Crystal garden. Last of five photographs showing growth of a crystal garden". A saturated solution of potassium aluminium sulphate (alum) is poured into the well at the base of the cardboard tree and begins to soak up through the card. The water in the solution evaporates from the surface of the card, especially at tips of the branches, and potassium aluminium sulphate crystals begin to form. The card at the tips of the branches has been impregnated with multicoloured dyes to colour the crystals, which are normally white. The crystals continue to grow for several hours, until all the water in the solution has evaporated. (See photo's A600/831 to A600/834)."