The test methods outlined are not necessarily a measure of how much a given test material will tarnish polished metals. When tested by these methods, paper that has less than 0.0008 % reducible sulfur, may be assumed to be nontarnishing as far as sulfur is concerned, but if more than 0.0008 %, this does not necessarily mean that tarnishing will occur, because sulfur compounds which may not cause staining are reduced by the treatment with the subsequent evolution of hydrogen sulfide. If more than 0.0008 % reducible sulfur is found, or if the effect of that or other materials causing stains on silver is desired, the paper should be subjected to an accelerated tarnishing test (see Test Method D 2043).

Note 1-If the pH of the paper is low (for example, 4.0 to 4.5 (cold extraction)) as little as 0.0002 % of reducible sulfur may cause tarnishing of imitation gold bronze prints; whereas, if the pH were higher, (for example, on brush-coated art paper) a much higher quantity of sulfur might be tolerated.

The test methods described do not give the total sulfur content, nor the total reducible sulfur content, nor are they intended to. They are limited to the reducible sulfur content determined under the conditions of the test (that is, a fairly mild reduction system).³

5.2.1 Those forms of sulfur believed to be "reducible" under the test conditions are (1) sulfide (released by the acid, not actually reduced), (2) elemental sulfur, (3) thiosulfate, (4) other polythionates or polysulfides, and (5) perhaps sulfite. Sulfate is excluded.