Using the concept of purity to reflect on the relationship between chemical practice and the philosophy of science, this article considers the philosophical significance of the chemical manipulations that aim to purify or otherwise transform matter. Starting from a consideration of the nature and role of pure (or idealised) examples in philosophy of science, the article underlines the temptation towards abstraction and theory for both scientists and philosophers. The article goes on to argue that chemistry, despite its increasing theoretical sophistication, (...) is a science that remains particularly close to laboratory manipulations. This point is made in reference to the work of Gaston Bachelard on the production of purity and with the aid of historical examples, notably exploring the interplay between techniques of purification and the definition of elements, with special attention paid to Lavoisier’s definition of elements as the final limit of analysis. The closing section concerns the manufacture of steel from iron ore in the eighteenth century, illustrating this process using texts by Pierre-Joseph Macquer and Denis Diderot. Steel production is used to illustrate the kinds of philosophical question that are raised by paying attention to the details of chemical practice rather than jumping straight to chemical theory and also suggests how scientific theory can emerge from this practice itself. (shrink)