Where do all the different brands of drying oils in artists’ supply shops originate? Artists’ materials manufacturers do not process their own oil, but rather purchase it in bulk from large industrial sources. For example, there are two major producers of linseed oil in North America and at least two in Europe, but there are many brands of linseed and stand oils offered by artist materials companies. Artist materials companies do not have the resources to process linseed oil, which today is a very specialized and large-scale industry.
About Linseed Oil
Linseed oil is obtained by various methods, including pre-expelling, followed by hexane extraction of the resulting press cake. The oil is refined to remove phosphatides and gums, which naturally occur in the seed. Subsequent refining through post-desliming with sulfuric acid and phosphoric acid yields oil with virtually no traces of phosphatides or gums.
Further post-treatments include lye neutralization and earth bleaching, which yields a very light drying oil. The natural smell of linseed is removed through vacuum steam distillation. Removing the thin layer of wax that covers each seed is the last stage of the refinement process. Dewaxing is done by cooling the oil to 4° C. The cooled oil is held at this temperature until the waxes crystallize, which is then removed by mechanical filtration.
Raw Oil
Raw linseed oil is linseed oil that has been extracted and packaged without any processing. It takes much longer to dry than boiled linseed oil. The former needs several weeks to cure, against 36–48 hours for the latter.
Having said this it is still a matter of opinion as alkali refined oils have come a long way but cold pressed is still considered the best because of quality not because of ease of extraction.
This is a belief held on by many artists but not shared by paint technologists and manufacturers. Refined and aged oil makes a better oil vehicle for paint making than cold-pressed unrefined oil, because refined oil does not contain “breaks”, combined water, waxes and other impurities that may cause the oil to yellow with age. The reason why cold-pressed linseed oil is recommended in artists literature is due to one fact: cold-pressed linseed oil has a high acid value compared to ordinary alkali-refined linseed oil. High acid value makes drying oil better for grinding pigments. Up until the last 100 years, only cold-pressed oils had a high acid number considered suitable for grinding colors. However, during the last hundred years, manufacturers have been producing high acid value refined oils and even high acid value stand oils or polymerized oils. Natural Pigments uses high acid value refined and aged linseed oil in many of its Rublev Colours Artists Oils, and offers these oils to artists for making their own paint through the Natural Pigments store.
Stand Oils
Stand oil is a misnomer in the industry today and the term is mostly used by artist materials companies selling heat-bodied oil to artists. The term “stand oil” originated in about the 18th century to denote oil that was purified by allowing it to stand and thereby letting “foots” or impurities in the oil to settle. Today, linseed oil producing firms offer a bewildering array of linseed oil products, such as polymerized oil, maleanized oil, boiled oil, aged oil, blown oil, conjugated oils, etc. These are interesting products that offer a wide range of performance variables that are not typically being offered to the artist.
Stand Oil A thickened drying oil; especially, linseed oil heated to about 600° Fahrenheit (315° C.). Stand oil got its name from the fact that, on standing, the mucilage coagulates and separates out from the drying oil.
Linseed oil that has been heated to about 300° C. under conditions that exclude oxygen is referred to as stand oil. It becomes polymerized—that is, it changes its molecular structure while retaining its chemical ingredients. It has a lower iodine number than the cold-pressed oil, dries slowly, yellows less than the other linseed oils, and forms a tough strong film. It imparts to the paint an enamel-like smoothness and tends to make the paint fuse and blend. Because of its heavy consistency and low acid number, it is used as an ingredient in the diluent or painting medium, in oil technique, rather than as a grinding oil, or binder. [Kay, Reed. The Painters Guide to Studio Methods and Materials. Englewood Cliffs, NJ: Prentice-Hall, Inc., 1983. p. 34]
I agree with most that was written above, except that stand oils or rather heat-bodied oils are now produced with iodine and acid numbers equivalent to cold-pressed linseed oil.
Stand oil is a partially polymerized but unoxidized linseed oil is made by heating the oil to abut 300° C. (570° F.) in the absence of oxygen. Stand oil is also called heat-bodied oil. It is not a good binder because it is too viscous, but it is an excellent addition to painting and glazing mediums. It yellows less than other forms of linseed oil and has good leveling properties. Its film gives a smooth, enamel-like surface without brushmarks.
Sun-thickened oil was the polymerized oil prior the 18th century and stand oil from the 18th century onward. To polymerize oil, the molecules are connected in chains (polymers) through a heating or oxidation process. This is the initial stage of the drying process, so polymerized oils dry faster. Polymerized oils have both higher viscosity (it is thicker) and is faster dying than oil (linseed, poppy or sunflower).
The European International Organization for Standardization (ISO) published a standard for linseed stand oil: ISO 276:2002 “Binders for paints and varnishes—Linseed stand oil—Requirements and test methods.” The standard includes acid value (mg KOH/g) and saponification value (mg KOH/g).
Polymerized Oils
Both non-conjugated and conjugated drying oils like linseed and Tung oil can be polymerized by heating in an inert atmosphere. These polymerized oils are then referred to as bodied oils. To achieve the higher viscosities of bodied oils, non-conjugated oils are heated up to 320° C. and conjugated oils to 240° C.
This increase in viscosity, or “body,” is caused from thermal decomposition of naturally occurring hydroperoxides. This decomposition yields free radicals that contribute to a limited amount of cross-linking.
Polymerized linseed oil dry faster, and are harder and more durable than raw oils. They also produce a smooth, glossy finish, whereas raw oils produce a matt sheen, a result of the natural expansion that takes place during polymerization. This expansion creates a finely-textured surface that appears to the naked eye as a matt finish.
Boiled Oil
Boiled linseed oil has been altered by adding chemical drying accelerators that is solvents and siccatives (driers). This treatment allows the manufacturer to reduce the volatile organic compounds (VOC) content, while maintaining the viscosity. Boiled linseed oil is often mixed with 10–15% polyermized oil.
Blown Oils
Bodied oils are drying oils with increased viscosity, produced by heating the oil without oxygen. To increase the viscosity of the boiled oil, air is sometimes “blown” through the oil at 60–100° C. Blown linseed oils are polymerized by oxidation to increase the viscosity and acid number. They are used in non-penetrating finishes and dry faster than heat-bodied linseed oils.
Yellowing
The yellowing of linseed oil is thought to be caused when conjugated, unsaturated hydro-peroxides are converted into conjugated unsaturated ketones. These unsaturated ketones can produce long-chain colored polyenes. Additionally, if 1,4-diketones are formed during the drying, enol tautomers can react with trace amounts of atmospheric ammonia. This produces a substituted pyrrole, which can be converted into a colored product by oxidation, or by condensation in the presence of formic acid. Colored metal siccatives can also contribute to the discoloration and/or yellowing of linseed oil. To alleviate the yellowing, saturated aliphatic aldehydes may be added to the oil.
The statement, "It is not a good binder because it is too viscous..." means that bodied oils do not alone make a good paint binder. It is very difficult wet pigments with a highly viscous oil. Higher acid values make it easier to wet very fine pigments, such as most modern artificial pigments. However, most paint manufacturers combine oils to obtain the best properties for their paint. For example, Natural Pigments uses bodied oils combined with aged oil in Rublev Oil Colors. We do this to reduce drying time, increase viscosity to avoid pigment separation without using stearates and other additives, and make a "longer" paint.
In review, here is what I know about bodied oils based upon direct observations and tests of over 50 commercial linseed oils: Most bodied oils (which include closed kettle, open kettle and vacuum or inert atmosphere cooked oils) are faster drying than cold-pressed and refined linseed oils. Most—but not all—form tough films and all yellow less than cold-pressed linseed oil and some yellow less than refined linseed oil. It is difficult to categorically state for certain that as a group they possess certain qualities, because of the variety of bodied oils available today.
Henry W. Levison Undoubtedly, yellowing in the dark is the result of thermally induced oxidative reactions. Some years ago, Huey described a method of storing standard reference paint panels in the cold and under an inert atmosphere in order to minimize their discoloration in the dark. Museums and artists may not wish to store the paintings in their collections at particularly low temperatures or in an inert atmosphere, but they might consider storing them at a lower-than-normal temperature. Moreover, they may also wish to consider the centuries-old practice of exposing yellowed oils to indirect daylight or to filtered fluorescent lamplight for brief periods to reverse the yellowing of the vehicle which may have occurred over several years of storage in the dark. The present work has demonstrated that very brief exposure to indirect north light will almost completely bleach out the yellowing that may have been built up over several years storage in the dark. Whether such a bleaching procedure would prove ultimately harmful to the paint, particularly if repeated often, is, of course, a matter that must be thoroughly investigated.
THE MOST IMPORTANT CONCLUSION from this investigation is that the net degree to which films are capable of discoloring in the dark does not seem to be a function of the age of the film or the extent and frequency of photochemically induced bleaching the films may have undergone in the intervening time. Moreover, in spite of as many as four cycles of discoloration in the dark and subsequent bleaching, the discolored films proved to be capable of being bleached, at any stage, practically back to their initial degree of whiteness.
The alkyd mediums exhibited the least yellowing throughout, followed closely by the paints made with safflower or poppyseed oil. The linseed oil paint yellowed the most, but recovered extremely well. The extent of discoloration in the case of the painting mediums apparently was related to the content of linseed oil. On the other hand, resins, other than the alkyds, do not appear to affect the degree of yellowing if the initial color due to the added resin is taken into account.
Glossary
[FONT=tahoma]Acid Value[/FONT]
This test method covers the identification of oils and oil acids in vehicles that have been separated from solventreducible paints. The test method is based on a gas chromatographic technique (of the methyl esters) applicable to products containing both saturated and unsaturated, animal and vegetable, unpolymerized or partially polymerized fatty acids having 8 to 20 carbon atoms.
[FONT=tahoma]Color[/FONT]
This test method covers the identification of oils and oil acids in vehicles that have been separated from solventreducible paints. The test method is based on a gas chromatographic technique (of the methyl esters) applicable to products containing both saturated and unsaturated, animal and vegetable, unpolymerized or partially polymerized fatty acids having 8 to 20 carbon atoms.
[FONT=tahoma]Iodine[/FONT]
This test method covers the identification of oils and oil acids in vehicles that have been separated from solventreducible paints. The test method is based on a gas chromatographic technique (of the methyl esters) applicable to products containing both saturated and unsaturated, animal and vegetable, unpolymerized or partially polymerized fatty acids having 8 to 20 carbon atoms.
[FONT=tahoma]Viscosity[/FONT]
This test method covers the identification of oils and oil acids in vehicles that have been separated from solventreducible paints. The test method is based on a gas chromatographic technique (of the methyl esters) applicable to products containing both saturated and unsaturated, animal and vegetable, unpolymerized or partially polymerized fatty acids having 8 to 20 carbon atoms.
[FONT=tahoma]Weight Per Gallon[/FONT]
This test method covers the identification of oils and oil acids in vehicles that have been separated from solventreducible paints. The test method is based on a gas chromatographic technique (of the methyl esters) applicable to products containing both saturated and unsaturated, animal and vegetable, unpolymerized or partially polymerized fatty acids having 8 to 20 carbon atoms.
References
Gettens, R.J. and Stout, C.L., Painting Meterials, Van Nostrand, New York, 1952, p. 42. Mayer, Ralph, The Artists Handbook, Viking Press, New York, 1970, p. 175.
Kay, Reed. The Painters Guide to Studio Methods and Materials. Englewood Cliffs, NJ: Prentice-Hall, Inc., 1983. p. 34.
Phillips, Morgan W. “Problems in the Reconstruction and Preservation of Old House Paints,” Proceedings of the North American International Regional Conference, Smithsonian Institution Press, Washington, D.C., 1976, p. 273.
Levison, Henry W., Artists Pigments, Hallandale, FL, 1976, p. 31.
Levison, Henry W., Color, Research and Application, Vol. 3, No. 1 Spring 1978, p. 7.
Levison, Henry W., "Yellowing and Bleaching of Paint Films," Journal of American Institute for Conservation, 1985, Vol. 24, No. 2, Article 2 (p. 69–76) See online.
Rakoff, H., Thomas, F.L., and Gast, L.E., Journal of Coating Technology, 51, No. 649, 1979, p. 23.
Tahk, Christopher, “The Recovery of Color in Scorched Oil Paint Films,” Journal of the American Institute of Conservation, Vol. 9, No. 1, Fall 1979, p. 3.
See reference 3, p. 10.
ASTM Test Method D 1644, Non-volatile Content of Vehicles.
Mayer, Ralph, The Artists Handbook, Viking Press, New York, 1970, p. 216.
ASTM Test Method D 1925, Yellowing Index of Plastics.
See reference 6, p. 5.
See reference 3, p. 31.
Huey, S.J., “Low Temperature Storage of Colored Standards Panels,” Color Engineering, September–October 1965 pp. 24–27.
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