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The Binder and solvents of oil paints (part 2)

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Among the film oils used in the oil paints mentioned above, linseed oil has the highest linolenic acid and iodine content (see table), so linseed oil dries the fastest and has the toughest film. Dry walnut oil is also fairly quick and even, with less yellowing, so it is used to grind light colors. Poppy seed oil is very clear, and hardly yellow, but dries slowly because it contains too little linolenic acid. The film of opiate seed oil is weak and brittle, so it should not be used as a primer. Safflower oil has similar properties to poppy seed oil but dries faster.

The ratio of fatty acids in some film oils

(compared to total fatty acid intake)

Linolenic (%) Linoleic (%) Oleic (%) Stearic (%) Palmitic (%) Number iodine
Flax oil 50 ~ 70 12 ~ 24 10 ~ 22 2.5 4 ~ 6 170 ~ 204
Hemp Oil 24 ~ 26 54 ~ 56 11 ~ 13 1 ~ 3 5 ~ 7 140 ~ 175
Opium oil 0 – 9 41 ~ 60 13 ~ 37 1.4 ~ 10 9 ~ 20 133.4
Walnut oil 1.5 ~ 5 60 23 1 ~ 4 3 ~ 6 152
Safflower oil 0.1 68 ~ 83 8 ~ 21 2 ~ 3 4 ~ 6 140 ~ 150
Sunflower oil < 0.7 44 ~ 75 14 ~ 35 1 ~ 3 3 ~ 6 125 ~ 140

 

Among the semi-dry oils (idodine number 115 ~ 130) are soybean oil, canola oil, cottonseed oil, sesame oil, etc. And olive oil, peanut oil, castor oil, coconut oil, etc. are non-drying oils. Non-drying oil cannot be used in oil painting because it does not create films.

Light causes an optical oxidation reaction (photooxygenation), which can speed up the drying rate of linseed oil by 900 times that of automated oxidation [5]. The automatic oxidation-drying process also releases a small amount of volatiles, causing a distinctive odor.

Temperature and humidity affect this evaporation. Temperature rise makes evaporation faster. Increased humidity impedes evaporation.

Metal catalysts such as cobalt, iron, manganese are capable of producing metal soap that increases the rate of oxidation reactions and the speed of high molecularization (making polymer films), should be used as a siccative ( desiccant). In particular cobalt is a very effective catalyst to speed up oxidation, while zirconium speeds up the process of high molecularization. But when used too much (over 0.5 ~ 1% by weight), the siccative creates a film like a skin on the oil paint, preventing the lower dry part from coming into contact with the air, causing cracking of the paint surface [ 6]. Metal catalysts also darken over time, so use siccative only when drawing primers. However, the safest way to quickly dry oil is to use sun-thickened linseed oil in combination with copal varnish.

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