Making Raw and Burnt Sienna — The Warm Iron Earth from Tuscany

Obtain raw sienna earth from a geological supplier or art pigment dealer. True sienna is a warm golden-brown, distinctly more orange-toned than yellow ochre and warmer than raw umber. The streak on white paper should be a clear warm brown with a golden or orange undertone — not greenish (which would indicate high manganese, closer to umber) or pale yellow (which is ochre, not sienna). Italian sienna from Tuscany is the traditional benchmark, though good deposits are found worldwide. Remove stones and organic debris.

Break the raw sienna lumps in a stone mortar and pestle, pounding and grinding until the earth is a coarse, gritty powder. Sienna is typically soft and crumbly — easier to crush than most ochres — because it forms in decomposed rock and soil rather than in hard mineral veins. Wear a dust mask during crushing. The coarse powder is now ready for levigation.

Add the crushed sienna to a glass settling jar with water at roughly 5:1 ratio. Stir vigorously, wait 30 seconds for coarse sand and grit to settle, then carefully pour the still-cloudy suspension through muslin cloth into a second clean jar. Let the second jar settle undisturbed for 4-6 hours. The fine pigment collects as a dense, smooth layer at the bottom. Pour off the clear water and repeat the settling if any grit remains. The levigated sienna should feel perfectly smooth — no grittiness at all.

Spread the wet pigment paste on a clean glass surface and dry completely (1-2 days). Grind half on a glass muller to a fine, smooth powder — this is finished raw sienna, a beautiful warm golden-brown with a luminous transparency that makes it one of the finest glazing pigments in oil painting. Store in a sealed glass jar. Reserve the other half for calcination.

Place the reserved raw sienna in a clay crucible and heat gradually to 300-500°C, holding for 30-60 minutes. As the goethite dehydrates to hematite, the colour transforms from golden-brown to a rich, warm orange-red brown — one of the most dramatic and beautiful colour changes in pigment chemistry. The exact temperature determines the final shade: lower temperatures produce a more moderate warming, while higher temperatures yield a deeper, more reddish tone. Do not exceed 600°C or the pigment dulls. Let cool completely.

Grind the cooled, calcined sienna on a glass muller to a fine, smooth powder. Burnt sienna is a warm, rich orange-red brown — compare it to the raw sienna from step 4 to see the transformation clearly. Store in a sealed glass jar. Both raw and burnt sienna are permanent, lightfast, non-toxic, and compatible with all media. Burnt sienna is particularly valued in watercolour for its granulating texture and in oil for its warm, transparent glazing quality. Mixed with ultramarine blue, it produces some of the most beautiful neutral greys and shadow tones available to the painter.