This paper explores major trends in visualization of medieval theories of natural and artificial transformation of substances in relation to their philosophical and theological bases. The function of pictorial forms is analyzed in terms of the prevailing conceptions of science and methods of transmitting knowledge. The documents under examination date from the thirteenth to the fifteenth century. In these, pictorial representations include lists and tables, geometrical figures, depictions of furnaces and apparatus, and figurative elements mainly from the vegetable and animal (...) realms. An effort is made to trace the earliest evidence of these differing pictorial types. (shrink)