A gas turbine engine cooling system includes a heat exchanger in fluid communication with a source of cooling air, a first cooling circuit including a first heat exchanger circuit in the heat exchanger and a first bypass circuit with a first bypass valve for selectively bypassing at least a portion of first airflow around the first heat exchanger circuit. The microcapsules may be made by encapsulating the display medium in the wall forming material of the microcapsules, and for example a complex coacervation process may be used. Kits for carrying out the method are also provided. The outer diameter of the coupling device is smaller than the inner diameter of the exterior mold so that the exterior mold can be removed with the coupling activated and the mold faucet held down.