A monolithic block of glass with a channel running through its interior. No seam, no glue line, no visible boundary. That is what diffusion bonding makes possible, and it works because atoms physically migrate across an interface until the join ceases to exist. Diffusion bonding is a solid-state process. Two surfaces are brought into intimate contact under pressure at elevated temperature, held well below the melting point, for minutes to hours. No filler, no adhesive, no liquid phase. The material heals itself across the boundary. For a transparent part with hidden internal channels, the sequence runs like this: • Machine, laser, or wet-etch the channel geometry into the mating face of one or both plates • Polish the bonding surfaces to optical flatness, often sub-nanometer roughness, and clean away every particle and hydrocarbon film • Bring the faces together so van der Waals forces pull them into spontaneous optical contact • Apply heat and modest pressure to drive interdiff...