Borax is an alkaline compound (Na2B407.10 H2O) used mainly as a cleaning agent, and in the manufacture of enamel and glass. It is commonly found in caviar outside the U.S., where it helps sweeten the flavor, improves texture, and makes it less likely to break down and become soupy. This is done in extremely minute amounts - with 500 parts borax per million parts of caviar. The only caviar without added borax distributed in Europe is the Azerbaijani Caviar.
According to my research, caviar with borax may actually be ordered in the United States. The Food and Drug Administration reportedly recently gave up their policy of detaining caviar containing borax after agreeing that the minute concentrations of borax, combined with the fact that most people do not eat caviar daily, make it a minimal health risk. It not has, however, been officially approved as a food additive, and thus must be labeled that it contains borax.
Caviar that contains borax is sometimes referred to as "ship" caviar, because of the fact that it was served many years ago on oceanliners crossing from Europe. The borax would help as an additional preservative for the long voyage.
Whether or not it truly is available in America, the practice of treating caviar with borax is actually used outside the culinaria, by the west coast winter steelhead fishermen. These fishers commonly use powdered borax on salmon eggs to toughen them, preserve their appearance, and prevent the growth of bacteria. The toughened clusters then stay on the hook longer than the fresh ones.