Casein Binder

Part of Pigments and Paint

Making casein paint binder from milk — a durable, waterproof coating system used for thousands of years.

Why This Matters

Casein paint is one of the best-kept secrets in the history of coatings. Made from ordinary milk, it produces a hard, smooth, waterproof finish that rivals modern latex paint in durability and far exceeds it in simplicity of production. Casein paints were used on everything from Pharaonic furniture to American colonial buildings, from Shaker woodwork to Russian icons. Some casein-painted surfaces have survived centuries of use with their finish intact.

For a rebuilding community with access to dairy animals, casein paint offers an enormous advantage. The raw material — milk — is renewable and available daily. The chemistry is simple: acid curdles the milk, separating casein protein from whey. The dried casein is then dissolved in an alkali (lime or borax) to create a powerful adhesive and binder that becomes waterproof when it dries. No petroleum products, no complex synthesis, no imported materials.

Casein binder is also exceptionally versatile. As a paint binder, it produces matte to semi-gloss finishes in any color. As an adhesive, it bonds wood with a joint stronger than the wood itself (casein glue was used in aircraft construction through World War II). As a sizing agent, it seals porous surfaces before painting. As a ground for painting, casein gesso creates a smooth, white surface superior to many alternatives. This single material, made from milk, serves as a complete coating system.

Understanding Casein Chemistry

What Is Casein?

Casein is a family of phosphoproteins that make up approximately 80% of the protein in cow’s milk. When milk is acidified, casein molecules aggregate and form solid curds. These curds, when dried and re-dissolved in an alkali, form a powerful colloidal solution that acts as both an adhesive and a film-forming binder.

Why It Works as Paint

Property	Benefit
Strong adhesion	Bonds to wood, plaster, stone, metal, fabric
Hard film	Dries to a tough, smooth surface that resists abrasion
Water resistance	Once dried and cured, casein film is largely waterproof
Chemical stability	Does not yellow significantly over time
Matte finish	Natural matte look; can be polished to semi-gloss
Pigment compatibility	Works with virtually all mineral and earth pigments
Biodegradable raw materials	Made entirely from renewable sources

Making Casein from Milk

Method 1: Acid Curd (Simplest)

This produces casein through acid precipitation — the fastest and most straightforward method.

Materials needed:

• 1 liter of skim milk (or whole milk with fat removed)
• 30-50 ml of vinegar (or lemon juice, or sour whey from a previous batch)
• A pot for heating
• Cheesecloth or fine cloth for straining
• A bowl

Process:

1. Skim the fat — if using whole milk, let it sit in a cool place for 12-24 hours. The cream rises to the top. Skim off the cream (use it for butter). Fat in the casein produces a weaker binder.

2. Heat the skim milk — warm to approximately 35-40 C (blood temperature — warm to the touch but not hot). Do not boil — excessive heat denatures the protein and reduces binder quality.

3. Add acid — stir in vinegar slowly. The milk will begin curdling immediately — white lumps (curds) form and separate from a yellowish liquid (whey).

4. Continue adding acid — keep adding vinegar while stirring gently until no more curds form. The whey should be relatively clear.

5. Let settle — allow the mixture to sit for 15-30 minutes. Curds settle to the bottom.

6. Strain — pour through cheesecloth. Collect the curds in the cloth. The whey can be saved for subsequent batches (it is already acidic) or used as a garden fertilizer.

7. Wash the curds — rinse the curds in the cheesecloth under clean water 2-3 times. This removes residual whey (which contains sugars that can promote mold growth in the finished paint).

8. Press — squeeze the cheesecloth firmly to remove as much water as possible. The result is a dense, white, crumbly mass — this is raw casein.

9. Dry — spread the pressed casein on a clean, dry surface in thin layers. Air dry for 2-5 days in a warm, well-ventilated area. Break up clumps as it dries to speed the process. When fully dry, casein is hard and chalky.

Yield

1 liter of skim milk produces approximately 25-35 grams of dry casein. For a day’s painting, you need approximately 50-100 grams, so plan on 2-4 liters of milk per work session.

Method 2: Rennet Curd

If you have access to rennet (from calf stomachs or certain plants), rennet-set casein produces a slightly different product:

1. Warm skim milk to 30-35 C
2. Add rennet according to the traditional cheese-making proportion (a piece the size of a grain of rice per liter)
3. Let sit undisturbed for 30-60 minutes until a firm curd forms
4. Cut the curd into cubes, let sit 15 minutes
5. Strain, wash, press, and dry as above

Rennet casein is slightly harder and less soluble than acid casein, which can be advantageous for waterproofing applications but requires more alkali to dissolve.

Dissolving Casein into Binder

Dry casein is not usable directly. It must be dissolved in an alkali to form a liquid binder.

Lime-Casein (Most Durable)

The strongest and most water-resistant casein binder:

1. Crush dry casein — grind to a coarse powder using a mortar and pestle
2. Soak — cover with warm water (2 parts water to 1 part casein by volume). Let soak for 2-4 hours until the casein has absorbed water and softened.
3. Add lime — stir in slaked lime (calcium hydroxide) at a ratio of approximately 1 part lime to 5 parts dry casein by weight. Add gradually while stirring.
4. Stir continuously — the mixture will begin to thicken and become smooth and glossy as the casein dissolves. This takes 15-30 minutes of stirring.
5. Adjust consistency — add water gradually until the binder has the consistency of heavy cream
6. Test — a properly dissolved lime-casein binder is smooth, slightly thick, and translucent when spread thin on glass. If lumpy, continue stirring or strain through cloth.

Working Time

Lime-casein begins curing (cross-linking) as soon as it is mixed. It has a working life of approximately 4-8 hours, after which it thickens and eventually becomes unusable. Mix only what you need for the day’s work.

Borax-Casein (More Forgiving)

If you have access to borax (sodium tetraborate):

1. Dissolve borax — add 15-20 grams of borax to 100 ml of warm water. Stir until fully dissolved.
2. Add soaked casein — stir softened casein into the borax solution
3. Mix thoroughly — the casein dissolves more slowly with borax than with lime. Stir for 20-30 minutes.
4. Advantage — borax-casein has a longer working life (12-24 hours) and is easier to work with
5. Disadvantage — slightly less water-resistant when cured than lime-casein

Ammonia-Casein

For areas where ammonia is available (from aged urine or sal ammoniac):

1. Soak crushed casein in water
2. Add ammonia solution gradually while stirring — the casein dissolves readily
3. Very long working life — ammonia evaporates slowly, giving hours of use
4. Produces a smooth, workable binder
5. Strong ammonia odor dissipates after the paint dries

Making Casein Paint

Basic Casein Paint Formula

1. Prepare the binder — dissolve casein as described above
2. Select pigment — any mineral or earth pigment works well with casein
3. Mix pigment paste — grind dry pigment with a small amount of water on a stone slab to create a smooth paste
4. Combine — add the pigment paste to the casein binder, stirring thoroughly
5. Adjust — add water for thinner paint, more pigment for more opacity, more binder for better adhesion

Casein Paint Proportions

Application	Casein (dry)	Pigment	Water	Notes
Interior wall paint	50 g	200-300 g	500 ml	Matte finish
Wood furniture	50 g	150-200 g	400 ml	Can be sanded and polished
Exterior wood	50 g	200 g	400 ml	Add extra lime for weather resistance
Artist’s painting	50 g	100-150 g	300 ml	Thicker for brush control
Primer/sealer	50 g	50 g chalk	600 ml	Thin, penetrating first coat

Adding Chalk or Whiting

For economy and better coverage on walls, casein paint is often extended with chalk (calcium carbonate):

1. Add finely ground chalk to the casein binder — up to 3 parts chalk to 1 part casein
2. The chalk provides body, opacity, and reduces the cost of expensive pigments
3. Too much chalk weakens the paint film — test on a sample area first
4. Chalk-extended casein paint is the traditional whitewash of European and American buildings

Application Techniques

Surface Preparation

Casein paint adheres best to:

• Clean, dry wood (sanded to remove any previous coating)
• Fresh lime plaster (excellent bond — the lime in the paint reacts with lime in the plaster)
• Clean stone and masonry
• Sized fabric or canvas

Surfaces that need preparation:

• Glossy or oily surfaces — sand to create tooth
• Very porous surfaces — apply a thinned first coat (seal coat) before the full paint
• Previously painted surfaces — sand lightly and test adhesion on a small area first

Applying

1. Stir thoroughly before each application
2. Apply in thin coats — casein paint builds coverage in multiple thin layers, not a single thick coat. Thick coats crack as they dry.
3. Brush technique — use a soft brush and work in one direction to minimize brush marks
4. Drying time — each coat dries in 1-4 hours depending on temperature and humidity
5. Number of coats — typically 2-3 coats for full, even coverage
6. Sanding between coats — lightly sand each dried coat with fine sandpaper or a pumice stone for the smoothest finish

Finishing

Dried casein paint can be finished in several ways:

• Matte — the natural finish; simply let the final coat dry
• Satin — buff the dried paint with a soft cloth or sheepskin pad
• Semi-gloss — apply a thin coat of drying oil (linseed) or beeswax polish over the dried casein paint
• Polished — sand very smooth between coats and buff the final coat vigorously with a felt pad. Historical furniture makers achieved remarkable glosses this way.

Troubleshooting

Problem	Cause	Solution
Paint too thick	Not enough water	Add water gradually while stirring
Paint lumpy	Casein not fully dissolved	Strain through cloth; stir longer
Paint cracking	Coat too thick or dried too fast	Apply thinner coats; avoid direct sun or heat while drying
Poor adhesion	Surface not clean or too glossy	Clean and sand the surface; apply a seal coat first
Mold growth	Residual whey sugars in casein; damp conditions	Wash casein curds more thoroughly; add borax as a preservative; improve ventilation
Yellow discoloration	Fat contamination from whole milk	Use fully skimmed milk; wash curds more thoroughly
Short working time	Lime-casein curing too fast	Work in smaller batches; use borax-casein for longer open time

Casein as Adhesive

Beyond paint, dissolved casein is a powerful wood adhesive:

1. Mix thick — dissolve casein to a thick, cream-like consistency
2. Apply to both surfaces — coat both pieces of wood to be joined
3. Clamp firmly — bring pieces together and clamp with maximum pressure
4. Cure time — 24 hours for handling strength; full cure in 1-2 weeks
5. Result — a joint that is often stronger than the surrounding wood. Casein glue was used in laminated aircraft propellers because of its strength and water resistance.

Historical Significance

Casein was one of the first plastics. In the early 20th century, casein was hardened with formaldehyde to create “Galalith” — a material used for buttons, buckles, knitting needles, and jewelry. While formaldehyde is not available in a rebuilding scenario, the underlying principle — that milk protein can be transformed into a durable, hard material — illustrates casein’s remarkable versatility.