The nominal density of a cocoon is calculated from the weight, the diameters of the circular samples, and the thickness, which is defined as the distances between two gauges at 1.5 MPa compressive stress (Table 1). The density results have contributions from the silk fibres in the structure and the calcium oxalate crystals on the surface of some cocoons. Cocoons with low porosity from SEM pictures are found to have high density (e.g. O. eucalypti, A. luna), although the density could be over-estimated due to the re-arrangement of fibres and sericin during the compression tests. On the other hand, calcium oxalate has much higher density than the silk fibres and has a significant effect on cocoon density. Unfortunately, initial analysis indicates that the density results from Table 1 do not show good correlation with porosity of the cocoon structures from SEM pictures.