Pigment Processing

Part of Pigments and Paint

Processing raw materials into usable pigments through physical and chemical methods.

Why This Matters

The gap between raw material and usable pigment is enormous. A lump of ochre-stained clay looks nothing like the smooth, vibrant powder that makes quality paint. A pile of iron rust bears little resemblance to the controlled red oxide that protects steel structures. Transforming raw materials into consistent, reliable pigments requires systematic processing — and the quality of your processing directly determines the quality of every paint, ink, and coating you produce.

Pigment processing is not a single technique but a chain of operations: cleaning, crushing, heat treatment, chemical transformation, particle size reduction, and purification. Some pigments require only physical processing (grinding and washing); others require chemical reactions to produce colors that do not exist in nature. Understanding the full range of processing methods gives you the ability to produce any color your environment can supply.

These processing skills also transfer to other critical technologies. The same calcination techniques used for pigments apply to making lime, processing ores, and producing ceramic glazes. The same levigation used to purify pigments works for refining clay and separating mineral concentrates.

Physical Processing Methods

Sorting and Cleaning

All pigment processing begins with removing contaminants from raw material:

1. Visual sorting: Separate pigment-bearing material from obvious waste rock, soil, and organic debris. Look for the most intensely colored pieces
2. Washing: Rinse in clean water to remove surface dirt, clay, and soluble salts
3. Picking: Remove individual contaminating particles — quartz grains, iron nodules in non-iron pigments, organic matter
4. Drying: Spread cleaned material on a flat surface in the sun. Must be fully dry before crushing

Crushing and Grinding

Reduce raw material to fine powder through progressive stages:

Stage 1 — Coarse reduction:

• Wrap material in cloth and strike with hammer
• Use mortar and pestle for controlled crushing
• Target: pea-sized fragments

Stage 2 — Intermediate grinding:

• Mortar and pestle with firm rotary pressure
• Target: fine sand consistency

Stage 3 — Fine grinding (mulling):

• Muller on flat stone slab with water
• Target: no perceptible grit between fingers
• Duration: 15-45 minutes depending on material hardness

See Grinding Techniques for complete detail.

Levigation (Water Separation)

After grinding, use water settling to purify and grade particles:

1. Suspend ground material in 10:1 water
2. Remove coarse waste after 30-60 seconds of settling
3. Collect medium-grade pigment after 10-15 minutes
4. Collect fine-grade pigment after 2-4 hours
5. Recover ultra-fine pigment after 24-48 hours

See Levigation for the full process.

Sieving

For consistent particle size, pass ground pigment through progressively finer sieves:

Sieve Material	Approximate Mesh	Use
Coarse woven cloth	500+ microns	Remove large fragments
Fine linen	100-200 microns	General paint-grade screening
Silk or very fine cloth	50-100 microns	Artist-grade pigment
Settled from water (levigation)	<10 microns	Ultra-fine watercolor grade

Chemical Processing Methods

Calcination (Heat Treatment)

Heating pigment materials in a kiln or fire transforms their chemistry and color:

Iron oxide color conversion:

Starting Material	Temperature	Result	Color Change
Yellow ochre (goethite)	200-300C	Light orange	Yellow to warm orange
Yellow ochre	300-400C	Red ochre (hematite)	Yellow to red
Yellow ochre	500-600C	Deep red	Yellow to dark maroon
Raw sienna	300-400C	Burnt sienna	Yellow-brown to orange-brown
Raw umber	300-400C	Burnt umber	Cool brown to warm red-brown

Process:

1. Place pigment in a fire-resistant container — unglazed clay pot, stone crucible, or iron vessel
2. Heat gradually to target temperature
3. Check color by removing small samples at intervals — hold the hot sample against white paper to judge color accurately (wet with water for true color)
4. When desired color is reached, remove from heat
5. Cool slowly to prevent cracking of the container
6. Re-grind the calcined material — heating often produces sintered lumps

Temperature Estimation

Without a thermometer: dark red heat is approximately 500C; cherry red is 700C; orange is 900C. For pigment calcination, you rarely need temperatures above cherry red. Most conversions happen below visible red heat.

Precipitation

Some pigments are produced by mixing two solutions together, causing an insoluble colored compound to precipitate (fall out of solution):

Example — Making verdigris (copper acetate):

1. Suspend copper sheets or scrap over warm vinegar in a sealed container
2. The acetic acid vapor reacts with copper over 2-4 weeks
3. Scrape the blue-green crust (verdigris) from the copper surface
4. Grind and levigate the collected material

Example — Making Prussian blue (if iron sulfate and potash are available):

1. Dissolve iron sulfate (copperas) in water
2. Add potassium ferrocyanide solution (made by fusing dried blood with potash and iron, then dissolving in water)
3. A deep blue precipitate forms immediately
4. Filter, wash repeatedly with clean water, and dry

Acid Treatment

Some minerals release pigment material when treated with acid:

• Dissolving iron in acid and then precipitating with alkali produces pure iron oxide pigments
• Treating copper ores with vinegar extracts copper compounds that can be precipitated as blue or green pigments
• The acid dissolves unwanted matrix material, leaving purified pigment

Oxidation

Controlled exposure to air transforms some materials:

• Iron filings left in moist conditions rust into iron oxide — a source of red and brown pigment
• Lead sheets exposed to vinegar vapor and then air produce lead white (basic lead carbonate) — historically the most important white pigment, but toxic
• Copper exposed to moist air develops a green patina (copper carbonate) — essentially natural malachite

Processing Specific Materials

Earth Pigments

The most common and forgiving pigments to process:

1. Collect colored earth or clay from deposits
2. Air-dry completely
3. Crush to coarse powder
4. Levigate in water — multiple passes for highest purity
5. Dry the fine fraction
6. Grind on slab to paint-grade fineness
7. Optional: calcine portions for color variants

Charcoal and Soot

Carbon pigments require minimal chemical processing but careful physical processing:

Charcoal black:

1. Select dense hardwood (oak, maple, beech)
2. Carbonize in a sealed container at high heat (see charcoal-making processes)
3. Select the hardest, most completely carbonized pieces
4. Grind in mortar (wear dust protection)
5. Wet-grind on slab to eliminate all grit
6. Very light particles — add a drop of alcohol to break surface tension during wet grinding

Lamp black:

1. Burn oil (any vegetable or animal fat) in a lamp
2. Collect soot on a cold surface held above the flame
3. Scrape collected soot into a container
4. Lamp black is already extremely fine — brief mulling with binder is usually sufficient

Chalk and White Pigments

1. Collect clean chalk or limestone
2. Crush to coarse powder
3. Levigate extensively — chalk benefits from long settling times (6-12 hours minimum)
4. Dry and grind to ultra-fine powder
5. For extra whiteness, calcine briefly at low heat to drive off organic impurities

Bone Black

A deep, warm black with good tinting strength:

1. Collect bones (any animal) and clean off all flesh
2. Place in a sealed iron or clay container (air exclusion is critical)
3. Heat to red heat for 2-3 hours — gases escape through a small vent
4. Cool in the sealed container
5. The result is charred bone — black, brittle, and easily crushed
6. Grind and levigate as normal

Quality Control

Color Assessment

• Always assess pigment color when wet — dry pigment appears lighter and different from how it will look in paint
• Rub a small amount of wet pigment on white paper or stone to see true color
• Compare batches against reference samples to maintain consistency

Purity Testing

1. Acid test: Add a drop of vinegar to a small sample. Fizzing indicates calcium carbonate contamination (in non-chalk pigments)
2. Fire test: Heat a pinch on a metal surface. Organic contaminants will smoke and burn off. Pure mineral pigments are unaffected
3. Settling test: Suspend in water. Pure pigment settles as a uniform layer. Mixed materials separate into visually distinct layers
4. Magnet test: Pass a magnet over spread-out pigment. Magnetite (iron ore) contamination is attracted to the magnet and removed

Consistency Testing

Process pigments to a standard using repeatable measurements:

• Weigh fixed quantities for each batch
• Time your levigation intervals consistently
• Record grinding durations and techniques
• Keep labeled samples of each successful batch as reference standards

Storage

Processed pigments, stored dry in sealed containers, keep indefinitely. Label each container with:

• Pigment name and color
• Source location
• Processing date
• Processing method (calcination temperature, levigation grade, etc.)
• Any special notes (oil absorption rate, lime compatibility, toxicity warnings)

Store away from moisture, direct sunlight, and heat. Keep toxic pigments (lead, mercury, arsenic compounds) in clearly marked, separate containers, out of reach of children.