Q: What does the term "acid number" in linseed oil mean?
A: The acid number, or acid value, is a measurement used in
laboratories to determine the amount of free fatty acids in a vegetable
drying oil (more specifically it represents the milligrams of potassium
hydroxide required to neutralize the free fatty acids in one gram of
oil). In general, oils with a higher acid number (5 to 10) are better
for grinding paints; oils with a lower acid number (1 to 3) are
considered more useful in varnishes and other clear coatings. When
making artists' oil colors, however, the color stability and
non-yellowing properties of the oils are more important than is ease of
grinding. Therefore, lower-numbered oils are more frequently used in
producing artists' oil paint.