Residuals - grease and ash

Scouring removes most of the unwanted material on greasy wool, but still leaves a very small percentage of dust and residual grease on the fibre. Both scour operators and users are interested in the amount of residuals as an indicator of cleanliness, and so almost all consignments are tested.

The amount of dust remaining with the fibre is determined simply by carrying out an ash test. This is part of the standard yield test, which is also usually carried out on the consignment (principally to determine the amount of vegetable matter remaining, which, because VM base is determined on the basis of the amount of greasy wool, actually increases as a result of scouring).

Separate samples are taken from the dried wool to determine residual grease using either NIRA measurements or dichloromethane (DCM) extraction (which remains the reference method in the case of dispute). These values are often compared with the DCM values obtained by the scour in their QC operations, which are also usually now obtained in New Zealand using NIRA technology.

For many years laboratories in the Southern and Northern hemispheres used different in-house methods for determining residual grease, which frequently led to confusion (see Info-bulletin 4.3). Unfortunately, because of the varying volatile fractions of many combing oils, the procedure used for scoured wool cannot be applied to sliver, and hence there are two separate procedures (for scoured and carbonised wool, and for tops and sliver) within the one IWTO standard. In November 2002, sampling of scoured and carbonised wool for residual grease was written into the IWTO Core Test Regulations, and the measurement has thus now become certifiable under IWTO auspices (for an explanation of this term, refer to Trading Certification services).