Aluminum Stearate

Aluminum stearate (aluminum distearate) is a white, wax-like powder (metallic soap) that dissolves in mineral spirits or hot oil. A small amount (2% or less) added to oil paint imparts a short, buttery consistency. It eliminates the separation of pigment and oil, thickens varnishes considerably. A concentrate of aluminum stearate and linseed oil can be prepared ahead of time and added to the paint whenever needed.

Aluminum stearate is made via the precipitation process using high quality stearic acid and exhibits the following properties: Good gelling and thickening action, excellent water repellency, transparency and a synergistic effect with zinc stearate or calcium stearate.

The effects of metal soaps on pigments have been extensively studied. Aluminum stearate was found to coat the surface of pigment particles and helped prevent settling as well as reducing the amount of oil needed to wet the pigment. The amount of aluminum stearate needed to coat pigments varied on a weight basis, but a solution of 2% by weight (weight of aluminum stearate/weight of oil) of aluminum stearate was more effective than 0.5% or 4% solutions in altering pigment surfaces (Gardner 1930). The soaps coat the surface of the pigments and by steric effects keep the particles from aggregating (Pilpel 1963), which helps to keep the particles in suspension.

With increasing amounts of aluminum stearate the oil pigment mixture becomes viscous, and by using an appropriate amount of aluminum stearate the paint can gel at a lower pigment concentration (Mayer 1965). This can be used to create a "cheaper" paint since a smaller amount of a costly pigment needs to be used. A significant advantage of using stearates is that the oil and pigment do not separate greatly over long periods of time in the paint tube. Manufacturers of artists' paints often use aluminum stearate in their formulations without listing it as a component on the product label.

Solubility

Aluminum stearate exhibits relatively high solubility in hydrocarbon solvents (such as mineral spirits) when compared to other metallic stearates. It is insoluble in water, alcohol and ether; but is readily soluble in benzene, acids and common solvents when hot.

Storage

Aluminum stearate has long storage life if stored in cool and dry location.

How to Use

Aluminum stearate dissolves in vegetable oils on heating and if a high enough concentration of the soap is used, gelling occurs on cooling. In the usual practice of making paints, the aluminum stearate is ground with the pigment before the bulk of the oil is added. To prepare a concentrated solution (10% w/v), add 100 grams of aluminum stearate (nearly fills a half liter measuring cup without compacting) to one liter of linseed oil. Heat the oil to about 150° C. and gradually slowly adding the white powder to the hot oil with stirring. Add one part of this solution to four parts of oil by weight of oil before adding to pigments and grinding.

References

Gardner, H. A., (1930) Physical and Chemical Examination of Paints, Varnishes, Lacquers, and Colors, 5th Edition, pp. 884-896.

Mayer, R., (1965) The Artist's Handbook of Materials and Techniques, Revised Edition, The Viking Press, New York, pp. 150-153.

Pilpel, N., (1963) Properties of Organic Solutions of Heavy Metal Soaps, Chemical Reviews, 63, pp. 221-234.