This exposition presents a traditional process of indigo vat dyeing. A fermented indigo vat is an ancient method of dyeing textiles. The process is based on bacterial action, which means that the vat is alive: it is a living design medium. With living design medium, is referred to all material production or means of modification that incorporate simple living organisms such as bacteria. A living design medium also includes and implies a designer perspective. Biomaterials and biotechnology-based manufacturing methods, often involving living organisms, are increasingly entering the designers’ workspace. This will affect design practices, especially in the more material-centric domains, such as fashion and textile. This study aims to investigate the ‘being’ of a living design medium. However, instead of looking at new biotechnologies or biomaterials, this paper turns to one of the world’s oldest traditions of textile modification, the fermented indigo vat dyeing. By choosing a fresh (or rather ancient) context, this study contributes a new perspective to the ongoing discourse on (bio)materiality. The article reflects on the practice of indigo vat dyeing from a new materialistic stance. Data consists of self-documentation and reflective writings from and of a dyeing workshop (2019) and experiences of maintaining a fermented indigo vat at home for six months (2020). Data is analysed in hermeneutical iteration and presented through different metaphorical perspectives: the vat as a spirit, the vat as a tool, the vat as a child, and the vat as a living medium. The main points of the discussion are (1) Indigo practices create awareness of worldly connectedness; in dyeing, I communicate with the whole of the ancient tradition and all of its practitioners. Relationship between dyer and indigo to transcends time and space. (2) While vat dyeing is appreciated as a challenging tool that dyers want to engage with and master, this risks reducing the meaning to its purpose. (3) The use of metaphor-full language within indigo practices grants agency to the vat for practical means; it works as a way to guide us towards interaction within the living medium. (4) The concept of a living medium, as proposed here, draws from Gilles Deleuze and Tim Ingold: it exists in a world of constant becoming and describing it aims to grasp the material flux of the world in its unfolding.