Bookbinding and the Conservation of Books
A Dictionary of Descriptive Terminology

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sulfur dioxide

One of the oxides of sulfur. Sulfur dioxide (SO 2 ), which is probably the most common cause of the deterioration of paper, is produced when sulfur, or materials containing sulfur e.g., coal, oil—is burned in air. It is present in varying amounts in the atmosphere of almost all industrialized nations. Sulfur dioxide, even at higher concentrations than are normally found in the atmosphere, is not in itself harmful to paper or bookbinding materials, but it can be converted into highly destructive sulfuric acid, either by being oxidized to sulfur trioxide (SO 3 ), and then into sulfuric acid (SO 3 + H 2 O = H 2 SO 4 ), or by reaction of SO 2 with water to form sulfurous acid, which in turn oxidizes to form sulfuric acid (H 2 SO 3 + H 2 O = H 2 SO 4 + 2H). Some authorities maintain that this conversion of sulfur dioxide to sulfuric acid requires the presence of small amounts of copper or iron, i.e., minute particles of metal broken from the refining apparatus and entering the pulp, as is almost always the case with papers produced during the past 150 to 200 years; whereas others maintain that the process will take place regardless of metallic content and that the copper and/or iron impurities simply function as catalysts and thus quicken the rate of deterioration.

Archival papers should be protected against atmospheric sulfur dioxide. insofar as possible, by: 1) removing dangerous or potentially dangerous impurities, e.g., catalytic materials, from the paper, or, alternatively, manufacturing paper free from them; 2) the addition of alkaline and buffering agents to neutralize any sulfuric acid that does form; 3) destroying the chemical (catalytic) activity of the metallic impurities by the use of inhibitors, such as magnesium salts; and 4) storing paper in an atmosphere as free as possible from sulfur dioxide.

Sulfur dioxide is also used as a reducing agent in the preparation of basic chrome tanning liquor from sodium dichromate in acid solution: Na 2 CrO 7 + H 2 O + 3SO 2 = 2Cr(OH)SO 4 + Na 2 SO 4 . (193 , 198 , 248 )

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