Making Yellow Ochre Pigment — Grinding and Levigating Goethite into the Oldest Painter's Yellow

Obtain raw yellow ochre earth — a naturally occurring mixture of goethite (iron oxyhydroxide, α-FeOOH), limonite, and clay. Good quality yellow ochre has a strong yellow-to-golden streak when rubbed on an unglazed ceramic tile (the streak test distinguishes it from brown or red varieties). The colour should be a clear, warm yellow without excessive grey or brown tones. Source from geological suppliers, art pigment dealers, or collect directly from exposed yellow clay banks and eroded hillsides where iron-rich deposits outcrop. Remove any obvious stones, roots, and organic debris by hand.

Break the raw ochre lumps into small pieces by wrapping them in clean cloth and striking with a hammer, or by pounding in a large stone mortar. Continue crushing until the ochre is reduced to a coarse gritty powder with no pieces larger than a grain of rice. Wear a dust mask during this step — ochre dust is non-toxic but prolonged inhalation of any mineral dust is irritating to the lungs. The goal is to break open the mineral structure so that water can penetrate and separate the fine pigment particles from the coarser grit and sand that always accompanies raw ochre deposits.

Place the crushed ochre in a large glass settling jar and add water at a ratio of roughly 5:1 (water to ochre). Stir vigorously for two minutes, then let stand. Within 30 seconds, coarse sand and stones will sink to the bottom — this is waste. After 30 seconds of settling, carefully pour the still-cloudy yellow water through muslin cloth into a second clean settling jar, leaving the coarse sediment behind. Let this second jar sit undisturbed for 4-6 hours. The finest pigment particles will eventually settle as a smooth, dense layer at the bottom. Repeat the washing if the first settling still contains grit — two or three cycles of settling and decanting produce a remarkably uniform, fine pigment.

Once the fine pigment has settled completely (the water above should be nearly clear), carefully pour off and discard the clear water without disturbing the settled pigment layer. Spread the wet pigment paste onto a clean glass slab or non-absorbent surface and let it dry completely in a warm, well-ventilated area — this takes 1-2 days depending on humidity and thickness. Do not attempt to speed drying with direct heat, as temperatures above 200°C would begin to convert the yellow goethite to red hematite (which is exactly the process used to make red ochre). The dried pigment will form a hard cake.

Break the dried pigment cake into small chunks and grind on a glass muller (a flat glass grinding surface with a heavy glass muller stone) until the pigment is a fine, smooth, uniform powder. The muller produces a finer, more consistent grind than a mortar — the glass-on-glass action shears particles down to optimal size for painting. Yellow ochre should be ground to a soft, floury consistency with no grittiness. Store the finished pigment in clean, sealed glass jars. Yellow ochre pigment is completely lightfast (it will not fade or change colour), non-toxic, and compatible with every painting binder — oil, egg yolk, gum arabic, wax, casein, lime, and acrylic polymer.