Hybrid practices are emerging that integrate creative materials like paint, clay, and cloth with intangible immaterials like computation, electricity, and heat. This work aims to expand the design potential of immaterial elements by transforming them into manipulatable, observable and intuitive materials. We explore one such immaterial, electric heat, and develop a maker-friendly fabrication pipeline and crafting support tool that allows users to experientially compose resistive heaters that generate heat spatially and temporally. These heaters are then used to couple heat and thermoreactive materials in a class of artifacts we term Thermoreactive Composites (TrCs). In a formal user study, we observe how designing fabrication workflows along dimensions of composability and perceivability better matches the working styles of material practitioners without domain knowledge of electronics. Through exemplar artifacts, we demonstrate the potential of heat as a creative material and discuss implications for immaterials used within creative practices.