Introduction: Tri-color Gum Bichromate Prints From Digital Images

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This instructable will demonstrate how to create a tri-color gum bichromate print from a digital image.  Gum bichromates have a history dating to 1858, when the first color images where printed.  Of course 150 years ago photographers working in large format black and white made their negatives individually using red, green and blue filters.  Take note that this required panchromatic film, presumably a rare commodity in the era of orthochromatic plates - serious handicap in any attempt at full-color printing.  Computers have been used to emulate that filter process for some time (remember the days before the internet, when things were printed?), so why not print those negatives using the very process that was novel before Lincoln was in the White House and remained en vogue through the early 20th century.  Best to avoid waxing philosophical, but many works from the heyday of gum printing are really beautiful, inspiring images.  And those works will endure the test of time, given gum bichromate's archival properties.

(Special eco-bonus: gum bichromate prints are "developed" using only water.  No developer, no stop, no fixer, no toner.)

That the reader has basic skills with PhotoShop, Gimp or some other photo editing software is assumed.  The purpose of this document is to explain in almost painful detail exclusively how to perform the printing portion of the process, although rudimentary information is provided, since it is vital in explaining portions of the methods used.

That the reader is skilled in working with potentially dangerous chemicals and understands the risks associated with such activities is also assumed.  Warning: dichromates are poisonous and should not be ingested.  Raw dichromate crystals look like orange-colored sugar but can be lethal.  Be smart.

This is the starting image of carabiners used for the demonstration: the raw image with the exception of its being scaled to 1000 pixels on the long side @ 72dpi.  No other changes have been made.  Total eye candy.

For the camera technoids: original 3Kx2K image produced using Nikon D50 with 100mm Sigma macro @ f8 and Nikon SB-60 flash  (-1 EV) with hood on Bogen 3021/3047 combination, but that's only so much nonsense.

(Yeah, what's the deal with the process being called a bIchromate but the photoreactive chemical being used is a dichromate?  At some point during its golden age, major discoveries were made in molecular chemistry, leading to a sort of unification amongst international chemistry unions regarding naming conventions.  Chemically, bichromates were dead in favor of the much zippier dichromate.  Photographers decided simply to ignore the chemical name change and stick with the deeply rooted bichromate.  (Trivia!  Get your trivia heeyuh!)

Step 1: Materials Required

This process may seem material rich, which may discourage some from jumping in and giving it a try, but reasonable alternatives exist at a lower cost that will still yield stunning results.

To create color separation negatives, acquire these items:
- a computer and PhotoShop, Gimp or other photo editing software
- a digital image of reasonable resolution to create a print of desired size
- transparency film
- photo printer

To create light sensitive coating, commandeer this gear:
- liquid water color pigment in tubes
- saturated solution of dichromate (ammonium, sodium or potassium): 3 x 5ml
- 3:1 cut gum Arabic solution: 3 x 5ml
- mortar and pestle

To create final print, jack this stash:
- watercolor paper
- reasonable quality paintbrush that won't streak (quality hake brushes are quite inexpensive)
- masking tape
- pins
- print frame  This does not have to be a deal breaker.  A crude print frame can be made by hinging two thick sheets of glass together to sandwich the paper/negative combination.  A local glass shop should be able to supply two 11x14" sheets of non-UV glass with polished edges for maybe $15  An even less expensive method that is limiting in that it permits printing with overhead lighting only is just using one pane of glass atop some flat backing.  Be creative, O Pioneer.
- light source or our star, Sol
- photo trays

What's pictured:

A work Ansel Adams print of Canyon de Chelly - 26 x 20.
A 16x20 contact print print frame.  This is a Bostick and Sullivan maple model.  Unbeatable product.
Two sheets of *gelatin-sized* 11x14 sheets Lenox 100 paper
Color separation negatives (10" x 7" approx.)
Mortar and pestle
Gum Arabic (250ml 3:1 cut)
Ammonium dichromate (100ml of saturated sol.)
Masking tape
Measuring droppers
2" Hake brush

(Off camera: three 11x14" trays and a pencil that was going to make a later appearance but ended up on the cutting room floor.)

Step 2: Creating Color Separation Negatives

This is going to be a brief description but enough information should be here to get anyone started...a kind of instructable-within-an-instructable thing.

Gimp 2.6.9 is used as the guide, but PhotoShop users should be able to follow along.

1. Open your image file.

An image of 3Kx2K resolution @ 300dpi works perfectly, since a 10" wide image is what's used here.  Do your own math for higher or lower resolution images.

2. Flip the image left/right (Image/Transform/Flip Horizontally).  This is to make sure that when the negatives are printed, they're orientated correctly.

3. Invert image colors (Colors/Invert).  This creates a negative image.

4. Separate the image into component layers (Colors, Components, Decompose).  This will break your image into red, green and blue layers.  If you can't see this, open the Layers dock with Ctrl-L.

5. Print each layer separately in BLACK INK ONLY onto transparency film at your premium print setting.  Make sure only one layer is visible at a time when printing.  A small eye is present next to visible layers in the Layer dock.  Clicking it toggles that layer's visibility.  Turn off all but red, and print.  Turn off all but green, and print.  Turn off all but blue, and print.

6. Mark each negative with its layer information.  *** Vital

This is a budget operation.  Yes, that's overhead transparency film.  The negatives produced and used in the article were printed on budget film on an old Epson Stylus Photo R340.

Step 3: Registering Negatives

In order to eliminate blur-causing negative creep, negatives must be registered, so that when stacked, they're aligned perfectly.  In keeping with being a budget operation, please note the use of push pins.


To register your negatives, just stack them in perfect alignment, and stab a corner completely through with a pin.  Recheck your alignment and perform the very same operation on an adjacent corner.

Note the "R" in the photo, denoting the RED negative atop the stack.  Labeling is good.  But, wow, that macro shot of the push pin is a special bonus!

These pinholes will act as your guide when printing each layer of pigment.

Remove the pins and stack your negative in BGR (blue, green,red) order to prepare for printing.  Expect a quiz on this later.

Step 4: Mixing Pigment - Starting With Yellow

Gum bichromate prints revolve about the reaction between gum Arabic and a dichromate solution.  Dichromates are light-sensitive and, when exposed to UV light, will harden gum Arabic - trapping any pigment floating in it  - and become insoluable in water.  After exposure to UV the insoluable dichromate and unhardened gum/pigment are washed away, leaving an image behind.  Magic in photographic printing discovered less than 20 years after the first permanent photographic processes were invented.

In further keeping with the budget operation code of fiscal responsibility, the chemicals used herein were mixed at home.

To make a saturated ammonium dichromate solution, add 11.7g ammonium dichromate crystals to 88.3ml distilled water in a 100ml dark brown glass bottle.  This ideally makes about 13.25%, but since we're dealing with a saturated solution, a good chunk of crystal will remain at the bottom.  A little of this is a good thing, since it guarantees a saturated solution; however, too much is just wasteful.  Tweak the amount of water added to minimize waste.  As long as the solution remains saturated and exposure times consistent, results should be as predictable as can be expected, especially when so very much can go wrong.  Mix thoroughly.  Can be used as soon as solution clears.  Mixed chemicals are obviously available online, but dry chemicals are cheap and easy. 

Dry dichromate crystals are very poisonous chemicals and look like candy.  Be smart are about storing these or any potentially dangerous chemicals, especially in a home with children under foot.

Gum Arabic
To make 3:1 gum Arabic solution, add 25g (which is going to seem like an extraordinary *volume*) gum Arabic powder to 75ml distilled water in a glass bottle  This stuff is ridiculous.  Keep adding and adding, and when it seems more powder can not possibly be added, the water mysteriously dissolves all the powder and asks for more.  Mixture will resemble thin wallpaper paste, but will clear with resting.  Mix thoroughly.  Make at least a day in advance.

This natural extract of the acacia tree is a benign substance commonly used in chewing gum and candy manufacture.
Next to pigment this is guaranteed to be a top expense if bought premixed at an arts and crafts store.  Recommendation: buy powder in bulk and mix at home to save 75%.

These are the only chemicals required and should be enough to make 60 prints, 20 sessions working 3 at a time.  Each will keep indefinitely in a cool, dark location.

Liquid watercolor pigments in tubes are easiest, though probably not the most economical.  Powders are least expensive to use, but can be problematic in getting a thorough mix.  Recommendation: stick with liquid pigments.  When producing larger quantities of prints, purchase pigments in larger tubes (21ml).  The fuel economy of the small tubes is pretty miserable.

Obviously any color can be used for any layer, but this demonstration is designed to provide as clear an example of RBG-CYM three color gum printing.  The "standards" for cyan, yellow and magenta that were used:

cyan: Cottman phthalo blue 327
magenta: Cottman alizarin crimson hue 003
yellow: Cottman cadmium yellow pale hue 119

This is as good a place as any to explain pigment choices.  When printing from RBG negatives, positive colors, i.e. colors negative to the negatives, are required to produce the final positive image.  The red negative is used when printing the cyan pigment layer; the green negative is used when printing the magenta layer; the blue is used for the yellow.

In this example, the pigment layers will be printed in yellow, magenta, cyan order, corresponding to the blue, green and red negatives, respectively.  (You did label and stack them properly, right?  Did you pass the quiz?) 

One place not to skimp is in paper selection.  So many watercolor papers exist from top manufacturers, it's hard to choose.  For this example Lenox 100 is used...a 250g/m2 100% neutral pH rag paper.  It has distinct differences in its surfaces.  The "back" is relatively smooth with excellent tooth.  The "front" has an undesirable texture for this application made to emulate the look of European mould-made papers.  The back is being used. Lenox tends to shrink (A LOT), so before doing anything, the two sheets used here were sized in gelatin.  Read about sizing; it is very important and quite necessary to this application.

Get good paper, although it won't come cheap.  Cotton, neutral pH, fairly heavy...something that will withstand a minimum of four washings.  Size your paper to eliminate paper shrinkage.  Yup.  Good paper.  Ain't no gettin' round it.  A 22x30" sheet of Lenox is about $4.  Buy in bulk.

To mix pigment for coating:

In a mortar squeeze about a gram of pigment from the YELLOW paint tube.  A gram is about an inch long line.
Add 5ml of gum Arabic.
Mix thoroughly with pestle until pigment is smooth and uniform.
Mix some more.
Add 5ml sensitizer to pigment.
Mix thoroughly again until smooth and uniform.
Mix for another five to ten minutes.
The more thoroughly mixed, the less chance of pigment speckles in final print.

Step 5: Coating the Paper

Perform this step in normal incandescent (non-UV) room lighting.

With pencil mark the corners of the blue negative on watercolor paper, and paint the pigment/sensitizer solution onto the paper.  While there are no awards for neatness, likewise expect nothing for slop.  Try to cover the paper quickly as the gum solution tends to set rapidly and without warning will cause a brush to stick and shed bristles.  Expect good things from high quality hake brushes.  Even coverage is vital to a quality print.  Avoid leaving streaks and long bush lines if possible.  As with anything else, practice makes perfect.  Patience, grasshopper.

Enough pigment is being mixed in these examples to create three full-color final prints, though only two are being made.

Hang to dry in dark place after coating.  Usually 20 -30 minutes.

Step 6: Cleanup - It's Important

While your paper's drying, clean mortar and pestle, brush, droppers and print frame.  This is a recurring theme.

Do NOT clean with Icy Hot or soap.  Just water.

Step 7: Registering Negative to Paper and Preparing to Print Yellow Layer

Perform this step in normal incandescent (non-UV) room lighting.

Register the blue negative with the paper you coated in yellow pigment.

Repeat: Register the BLUE negative with the paper you coated in YELLOW pigment.

If truer-to-life colors are the desired effect, keeping your paper and negative orders correct is essential.

To register the negative to the paper, tape the negative at the top center with masking tape.  A 2-3 inch strip should do it.

Poke your pins THROUGH THE EXISTING HOLES IN NEGATIVE, continuing through the paper.  Need it be said to look at the pictures?

Remove pins, BUT LEAVE THE TAPE, and place the paper/negative stack in print frame.

Step 8: Printing and Washing Yellow Layer

Place assembled print frame in front of light source.  For this demonstration, a 100-watt metal halide light is being used.  With its high ultraviolet output, a metal halide light is an excellent light source for gum bichromate printing.  Let your light warm up in advance to ensure achieving optimum output in intensity and color temperature.  Consistency is paramount.  With a 100-watt bulb, print times of about a minute are to be expected with the print frame placed about 18 inches from the bulb, that is, given typical negative densities.  AAYMMV.

Short of a metal halide, the best available light source is probably the sun, although consistency when using it is a most Sisyphean task.  Fluorescent or BLB tubes also work, but print times are dramatically longer.

After exposing (printing) for 60 seconds, remove print from print frame and slide face-up into a tray of warm water (start around 80 degrees F) in normal room lighting (no fluorescent).  After a couple minutes, flip the print to face down and leave for 5-10 minutes.


The pigment is EXTREMELY FRAGILE at this point, and while other methods of "development," including manipulation using brush or water, have artistic advantages, this example is of a more-or-less straight print.  Gums are unpredictable enough, thank you.

Fill another tray with slightly cooler water, and transfer the print to it face up.  Gaze upon it, and wonder, "WTF is that?"

Soak print face down for another few minutes.  Fill another tray with even colder water, soak, continue...

...until you have gotten all the dichromate stain out of the print but have left enough (usually the maximum) yellow pigment behind to have top impact.  The problem is that dichromate stain is also yellow, so use judgement to determine when the print starts to fade too much.  This also gets easier with practice.  A good starting point is three or four progressively cooler 10 minute soaks in clean water.

Hang to dry.  Drying time is 60 - 90 minutes.

Step 9: Coating the Magenta Layer

Now that the yellow print layer's dry, time to print the magenta pigment layer, which requires the green negative.

Repeat: Time to print the MAGENTA pigment layer, which requires the GREEN negative.

Mix magenta pigment the same way as yellow: one gram pigment plus 5 ml gum Arabic.  Mix, mix, mix.  Add 5ml sensitizer.  Mix, mix, mix.  This is being treated here with brevity in the interest of humor and well, brevity.  Be nice to your print.  Don't rush.  You're going to be here a while.

Perform this step in normal incandescent (non-UV) room lighting.

Coat evenly within pencil marks directly over the previously printed yellow pigment layer.  This process is astounding in that the deeper into it one progresses, the uglier the print becomes...until the very end.  It's horrible.

This layer will be more difficult to smooth than the yellow layer.  Notice the pictured wide hake brush that was used to lightly brush over the coated surface of the print to blend the finish.  A light touch is required.

Hang to dry thoroughly about 20-30 minutes.  In the meantime, clean your mess, and clean your print frame.  Dust is your enemy.

Step 10: Printing the Magenta Layer

Perform this step in normal incandescent (non-UV) room lighting.

Registering paper and negatives and print methods need not be reiterated.  Align the pinholes in the (now REALLY fugly orange) paper with the pinholes in the GREEN negative.  Poke through.  Check alignment and place a strip of masking across the edge of the paper.  Remove pins, and place assembly in print frame.

As a starting point, expose the magenta layer for around, 10% longer than the yellow layer.  The yellow layer was exposed in step 8 for 60 seconds; the magenta layer shown here was exposed for 66 seconds.

A lower print time can be a crude way to affect contrast for a given layer, but only to a very critical tipping point.  At a certain (short) print time, most pigment will just wash away as soon as it touches the water, leaving only the thickest blobs where the corresponding negative was thinnest.

Step 11: Wash Magenta Layer, Wonder Why Am I Doing This?

The prescription for washing is the same here as in washing the yellow.

Fill a tray with water, soak face up for a couple minutes.

Flip.  Hey!  I can REALLY see the soluable dichromate solution wash out now!

Soak for 10 minutes or so.

Do this three or four times with progressively cooler clean water until the desired lightness in the print is achieved.

Hang to dry...60-90 minutes.

Step 12: The Blue Layer: Home Stretch

Mixing, coating, registering, printing and washing should be old hat by now, but here's a refresher:

1. Mix your pigment: a gram cyan watercolor, 5 ml gum, 5 ml dichromate

2. Coat paper.  Smooth and slow, there, Pancho.  Coating screw-ups can not be undone, and at this point hours of effort are at stake. Perform this step in normal incandescent (non-UV) room lighting.

3. Hang to dry 20 -30 minutes.  Clean your stuff up.

4. Register red negative to cyan-coated layer.  RED negative to CYAN layer. Perform this step in normal incandescent (non-UV) room lighting.

5. Tape (remove pins) and place in print frame.

6. As a starting point, expose for 10% less than yellow layer: 54 seconds was used here.

7. Wash.  Wash.  Wash.  No cutting corners.  At least 30-40 minutes.

8.  Hang to dry.  Pat yourself on the back.  You deserve it.

Step 13: The Finished Print

After drying, it's best practice to expose your print to a minute of UV and another thorough wash to completely eliminate any dichromate stain.  This is the final print after being scanned in RGB at 300dpi on a flatbed scanner.  The image was corrected in no way except to scale to 1000 x 700 (approx.) pixels @72dpi.