During the preservation process of oil paintings, they are prone to yellowing and darkening, which is a result of the oxidation of the surface varnish. To date, there is no varnish that does not oxidize. Removing the old varnish layer and reapplying a new coat of varnish is the only way to restore the original appearance of the work. The commonly used cleaning solvents are:

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(1) Turpentine. It has a weak effect on old oil paintings but a strong effect on new color layers. It works even better when heated with hot water.

(2) Refined gasoline and benzene (methylbenzene, xylene). It has a stronger solvent power than turpentine for resins.

(3) Ethanol (anhydrous alcohol) and methanol (wood alcohol). They are powerful solvents for resins.

(4) Acetone and chloroform. They can quickly dissolve resins and the oil layer. The cleaning agents sold in art supply stores are usually a mixture of several solvents. Strong solvents cannot be used alone. Modern oil paintings mostly use resin varnish, and turpentine can remove it. The older generation of Chinese oil painters mostly used oil-based varnish, and needed to use a mixture of turpentine and anhydrous alcohol. While acetone is only used as an additive when cleaning ancient works.

(5) Hydrogen peroxide. Many precious white pigments are substances of lead salts. In the air containing a trace of hydrogen sulfide gas, when hydrogen sulfide encounters lead ions, a chemical reaction occurs, producing black solid sulfuric lead, which makes the oil painting darken. Hydrogen peroxide can oxidize sulfuric lead into white sulfuric acid lead.

The wiping method: Use a white absorbent cotton ball (available at drugstores) dipped in the cleaning agent to make circular wiping from one corner of the picture to the other. Each part should not be wiped for too long to avoid damaging the color layer. The cotton ball should be replaced with a new one as soon as it is stained with yellow varnish. Most of the old varnish can be removed in the first cleaning. When cleaning a second time, for stubborn parts, strong solvents can be added. Stains caused by squeezing and fly droppings can be removed with a knife.

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