: After soaking, samples (often in "dumbbell" shapes) are tested for tensile strength and elongation to see how much they have weakened.
When rubber is submerged, several properties can change: volume, mass, tensile strength, elongation, and hardness. JIS K 6258 standardizes how to measure these changes so that results from a lab in Tokyo can be compared with results from a lab in Detroit.
Note: “JIS K 6258” is a Japanese Industrial Standard in the JIS K series (mechanical engineering / industrial machinery). Below is a structured, practical, and actionable analysis assuming you are looking for an English translation or English PDF version and need to understand, use, or evaluate the standard.