Understanding Oil Absorption (OA) in Artists' Pigments

George O'Hanlon | June 23, 2025 | 5 min read 690

Oil absorption (OA) is the volume of oil, expressed in milliliters or in grams, that 100 grams of a dry pigment requires to become a smooth, workable paste. OA underpins every paint formula because it governs the pigment‑to‑binder ratio, sets the pigment‑volume concentration (PVC), and shows you where the critical PVC (cPVC) lies—the tipping point at which a dried film shifts from strong and flexible to dull, chalky, or powdery. Too little oil underbinds the pigment, causing it to become dull, permeable, and lift. Too much oil pushes the mix past cPVC, leading to wrinkles, yellowing, and extended drying times. Measuring OA allows you to dial in binder ratios with confidence, predict open time, and create stable, archival paints—whether you work in oil, watercolor, gouache, or egg tempera.

Historical note: British paint chemist H. Labers first proposed standardizing OA in the 1920s to curb failures in industrial coatings. ASTM D281, the test paint chemists use today, descends directly from that early research.

Why Measure Oil Absorption?

Published tables list only one value per pigment, yet particle size, moisture, and surface treatments vary from batch to batch. Your numbers are more accurate for your studio.
Studio testing enables you to fine-tune medium ratios according to your grinding style, local climate, and preferred viscosity.
Accurate OA data prevents wasted oil and avoids under‑ or over‑binding, two leading causes of cracking, sinking‑in, and wrinkling.
Understanding OA is the first step toward mastering PVC and cPVC, concepts that control gloss, permeability, and mechanical strength in every paint system.

What You’ll Need To Perform A Scaled ASTM D281 Method

Measurement Tools and Supplies

Raw linseed oil — the ASTM D281 reference oil; store at 20–25 °C, out of direct light.
Digital scale (0.1 g, ≥ 500 g capacity) — weigh pigment and track oil additions; tare accuracy is more important than capacity.
10 g pigment sample — a quicker surrogate for the 100 g lab procedure; oven‑dry hygroscopic pigments at 60 °C for two hours.
1 mL syringe or graduated pipette — dispenses oil in 0.05 mL steps; mark 0.02 mL gradations for high‑OA pigments.
Glass slab and palette knife — mixing surface and draw‑down tool; any non‑porous, inert slab works.
Timer and notebook — keeps grinding intervals consistent; a spreadsheet aids long‑term comparison.

Safety and Clean‑up Tools

Lint‑free wipes and citrus solvent — quick clean‑up between batches; gentler than mineral spirits.

Studio hint: Warm the room—not the oil—to 25 °C (77 °F). Stable temperature keeps viscosity within ASTM tolerances and prevents artificially low OA readings.

Step‑by‑Step: Determining OA in the Studio

Weigh exactly 10 g of dry pigment on the scale, then zero it.
Add oil dropwise, one drop (about 0.05 mL) at a time, folding the mixture with a knife.
After each two-drop addition (approximately 0.1 mL), mix for 30 seconds to ensure every particle is wetted.
Draw‑down test: scrape a thin smear with the knife. If the edges crack or appear grainy, add more oil and repeat the process.
Stop as soon as the smear glides smoothly and appears glossy without tearing or crumbling.
Record the total oil absorbed (e.g., 1.8 mL). Multiply by 10 to convert to mL / 100 g.
Clean up tools immediately to avoid cross‑contamination.

Quick conversions: (1 drop ≈ 0.05 mL, 2 drops ≈ 0.1 mL, ¼ tsp ≈ 1.25 mL, 1 tsp ≈ 5 mL)

OA Example: 1.8 mL for 10 g → OA = 18 mL / 100 g.

PVC, cPVC, and Why They Matter

Pigment‑Volume Concentration (PVC) sets a paint’s gloss, hardness, and permeability—knowing it tells you how your paint will look and how long it will last. Once you have OA, you can estimate PVC with:

PVC = Vp / (Vp + Vo) × 100

where Vp is pigment volume and Vo is oil volume after converting weights to volumes. Most oil paints reach their cPVC between 45–55%; films below that range remain glossy and flexible, while those above turn matte and porous. Your OA measurement shows exactly how close you are to that line.

Interpreting Your Results

OA (mL / 100 g)	Typical Pigments	Handling Tips
< 20	Lead white, ochres, iron oxides	Use raw oil sparingly; keep PVC low to maintain adhesion.
20 – 35	Cadmiums, cobalt blues, viridian, siennas, umbers	The standard linseed ratio works; tweak it with small additions of bodied oil or wax.
35 – 50	Quinacridones, dioxazine violet	Try bodied oil for better brush control.
> 50	Phthalos, carbon black, lampblack, alizarin crimson	Add bodied oil to maintain film strength and prevent wrinkling.

Common Pitfalls and Fixes

Symptom	Likely Cause	Remedy
OA jumps between batches	Moisture or particle‑size variation	Oven‑dry sample; sieve pigment for consistency.
Paste cracks while mixing	Oil added too quickly or under‑grinding	Reduce the increment to 0.02 mL; mix for 60 seconds per addition.
OA reads unusually low	Solvent carry‑over from pre‑wetting	Avoid alcohol or spike oil during OA testing.
Paint film stays tacky for days	Ignoring OA in the final formulation	Re-calculate PVC; add inert extender (such as chalk) to balance.

Beyond Oil: Applying OA to Water‑Based Media

OA principles translate directly to gum arabic solutions, acrylic emulsions, and egg yolk tempera. A high‑OA pigment such as phthalocyanine blue (PB15:3) (≈ 52 mL / 100 g) will likewise demand more gum or egg solution to wet every particle. If your egg tempera feels greasy or lifts when rewetted, check whether you underestimated its oil absorption (OA) and shorted the binder or used too much egg yolk.

Medium	Binder‑to‑Pigment Rule of Thumb	Adjustment for High‑OA Pigments
Watercolor	1 part gum solution with 1 part pigment	Increase binder 15 – 20 % and add 3 % glycerin; expect slightly longer drying time due to extra gum.
Gouache	1.5 parts gum solution with 1 part pigment	Add precipitated chalk for body and opacity; drying slows modestly as binder rises.
Egg tempera	1 egg yolk emulsion with 1 part pigment	Thin emulsion with water; extra binder may extend set‑up by a few minutes.

Drying‑time note: In water‑borne media, more binder means a thicker vehicle layer that must release extra water (or coalesce for acrylics) before the film sets—plan for a longer open time when you boost binder to suit high‑OA pigments.

Safety and Cleanup

Wear an N95 dust mask when handling dry pigments.
Store oil‑soaked rags in a sealed metal bin—linseed oil-soaked rags and paper towels can spontaneously combust.
Label jars with the pigment name, OA value, and date. The future‑you will thank present‑you.

Important Notes

OA is the metric for mastering PVC and cPVC.
Studio‑scale ASTM D281 testing is quick—10 g pigment and a syringe.
Consistent temperature, raw linseed oil, and gentle increments yield repeatable data.
Record everything: OA values feed directly into recipes for oil, watercolor, gouache, and egg tempera.