"The medium on which yantra is drawn is either paper or metal, and the medium used is sometimes said to determine the relative effectiveness of the yantra – in this, the rarer, precious metals such as gold and silver are generally considered to be more effective, but for some yantras (often with the purpose of mastering the baser powers of black magic such as killing or invoking ruin on an enemy), a baser medium such as iron is prescribed. Less permanent materials are also used in some ritual contexts, such as inscribing of a yantra in sand or with certain powders – these are most often normally dispersed at the conclusion of the ritual."