Casein is a colloid derived from milk. For those already versed in gum printing, casein printing is done much the same way: a colloid is mixed with a light-sensitive dichromate and watercolor, brushed onto paper and exposed under a negative to UV light. Where the light hits the most, the casein hardens the most. Where the light hits the least, the casein and pigment wash away in a simple water bath, and thus an image emerges. For each print, this exposure and development process is done multiple times, particularly for a tricolor print, which will require a red, yellow and blue exposure to complete it.
Casein printing differs from gum printing in a number of ways however.
• It is quicker to expose, from as little as 15 seconds to four minutes
• A lesser-strength dichromate is best such as 10% potassium dichromate
• It is quick to develop, in as little as several minutes to 15 minutes
• It is very hardy, and alkalies such as ammonia or sodium carbonate as well as rougher methods of brushing can be used during development
• The layer is thinner, resulting in a faster layer, a sharper layer, and a finer-grained layer, but one that still carries a high pigment load
• Casein is dead-matte, not glossy, when powdered pigments are used
• Ammonium caseinate powder kit from Photographers’ Formulary or fat-free cottage cheese and nonsudsy ammonia 10% solution
• Potassium (or ammonium) dichromate
• Powdered pigments or tube watercolors (suggestions: Daniel Smith or M. Graham quinacridone rose, thalo blue, nickel azo yellow, lamp black and/or burnt sienna)
• Gamblin PVA (Polyvinyl Acetate) Sizing
• 140lb watercolor paper like Fabriano Artistico, Arches Aquarelle or watercolor paper sold in convenient watercolor pads
More than likely, multiple printing will be done, so the paper should be preshrunk before the initial printing or successive printings will be slightly out of registration. Once the paper is preshrunk, it can be easily sized with non-toxic Gamblin PVA Polyvinyl Acetate Sizing and ready in minutes.
1. Cut all paper to the size needed. Save time and do a large batch at once.
2. Soak paper in hot water for a minimum of 1⁄2 hour.
3. Drain and hang the paper to dry.
4. Dilute PVA 1 part + 2 parts water.
5. Measure 5ml (1 tsp) of solution per each 11×14″ paper sheet and brush the PVA on the paper surface carefully and thoroughly. Some papers benefit from two coats; dry slightly between coats. Dry paper. Once bone dry, paper can be used immediately. Use a hair dryer to speed up drying time.
10% Potassium Dichromate Preparation
10g potassium dichromate (11⁄4 tsp.) Water to 100ml Measure out the dichromate crystals wearing gloves to minimize exposure. Do not breathe in the dust! Add the crystals to the water and bring the water to 100ml. Store this solution in a brown glass dropper bottle if desired.
Photographers’ Formulary is the only source for am- monium caseinate powder to date. There are many sources for sodium caseinate, but the sodium form
is not water soluble like the ammonium form. The Formulary sells a casein kit that contains ammonium caseinate, potassium dichromate and sodium benzoate preservative. Mix small batches of casein at a time. Approximately 1ml covers 30 square inches, so a little goes a long way!
10g ammonium caseinate powder (2 Tb.) 90ml water (3 ounces) 1g sodium benzoate (about 1⁄4 teaspoon) 10ml hot water (2 teaspoons)
Stir the casein powder into the 90ml water. It’s non- toxic and can be mixed in a blender if desired. Blend on high for several minutes until thoroughly dispersed. It doubles in volume, but within a couple hours reduces from foam to a milky liquid. Pour the casein into a container, add the sodium benzoate dissolved in the 10ml (2 teaspoons) hot water and store in the refrigerator if desired. This is easily enough for 20 tricolor 8×10 inch prints.
Cottage Cheese Casein Preparation
An alternative to caseinate powder is cottage cheese.
60g nonfat cottage cheese (1⁄4 cup)
15ml ammonia 10% solution (1 tablespoon)
20ml water (4 teaspoons)
Preservative, If Desired (See Below)
Place the cottage cheese in a strainer and rinse off any whey. Drain. Put in a container and add the ammonia and water. Stir. This will liquefy to a lump- free opalescent solution in a short time. Even though a container of cottage cheese contains four times this amount, it is best to mix smaller amounts of casein at a time and store in the refrigerator because it spoils fairly quickly unless preserved. If desired, add a preservative such as a gram of sodium benzoate dissolved in two teaspoons hot water.
Coating Mix Preparation
At time of use, thoroughly mix together 1/8 teaspoon powdered pigment or tube watercolor +1⁄2 teaspoon casein +1⁄2 teaspoon potassium dichromate to coat up to two 8×10 prints. Brush the coating on the paper. Smooth out the brush strokes with a dry brush held at a 90o angle. Let dry away from light.
With such a thin layer the paper is usually dry within minutes to a half hour. For a tricolor casein print, start with either the blue or red/magenta layer. Be sure to clean the brush immediately after use so the casein does not harden and ruin it. Tip: if the brush
has gotten crusty, suspend the bristle tips in water with a bit of ammonia or sodium carbonate.
Note: Pigments that contain nickel, copper, manganese, chromium, iron or other metal salts will precipitate or throw down flakes or chunks of insoluble casein so the practice of making stock color solutions is not recommended.
Register the negative on the paper with registration marks before exposing. Blue is the fastest; yellow, red and black are slower, although this is not a hard and fast rule and dependent on pigment brand and amount. As little as one half to four minutes under UVBL is sufficient for all colors. Generally blues expose in under 2 minutes, reds perhaps around 2-3 minutes and yellows perhaps a minute longer or 3-4 minutes, but lately I have had much success exposing all colors for 45-90 seconds. Casein is fast! If using the sun, an always-variable light source, these times should be cut way back, even by as much as 3⁄4 those times!
(Making an appropriate digital negative is beyond the scope of this article. See Gum Printing and Other Amazing Contact Printing Processes or PrecisionDigitalNegatives.com to learn more).
1. Place the print face up in a tray of warm to hot water for a minute or two until all dichromate leaches out. This can be seen as yellow streaming out into the water from the borders of the print. Handle the print with tongs in this tray to minimize exposure to dichromates.
2. Transfer the print to a fresh tray of warm water for the remainder of the development time. Since the dichromate is all but gone from the print, it can now be handled. If necessary, prop the print up on a piece of Plexiglas and give it a gentle spray of water to release pigment in the highlights. Then return the print to the tray face down for the remainder of the development time.
3. Oftentimes, the development of the casein print is immediate, but more development is needed to clear yellow dichromate stain. If the print is not developing quickly after the water spray, there is recourse: run a soft foam roller or a hake brush over the surface to clear the highlights. With casein this measure does not risk as much flaking and grain as with gum. The grain, if revealed, is much finer. Automatic development (not touching the face of the print but merely letting the water do its thing) produces the smoothest print.
4. If the print is not developing, mix up a separate alkaline tray with up to 10–15ml ammonia per liter of water, or two teaspoons sodium carbonate per liter, or even a few teaspoons of baking soda. Alkalis speed up development. The print can be soaked in this bath for a minute, re-sprayed and returned to the water bath trays for the remainder of the development time. If these kind of chemical measures are always necessary though, cut back on exposure time and/or use a denser negative with greater contrast.
5. Once the development is complete, hang the print to dry. When the print is completely dry, coat and process as per above once again. It may be necessary to use a size replenisher if alkalies have been used.
The coating develops white circles that look like popped bubbles.
• Called “fisheyes,” coating mix may be too thick but keep brushing until they disappear
• Sizing is too heavy
The print does not develop but looks like a square of low contrast nothing.
• Negative is too low contrast or thin
• Use a stronger size, even full-strength PVA
• Dilute dichromate to 5%
• Cut exposure to even as little as 15 seconds with some colors (blue)
• Use hotter development water
• Use an alkali in the water
• Use a roller, scrubber, brush, or Scotch Brite pad to aid in developing the print, but replenish size before the next layer
• Use a size replenishing coat between layers
The print is too high contrast/the coating flaked off.
• Size is too heavy
• Negative is too contrasty
• Too much pigment is used in the coating mix
• Layer is too thickly applied
• Not enough exposure
Resources: Ammonium caseinate, dichromate, benzoate- photoformulary.com; Watercolor paper, Gamblin PVA, watercolors, powdered pigments- JerrysArtarama.com & DanielSmith.com; PrecisionDigitalnegatives.com