Cast materials are typically very malleable, and thisadaptability is retained to various degrees depending onthe particular material and how it is subsequently treated.This category includes materials such as clay, plasticineand plaster – the latter also commonly known as gypsum.Modellers only usually work with one of these materialsin a model at a time, since their different properties donot make them easy to combine. The ‘hands on’ natureof this type of materials means that architectural modelsmade from them tend to focus on exploration of formand relationships between mass and void rather thanincorporating detailed design information. Despitethis, it should be pointed out that it is possible to castplaster with considerable accuracy if required, in order toproduce complex and ﬂuid three-dimensional forms witha smooth ﬁnish. In the construction process gypsum,also known by its chemical name of calcium sulphate,is used at full-scale – typically, though not exclusively,for internal ﬁnishes. Working with this material for architectural models involves producing a mould forthe liquid plaster to be poured into. Although this cansometimes be a time-consuming activity, the beneﬁt isthat once the mould has been produced it can be usedover and over again as necessary. This is particularlyuseful if a designer needs to produce a lot of repetitivecomponents for a model. Modelling plaster is a whitepowder mixed with water for use in liquid form asdescribed above, or may be used in thicker consistencieswith craft tools. Whilst plaster in its liquid state cansometimes be messy to work with, it quickly sets andhardens. When it has transformed into its solid state, itcan be worked on further with a craft knife or sandpaperfor more detailed work on surfaces – and it may bepainted or varnished to extend the possibilities ofits appearance.