By studying X-ray diffraction photographs of crystals, scientists could calculate the arrangement of atoms within molecules. Some of the materials analysed occur naturally as crystals. Others, such as haemoglobin, have to be crystallised first. Crystallising a substance orders the arrangement of its atoms into a repetitive pattern, which makes it possible to interpret accurately the diffraction patterns it produces. Making a crystal of adequate quality to study is a complex process and can take considerable time.