A test performed to measure the bursting strength of paper or paperboard. In a Mullen test (also called a pop test), the paper sample is placed between two ring-like clamps in a device called a Mullen tester, and hydraulic pressure is used to inflate a rubber diaphragm, which expands against the sample stretching it. The measure of the total hydraulic pressure expanding the diaphram at the time the sample ruptures (usually expressed in either pounds per square inch or kilopascals) is its bursting strength. Mullen tests are performed for each side of a paper or paperboard, and the bursting strength can be expressed as the average of both sides. Bursting strength expressed as a percentage is called the percent Mullen. Mullen tests are typically performed on papers and boards designed for use in packaging, bags, and envelopes. It is rarely performed on printing or writing papers. See Bursting Strength.