In this architectural research exploration, we challenge the notion of an interactive architectural surface as single-layered, two-dimensional interaction interface. Instead, we propose the notion of Interactive Voluminous Substance, which moves the interaction experience into four dimensions, shifting it from far-field, proximity-based interaction to a near-field, tactile one. We present four features of architectural expression that could potentially sustain the embodiment of this Substance: spatial positioning, geometry, expression, hybrid material composition and interaction design. If the future architectural interiors and exteriors are made from Voluminous Architectural Substance, how will it be to dwell with them? We propose two physical prototypes and two interaction stories as speculative objects probing this question.