In order to make a gum print, you need to mix your own photographic emulsion and coat it on paper. This requires various tools and substances. Most tools you may already own and if not, they are all fairly inexpensive and easy to find at local arts and crafts stores. Ammonium Dichromate and Gum Arabic, the solutions needed to mix an emulsion, are easy to obtain through a photography chemistry supplier such as Photographer's Formulary
- Gum Arabic
- Ammonium Dichromate
- Watercolor Paint or Pigment
- Watercolor Paper
- A mixing tray or bowl
- Sponge brushes and other various brushes
- Transparencies / Inkjet Printers
- A sink or development tray
- A Contact printing frame
- The Sun (or other ultraviolet light source)
Gum Printing Supplies*
*Please note that this is not a comprehensive list, only the basic materials needed. In later sections other optional materials will be covered.
Gum Arabic was once known as Gum Acacia and its use dates back in history about 5000 years. Basically the substance is tree sap from the Acacia Senegal trees. The Ancient Egyptians were some of the first to use it for hieroglyphics and mummification. The substance was introduced to Europe through Arabian ports and became commonly known as 'Gum Arabic'. Today it is used in foods for fiber supplements, candies, and all sorts of things. Watercolor paint is pigment mixed with Gum Arabic as a binder.
Gum Arabic is a gooey semi-transparent sticky brown liquid. You can also find it in a dry powder form for mixing with water to a desired consistency, however Photographer's Formulary sells gum arabic in liquid form which works superbly for Gum Printing. I have never mixed my own gum arabic.
Ammonium Dichromate was once called Ammonium Bichromate during the late 1800s. However, due to modern naming conventions in chemistry the 'Bi' prefix became 'Di'. In photographer slang the term 'Bichromate' is still widely used, while officially it is 'Dichromate'
Ammonium Dichromate is a dangerous substance! It can cause cancers with prolonged exposure, and is poisonous if ingested. I get mine from Photographer's Formulary where it comes in crystalline form. The crystals themselves can explode when exposed to heat like a flame. When burned, dichromate will expand, releasing smoke and heat causing a chain reaction in any nearby dichromate. This is sometimes used in volcano models for science projects. We will not be burning dichromate, just getting it wet.
Another chemical used by printers is Potassium Dichromate which essentially looks and acts the same as ammonium dichromate. I have only used ammonium dichromate, and for all purposes it has suited me well. Ammonium Dichromate is said to be less sensitive to light then Potassium Dichromate. Since Ammonium Dichromate exposes slightly slower, I feel I have more control over exposure times.
To use the dichromate for gum printing, the raw crystals need to be dissolved into water making an Ammonium Dichromate solution, ready for printing. I mix and store mine in an 8 oz. cheap plastic shampoo bottle from Wal-Mart. (however these travel sized bottles may prove difficult to find as I got mine in 2003 before the airport liquid container problems), I mark the bottle with a clear skull and crossbones and keep it out of reach of any curious children. Be sure to treat the chemical responsibly. Also remember that it is reactive to heat. If you happen to live in 105F degree summers like me here in Texas, certainly do not store it in your garage! (see the MSDS).
Pigment is any material crushed into a fine powder that is insoluble to water. Usually pigments are from rocks, bones, ashes, perhaps even metals. Some synthetic pigments are out there as well. I have heard of pigments such as crushed brick being used in gum printing, or even cigar ash. Most pigments are absolutely stable and wont fade for thousands of years. When the paper of your image finally crumbles to dust centuries from now, your pigments will still be the same color they were the day you made your print!
Watercolor paint is pigment with Gum Arabic mixed in as a binder (or a glue to hold the pigment to the paper surface). Water is watercolor's medium, or the vehicle which transports the Gum Arabic-pigment mixture into place on paper. The little tubes of watercolor paint you can find in any arts and crafts store work just fine for gum printing. The more expensive brands contain a greater ratio of pigment to Gum Arabic then the cheaper 'student grade' varieties. I use Grumbacher water colors because they are inexpensive and readily available.
I use the little tubes of paint and mix a new emulsion with every printing. It provides a greater flexibility to the creative process. Many others prefer to buy pigment in bulk and mix their own gum Arabic / pigment solutions ready for printing. The upside to this way of working is you know exactly what ratios of pigment to gum arabic you are using at all times and can be even more consistent in your printing. Although, this degree of precision is absolutely unnecessary.
You will learn soon that some pigments have different tactile and visual properties. Some colors of pigment are very transparent, but not because they are diluted. Other pigments are thick and chalky, very opaque. Often I will use gouache paint rather than watercolor paint. Gouache paints have more pigment content than watercolors (thus they are more expensive) and they also are loaded with chalk giving them a bright vivid, yet dense opaque look. They print easily with gum bichromate.
Some pigments have been blamed for staining paper. I have often come across this with very fine pigments such as lamp black, made from soot. The pigment gets caught in the fibers of the paper and this can be an annoyance if you do not desire the effect. There are ways of preventing staining and maintaining clear highlights by sizing your paper. You can also embrace staining, mastering the 'accident' to choose when to stain your paper purposefully. It would be a good exercise to find out what pigments stain and which do not. I can not be specific about which pigments will and will not stain because it all depends on what pigments, paint, paper, sizing, or development that YOU use.
For Color Separation Printing I use Thalo Blue, Thalo Crimson, Cadmium Yellow Pale Hue, and some kind of gray like Charcoal Gray (made with, you guessed it, charcoal dust). Often I will use Paynes Gray as it has a slight blue cast that diminishes the yellow haze ammonium dichromate sometimes leaves behind. Occasionally I will use an intense dark color like Lamp Black to further darken shadows of an image.
The paper you choose is very important to the process of gum printing. The paper affects the texture of the image, how many layers you will be able to print, how rapidly a print develops, and the lifetime of your print.
Factors about your paper choice are:
- Sizing - treatment to prevent staining
- Weight - thickness of paper
- Color - tone or color of paper fibers
- Texture - visual pattern of paper
- Tooth - roughness or smoothness
I was a poor college student when I began printing in 2003 (and am not much richer as the years go by). I tend to use inexpensive materials and find ways for them to work for my purposes. There is no sense in spending big money on grade A materials if you are not going to be using them to their full extent. I used the 11x15 inch Strathmore watercolor 140 lb cold press paper for several years. It generally has suited me well, although recently it seems Strathmore may have changed the manufacturing process and it is no longer smooth as I prefer. I now use Canson Aquarelle paper in both 11x15 and 18x24 inch sizes. The Canson paper is more delicate and has torn occasionally during development causing stings of expletives to spray from my mouth! However I've got a handle on it, and now develop my prints more carefully.
Whether a kind of paper is good or bad depends on how you use it and what you want your images to look like. Don't blame the paper! Find a paper that suits you.
Papers come in different textures or 'tooth'. In other words they are rough or smooth and can take a different 'bite' onto your colors. I find rough papers hold onto your emulsion stronger and require more aggressive development. Smooth papers can handle some aggressive development, but are suited for retaining detail in your images. Sometimes the backside of water color papers are smoother than the front.
For several years I've used 11x15 inch paper because it comfortably holds an 8.5x11 transparency, which I can easily develop in the sink. Recently I have begun printing on large 18x24 paper with 13x19 transparencies. Printing small requires people to view images from very close and creates an intimate connection. Larger prints will draw viewer from across a room and seem suitable for gallery settings or for images with small details in context of a larger scene.
There is a number associated with paper which indicates the weight of the paper, or how thick it is. 140 lb paper (or 300g/m2) is durable enough to work with multiple printed layers. The higher the weight, the stiffer and thicker (and more expensive) the paper is. 90 lb paper is a bit too flimsy and dries very wrinkly although i have used it in the past. You will most likely want a heavy paper because you will be re-wetting the paper perhaps up to a dozen or more times, but always at least once or twice. Some thin papers will shrink drastically making the over printing of layers something of a nightmare!
There are toned papers around. It is not something very attractive to me personally, I prefer to make all tones and colors myself during printing. I have seen many specialty papers used, some papers were so eccentric there were butterfly wings incorporated into the fibers!
One last thing to mention, some papers will sink in water. Part of the development process requires that you float your paper on the water surface. Any paper that sinks is therefore useless.
Painters have pallets to mix their colors, gum printers should have a bowl or tray to mix their emulsions. I used a sheet of glass for a brief while, mixing my emulsion in a little puddle in the center. It was messy and a pain.
One night in 2003 my girlfriend at the time made some friends and I spaghetti, and brought a few of those plastic microwaveable containers to hold the leftovers. A few days later I was musing to do some printing, and found the container on the top of the trash. Perfect! I rinsed it off and still use for mixing my emulsions today!
This tray works so well because in each of the four comers there is a little valley which I use as a rough measuring device for my emulsion. I pour a bit of ammonium dichromate in one corner, gum arabic into the opposite corner, and squeeze a 'worm' of water color paint in the center. It is marvelously easy to mix the emulsion using a brush and the entire tray rinses off in an instant. I will still have the little tray when I'm 92 I am sure.
As you might be gathering, there are no correct tools to use with gum printing and I generally use what I've got at hand. That goes for which brushes as well. For mixing the emulsion I use an old 1 inch furniture painting brush. I once used a paring knife for some reason (perhaps it was my version of a pallet knife), but it would scratch up my plastic tray. I find the small brush does a faster more efficient job and mixing everything together.
For coating the emulsion on the paper I use sponge brushes 95% of the time. Sponge bushes really help to make an even coating of emulsion on the paper. I use a 3 inch sponge brush for smaller papers and a 4 inch on Larger papers. I sometimes cut a tiny bit of the corners off to give the brush a rounded edge that is less likely to drag streaks into my emulsion layer.
I also use some chinese ink brushes or stiffer acrylic brushes occasionally during development to brush away unwanted tones or for other effects. Some printers even use feathers during development. This technique is discussed later in the free-form printing section.
When I began printing in 2003 I used generic clear transparency sheets meant for overhead projectors. They were relatively inexpensive and could be found at any office supply store. I was also using a printer I literally found in the garbage. (my friends and I had a habit of dumpster diving toward the end of college semesters when many students preparing to move emptied their rooms of perfectly clean functional electronics and furniture) The printer worked fine but I eventually bought a dedicated Epson photo quality printer in 2004. The Epson inks happened to be pigment based so they blocked UV light easily and the negatives worked great. A few years later, however, the printer nozzles had clogged, the ink was expensive and the printer ended up in the garbage one day. I don't think anyone picked that one up!
I looked to buy a new printer, but the new technology could no longer print on clear transparency sheets! I guess an infrared sensor caused the new printers to not recognize there was something in the tray to print on. After a few purchases and returns, it's needless to say I was quite frustrated. Eventually I began using a combination of a Canon inkjet printer and Pictorico OHP transparency Film. Although, the canon inks were not pigment based and brought about more problems which eventually led to some insights about making calibrated inkjet negatives. I now use a large format Canon printer that uses a black pigmented ink. If I could give one piece of advice on printers, go with Canon.
The Pictorico OHP transparency Film has worked amazingly well! The one and only drawback is the price which for an 8.5x11 comes in at about $1 per sheet. The larger 13x19 sheets are $2.50 a sheet! This is of little concern if you are selling your work, but if you are just learning, it can be a bite to the wallet.
To make a gum print it is essential to have a place to develop your print. Large Photo darkroom trays would be ideal. But, if you are like me and have very little space, then you might have to settle for the kitchen sink (ERRRKK!!) I know, I know, the kitchen sink is not a very safe place to have dangerous chemicals.
Perhaps I am flirting with disaster here due to the toxic nature of dichromates, but I am as careful as I can be, and rinse the sink out afterwards. If you do use your sink, PLEASE take precautions not to poison yourself or others with dichromate!!
Currently for my large prints I am using the bathtub for a development tray which holds an 18X24 inch piece of paper comfortably. (although not so comfortable on the knees!)
When you expose your print, you need glass to hold your negative or transparency firmly against the emulsion, otherwise you'll end up with a perfectly exposed blurry image. Some people have custom made contact printing frames with latches and pins and foam to hold everything in place. I have always used plain old glass, tape, and those black office binder clamps.
I started out using cheap 1/8 inch sheets of glass which worked perfectly well during the printing process, however the thin glass had a tendency to break! For years I've used two 16x16 panels of thick furniture glass taped together at one edge for a makeshift hinge. The glass is about 3/8 inch thick with grounded edges (no worries about painful cuts!). My thrifty mother found the glass at a garage sale for a couple bucks.
Now that I'm printing larger I have built an 18x24 inch frame using one sheet of 1/4 inch glass taped to a solid panel of oak on one edge. The taped hinge allows me to open and close the glass and wood like a book. Many online businesses sell custom size glass panels, although the shipping costs for glass are steep!
Gum printing has so many variables that affect the outcome of the image, it is often jokingly said that even the phase of the moon can get involved! While this is of course nonsense, the Sun does play an important role in printing. Exposing a print requires ultraviolet light and a huge source of that kind of light gives many of us beautiful tans (or in my case usually burns me red).
The image to the right shows the sun in 4 month intervals. You can clearly see its activity changing over time. The visible light we see from the sun is pretty steady, however, UV and x-ray radiations fluctuate over the years.
In addition to the sun's weather patterns, we have our own earthly weather patterns to worry about! Cloudy days, windy days, hazy days, time of day, altitude, seasons, and the list of factors affecting sunlight goes on. If you do print using the sun, you should live somewhere that's sunny a good portion of the time, say like here in Texas during the summer. A general rule of mine is: If you could get sunburned, or at least a tan, you can expose a print. I have seen a print exposed on a cloudy day, but the results were not what you would call remarkable.
I gave up on the variability of the sun and expose my prints using a homemade ultraviolet exposure unit, basically an array of uv lights like a tanning bed. It has never failed me once, and I now can print with consistency all night if I wished, something the Sun or the Moon won't let you get away with!