I’d seen volume gradients used in a few places and not really understood them. Houdini help says: “the gradient is a vector pointing in the direction of increasing volume values.” Hmm. Nothing if not laconic.
Really excellent tutorial — very clear, well-paced, and with solid workflow — by Armin Lofti showing how to abuse the pyro source spread node (introduced in Houdini 18) as a quick and dirty way to drive noised infections and growth patterns, used here to grow a sheath of ice around a freezing object.
Converting a mesh into a highly viscous FLIP fluid in Houdini, then selectively lowering the viscosity of certain areas by raising their temperature, is a fairly simple way to simulate a melting object, with reasonable art-directability by means of noising and animating an invisible heat source.