious choice for first timers. I would recommend using a high quality artist grade water color that comes in small tubes. The amount of pigment that you add to the emulsion is critical and there is no easy way to measure. For most pigment colors I add a small pea size amount to the 4 tablespoons of emulsion, but some colors require a bit more or less and only experience will tell what works for you. Mix the color thoroughly into solution. A little pigment goes a long way and if you are having trouble getting the emulsion to rinse away from the paper then you probably are adding too much pigment.
Apply emulsion to paper
Brushing the emulsion onto the paper requires a little practice. If the emulsion is brushed on too thick it will require longer exposure and can cause flaking when it is rinsed. Heavy brush marks will show up in the final print, but this is sometimes a desired look. If you do not want brush marks then the emulsion needs to be brushed on as smooth and evenly as possible. I like to use a soft brush to apply the emulsion to the paper and then I quickly brush with a much larger and softer brush to smooth out the finish. The gum mixture starts drying quickly so you have only a few minutes to get the desired finish. If you continue to brush after the emulsions starts to set up it will cause spots in the finish so work fast. Let the sheet air dry for about 20 minutes. A hair dryer can be used to speed up the drying process if you use low heat. High heat will damage the emulsion.
If you are an avid photographer you may already have the ability to make full size negatives, but the majority of us do not. Because of digital technology it is becoming very difficult to get full size negatives to make contact prints. There are services out there that can produce large negatives on image setters if you are willing to pay the price. If you are doing a multi-color print and require color separations the cost can be well over $100 for 8×10 negatives which is way too much for most hobbyists. There are other options.
If you have access to an inkjet printer you can easily print your own negatives onto transparency sheets. You must first either scan your photo or import a digital file into editing software that has the ability to reverse or invert the image so that it is a negative. Print the photo onto a transparency and you have your negative! The one big downside of this method is the cost of inkjet transparencies and ink (negatives use lots of ink).
If you plan on making multi color prints than you will need some method of separating out the colors. I have experimented with CMYK separations, but RGB separations have become my favorite. If you don’t understand what these terms means http://en.wikipedia.org/wiki/CMYK_color_model should help give you a little insight.
To make the different color negatives I use Photoshop to separate the Red, Green, and Blue channels on my computer. The channels print as follows:
- Red Channel = Cyan Watercolor
- Green Channel = Magenta Watercolor
- Blue Channel = Yellow Watercolor
My preferred way to make a negative is oiled paper. The process involves using a regular sheet of paper with the image printed onto it in reverse as described above. To make the paper transparent to light you wipe the paper down with vegetable or baby oil. This process makes the image slightly more contrast, but I prefer the look.
Exposing the image
The negative needs to be tightly pressed against the emulsion surface to get a detailed image. Two plates of thick glass with the emulsion/negative sandwiched in-between works very well, but a vacuum frame is best. The printed surface of the negative should be face down in contact with the emulsion for best results. If you plan on making more than one burn you will need to place the negative in exactly the same position each time. If you have trouble seeing the previous image through the negative I would recommend making pin holes in the four corners that can be easily re-positioned with thumb tacks over a cork board and then taped in place.
The UV exposure time of a print will vary greatly depending a multitude of conditions. The only way you can figure out the exact exposure for the density you are trying to achieve is through trial and error. If you are using sunshine to expose your print it will take a lot of approximation as to how long to leave it out. A high noon exposure will be much different then a late afternoon exposure and the only way to know is to try it. 10-20 minutes under bright sun is a good starting point.
I prefer to use a more controlled UV source such as a mercury halide lamp. The exposures are easy to calculate and repeat (and it works on cloudy days too). For my lamp I use a 9-12 minute exposure depending on which color I am printing.
There are a lot of variables when it comes to exposure times and I would recommend exposing test strips to see just how long you can expose a particular emulsion and still get the highlights to clear in a reasonable amount of time. Keep your test strips and write down the exposure times for future reference. I have found that even the same formula requires different exposures for different colors of pigment. You will eventually find your favorite colors and develop preferences.
One of the benefits of working with gum is that the print can be developed with just pure and simple water. I have tried several techniques of developing my prints and each one will offer a different look to the final piece. The one thing to remember is that when the emulsions wet it is fragile and very easy to damage.
There are many rinsing techniques such as wet brushing, pouring or streaming water, etc., but I would recommend just a soak in warm water until you are comfortable with how delicate the emulsion really is. I like to use room temperature water for the first soak to remove the majority of the dichromates which rinse away and turn the water yellow. After about 10 minutes in the first bath I move the print to a second bath of moderately warm water. In the warm water I gently agitate the print until all of the highlights are clear.
It is very difficult to get a good gum print with just one coat of emulsion. Most prints will require two or more coats and with each coat you risk damage of the previous layer. When brushing on additional coats of emulsion be sure to brush lightly. Aggressive brushing can damage the layer underneath even though it has been exposed and dried.
I still have difficulty understanding the theory behind the additive and subtractive process of blending colors using RGB and CMYK and the only way I have found success is by trial and error. When I experiment with a new watercolor pigment I usually will make a test print with layers of several different colors to visually see how they blend. Some colors do not blend well and create a dark muddy pigment no matter how thick or thin they are applied to the paper.
Variations and errors
Note: some of the grain in the following photos was caused by the poor quality of my scanner.
This photo of the barn was an experimental print that I made with CMYK separations printed as a tri-color. On my computer I mixed the Black channel equally into the C,M,and Y channels and output only the CMY negatives. Wherever the three colors overlapped in the shadow areas it would give the appearance of black. This is the same concept that RGB uses to create black, but when I was printing with Red, Green, and Blue channels (CMY watercolor pigments) I would get a muddy brown color instead of black. In this photo the use of CMYK channels did produce darker blacks, but obviously the bright colors were altered as well. My solution was to stick with RGB, but use darker pigments.
This is a photo of my stepdaughter that was printed CMYK color. The light spotty areas are the result of using too much acrylic sizing. This was an experimental print on which I used a very different formula and technique. Instead of just letting the print soak during the development, I used a paint brush to wipe away the unexposed emulsion in the bath of water. The formula that I usually used would just wipe clear away with the aggressive developing, so I had to use a much less saturated solution of gum by adding more water to the gum solution. The mix was very watery which made for a very thin emulsion. The thin emulsion had a short density range which required many layers to build up any detail at all.
This photo of the flower is another attempt at using a paint brush during development to clear the unexposed emulsion. The formula was again very watered down so that the emulsion was thin and many coats were used. This method makes for a very low detail high contrast print.
The gum print below is a sample of a bad emulsion formula. This is a 4 layer CMYK print using all for channels. The small white spots are actually clear areas where the emulsion flaked off the paper. I have researched a few theories as to why this happens, but I believe that in this case it was the amount of gum in my mix. I was attempting to get a more viscous emulsion by reducing the amount of water and increasing the gum arabic. The first two layers went down successfully and then the flaking started during the third rinse.