An extract from Laura Blacklow’s book New Dimensions in Photo Processes.
Always be careful when handling chemicals. Read the health and safety instructions.
“I chose casein printing for this image because it is a delicate process, which creates subtle colors and forces me to slow down as I work.”
Some of the color has been made by building up casein coatings of different hues, each of which was printed with a negative. Some of the color has been achieved with casein coatings and no negative, while other hues come from colored pencils.
This process, which utilizes curdled milk to bind the component materials, was patented in 1908 as a technique to use with other photo-printmaking methods, such as Van Dyke brown printing. Used by itself, however, casein printing can yield as subtly graded, colorful results as the watercolor paints that make up part of the emulsion. The translucency of each color coating permits a new hue in a new layer of emulsion to change the pigment color underneath. One of the least expensive of the hand-applied emulsions, casein printing is closely related to gum bichromate printing is recommended.
Bichromate (also called dichromate) can cause skin inflammation, similar to an allergic reaction, upon repeated exposure. Wear protective gloves and goggles at all times when mixing and coating bichromate as well as when washing a bichromate-sensitized print. Wear a respirator with a toxic dust filter when handling bichromate crystals.
Bichromates can be poisonous when ingested, even in small quantities. Keep chemicals and chemically-contaminated materials away from your mouth and out of the reach of children and pets. Bichromates are suspected carcinogens.
Dispose of excess solution or chemical by flushing it down the drain with a large volume of water, not in a wastepaper basket.
Certain paint pigments, such as emerald green, cobalt violet, true Naples yellow, all cadmium pigments, flake white, chrome yellow, manganese blue and violet, Verona brown or burnt umber, raw umber, Mars brown, lamp black, and vermillion, can lead to poisoning and other complications if they are ingested or inhaled frequently. Wearing a respirator, working in a ventilated area, and carefully washing hands and cleaning fingernails after using these pigments can prevent accidentally carrying them to the mouth and ingesting them.
- Paper or fabric is preshrunk, sized, and dried.
- Under subdued light, the paper or fabric is coated with casein pigment emulsion and allowed to dry.
- The coated surface is placed in contact with a negative or object, and ultraviolet light is shone through the negative.
- The negative is removed. The paper or fabric is developed in water, where unexposed areas of color are dissolved, leaving a reversed image to dry and harden.
- One coating must dry before a new layer may be applied, exposed, and processed.
Negatives the same size as the print you wish of low density (not heavy black) offer the possibility of full tonal range after one exposure to casein pigment emulsion. More often, though, the density of the negative exceeds the range of the emulsion, requiring multiple exposures to build up highlights, middle tones, and shadows. Posterized negatives or color-separation negatives versatility. High-contrast negatives also work quite well. If more than one printing is desired, negatives will need to be registered, and in Gum Printing.
Try stencils, found objects, torn paper, lace, drawings, and photocopies on acetate.
You will need either potassium or ammonium bichromate, which can be purchased in 1 lb (0.5 kg) or smaller bottles from merchants listed in Supply Sources. Ammonium bichromate is preferred for its greater sensitivity to ultraviolet light.
Instant powdered milk, available in grocery stores, provides the casein binder. Undiluted lemon juice or 28% acetic acid, sold as stop bath in photo stores, curdles the milk. A bath of clear ammonia (nonsudsy) from a grocery store or ammonium hydroxide diluted with water to a 1% strength helps clear a muddy print and also is one of the elements in the emulsion.
Professional-grade watercolors or gouache in tubes work best. Liquid casein colors have been used with some success. Make sure that the pigment you use has no chromium (such as chrome yellow) in it, because this will react negatively with the bichromate.
4. Distilled water.
Sometimes tap water is fine for making the bichromate solution, but the use of distilled water eliminates the frustration of possibly mixing bad solution.
Finished prints should be produced on rag paper of medium absorbency for longevity, but practice prints can be made on less-expensive paper. All paper should be sized, then preshrunk, for more than one coating. James R. Collins, author of
The Gum Print
in Darkroom Photography (Oct. 1986), suggests the following method: Size the paper, then air dry it, apply the emulsion to an area about 1 in. (2.5 cm) larger on all sides than the image, and hang the paper in front of a cold-air fan to dry. When the emulsion is just barely tacky to the touch, put two pencil dots 12 in. (30.5 cm) apart and about 1/8 in. (5 mm) inside the emulsion edge. Hold a hair dryer about a foot (30.5 cm) from the paper, and evenly blow warm air over the paper until it has shrunk the paper by 1/8 in. (5 mm) and the pencil dots measure 117/8 in. (30.25 cm) apart. Make the exposure, develop the image, and repeat this preshrinking (without resizing) procedure with each coating.
Preshrink synthetic or natural fabrics as described above. Practice making prints on paper first, because working on fabric is more difficult.
Because sizing paper and fabric is necessary, messy, and time-consuming, you will find it more practical to size several pieces at once. The acrylic-polymer medium method works well. It is easy, but it stiffens the paper and will not allow for shrinkage described in item 5 above. Todd Walker’s alum sizing is more appropriate.
7. Ultraviolet light.
You cannot see a color change with the emulsion, as you can in cyanotype or Van Dyke brown printing, so artificial lights such as sunlamps, photofloods, or fluorescents are recommended because the exposure can be exactly timed. However, indirect sunlight can be used; avoid direct sunlight, where bright radiant heat output can cause the emulsion to fog with exposures over 5 minutes.
Try fine flat-bristle brushes or nylon paint brushes about 1 in. (2.5 cm) or more wide, a sponge brush, or a foam paint roller from the hardware store. A soft brush for coaxing the development of the print in water can be handy. Keep the metal part of the brush out of the bichromate solution. If you use an atomizer or air brush, be sure to wear a respirator, and work in a well-ventilated area.
9. Brown bottle.
Since one of the emulsion’s components, diluted potassium dichromate, is light sensitive, it should be stored in a dark bottle, such as a clean, used fruit juice jar.
10. Cheese cloth and mesh food strainer.
Inexpensive cheese cloth, purchased at a hardware or grocery store, is used to line a food strainer when cleaning the casein base.
11. Printing frame.
Use glass with Masonite, another sheet of glass, or foam core backing.
12. Two trays or tubs.
You will need glass or porcelain trays larger than the print when you develop the image and fix it.
13. Timer, watch, or clock.
14. Measuring spoons.
Cooking utensils are fine. Small clear 4-oz. (102 ml) medicine measuring cups, available at pharmacies, are useful.
15. Measuring cup or beaker and stirring rod.
You will need a cup or beaker that measures 16 oz (455 cc) in 1/2 oz (14 cc) increments.
16. Two registration pins, masking sheets, masking tape, 1/2 in. (11/4 cm) heavy black tape, ruler, hole punch, scissors, stencil knife (optional).
If you are using more than one negative or more than one color, and you want the layers of the image to line up repeatedly.
17. Spray nozzle for sink hose and kitchen blender (optional).
These two items make the job of mixing the casein and the dichromate easier. Do not use the blender for preparing food afterward.
18. Protective gloves and respirator.
19. Shallow bowl.
A small glass or china bowl or plate becomes the palette for mixing the emulsion.
20. Newspaper, hair dryer.
Tip for Making the Bichromate Solution
- Should the crystals separate from the water, they will reblend when warmed to 90°F and stirred.
Tips for Making a Casein Print
- You must thoroughly mix the pigment with the casein and bichromate solution, or the emulsion will be streaked.
- Because the casein curd will dissolve three hours after adding the other chemicals to it, plan on doing multiple coatings during one print session.
- The pure casein can be refrigerated and preserved for a few days.
- Exposure times for casein printing must be determined by testing, so before you start on an actual print you can lower the frustration level by running the following test: Coat with emulsion and dry a sheet of 8×10 in. (20×25 cm) paper. Cut this sheet into 4×5 in. (10×121/2 cm) strips, marking each strip to indicate the emulsion color and formula used. Using a different strip for each exposure, make tests of different lengths of time, being sure to note the exposure time and light source on the strip. Make sure the same part of the negative is used for each test. Develop, dry, and evaluate each test for each emulsion used, and keep a notebook with these tests strips for future reference.
- If the image is overexposed or the light source is too hot, the emulsion will harden and will not wash off. If the image is underexposed, the emulsion will float off, and if you use too much pigment when mixing the emulsion, the color will flake off.
- While the print is developing, you can lighten areas of it by rubbing very gently with a soft brush or cotton swab, but be careful because the emulsion is fragile and severe agitation can remove the image altogether.
- In the Jan./Feb. 2000 issue of
, Bob Whitfield describes “a new process: gelatin acrylic.” He uses posterized high-contrast positives with semigloss house paint on paper sized with gelatin and coated with bichromate.
Making the Bichromate Solution
Equipment You Will Need
- Ammonium bichromate crystals
- Measuring cup or beaker
- Bowl or tub
- Stirring rod
- 1 oz (30 ml) measuring cup
- Distilled water
- Brown bottle
- Neoprene gloves
- Respirator with toxic dust filter
- Pour water into measuring cup to make 1/4 cup plus 1 tablespoon (43.5 ml, 21/2fl oz).
- Adjust water temperature to 90°F (32°C).
- With constant stirring, add 1 tablespoon (14.5 ml, 1/2fl oz) bichromate crystals.
- Add more bichromate until crystals will not mix in but precipitate to the bottom of the liquid.
- Adjust water temperature to 68°F (20°C).
- Stirring constantly, add water to make 11/2 cups (355.5 ml, 12 oz).
- Pour the bichromate solution into a labeled brown bottle.
Making a casein print
Curdling the milk
In a measuring cup, mix 1 oz (30ml) instant powdered milk with 7 oz (199 cc) hot water. By adding a few drops of 28% acetic acid or lemon juice to this mixture, curdle the milk.
Arrange two layers of cheesecloth in a strainer, pour the curdled milk through it, catching the curd in the cheesecloth, and rinse the curd with cold water.
Squeeze the cheesecloth and curd.
Mixing the emulsion
Crumble the curd into a measuring cup, add 2 oz (50 cc) of 1% ammonia, and mix well by hand or electric blender. Thoroughly mix 1/2 teaspoon (2.5 ml) of the casein (or curd) mixture with 1/2 teaspoon of the dichromate solution and 1/4 teaspoon (1.25 ml) watercolor pigment.
3Coating the paper
Use the emulsion to coat one 8×10 in. (20.25×25.5 cm) piece of paper by pouring it from the cup onto the sized paper and quickly working that emulsion on evenly with a brush to an area beyond the image. The emulsion will not look like the watercolor pigment at this point, but may have a yellow-orange tint to it.
Exposing the print
Use a hair dryer on the cool or warm setting, and when the emulsion is dry, make a sandwich with the backing board on bottom, paper with emulsion facing up next, negative reading correctly on the paper, and clean glass on top.
Place the loaded print frame in shade for 10–30 minutes, under a sunlamp or photoflood bulb 3 ft (1 m) away for 3–12 minutes, or under ultraviolet fluorescent bulbs 6 in. (15.25 cm) away for 6–15 minutes. Exposure times vary according to the light source, pigment color, and pigment density. You will have to experiment and keep notes.
Developing the paper
The reaction that occurred during exposure continues after the emulsion has been removed from light. Therefore, immediately place the paper face down in a tray of cool water for 5 minutes. Move the print to a tray containing 2 oz (57 ml) of 1% ammonia and 32 oz (909 ml) cool water for 5 minutes. Development may take several dips back and forth; it is complete when the unexposed areas (highlights) of the emulsion dissolve and float off the paper, and the exposed areas (shadows, middle tones, and highlights with detail) are the same color as the pigment you used.
Fixing the print
Transfer the print to a final tray of gently running cool water for 15 minutes, remove the print, and air dry it flat or use a hair dryer. Resize the print if you want to add another layer of emulsion.
New Dimensions In Photo Processes
by Laura Blacklow
A step-by-step manual to many of the processes.