Measurement of density and medullation in wool

Density is one of the fundamental physical characteristics of wool which is assumed to remain constant, yet medullation, when present, is an obvious source of variation in the density of wool fibres. It was proposed that a standardised and commercially viable method of measuring the density of wool fibres should be developed. Adaptation of an existing method of measuring the volume and density of finely powdered and porous solids, using a rapid, automated helium pycnometer, was pursued. Preliminary studies had identified sample mass constraints and the number of sequential measurements required for the technique. In this thesis, the effect of fibre length on the measurement of volume and density of wool fibres was investigated, to complete the development of the standardised measurement method. The helium pycnometer was then used to explore: an hypothesised relationship between medullation and fibre density; seasonal differences in fibre volume and fibre density of four types of wool, with varying degrees of medullation; and an unsubstantiated suggestion of a difference in specific gravity between lambswool and that of the adult sheep. A rapid, standardised method for measuring the volume and density of wool fibres was developed. Recommendations constituted use of: a sample mass between one and ten grams; fibre lengths greater than or equal to ten millimetres; and the fourth sequential sample measurement, for the measurement of the volume and density of wool fibre. The fibre density of non-medullated wool, measured using a helium pycnometer (1.29 g/cm3) was similar to the density and specific gravity, measured using a density gradient column or benzene pycnometer (1.30 g/cm3). Fibre density of medullated wool was significantly lower than that of non-medullated wool. There was a significant (p<0.001) relationship between percent medullation by volume and fibre density. However, prediction of medullation using helium pycnometer measurements of fibre volume was less accurate than using the projection microscope, OFDA, WRONZ Medullameter or NIRA methods of measuring medullation. There was significant seasonal variation in fibre volume and fibre density, for both non-medullated and medullated wools. The degree of medullation also varied significantly between seasons. Seasonal variation in fibre density was only partially explained by variation in medulla content. The maximum fibre volume and percent medullation by volume occurred during the summer months, while the minimum for both occurred during the winter months. The reverse situation applied for fibre density. The fibre density of lambswool did not differ from that of the adult ewe, once the effect of medullation was accounted for. This study has contributed new work to the area of wool metrology. This includes: a rapid and economically viable method of measuring the fibre density of medullated and non-medullated wool types; identification of seasonal trends in fibre volume and fibre density; and it questions the assumption that the density of wool keratin is constant.