Methods and apparatus for controlling a digital power supply are disclosed. The slurry comprises a binder, a gelling agent, a first refractory material having a density of 1 and a mean particle size of 1, and a second refractory material having a density of 2 and a mean particle size of 2, wherein 12 and 12. A first linkage member is operatively connected to the shutter to open the shutter upon being actuated by a first engagement feature.