Scouring or washing is the first mechanical process that wool goes through, to remove grease and dirt from fleece. Carbonising is unique to the woollen system and breaks down any remaining impurities after scouring.
Scouring
Wool which has been shorn is known as greasy wool, which not only contains grease, the basis of which is lanolin, but also other contaminants including suint, dirt, dust and sand.
Factors such as the terrain and environment upon which the sheep graze can affect the amount of contaminants found in fleece. Typically the total amount of contaminants present in greasy wool is around 35% to 40% of the total fleece weight.
These contaminants need to be removed before the greasy wool can be processed and this is done by a process known as raw wool scouring.
Scouring involves the greasy wool being fed into a machine where it is continuously transported through a series of bowls, typically between 6 and 8 bowls, containing wash liquor. In the initial 2 to 3 wash bowls the wash liquors comprise warm water*, containing detergent and sometimes an alkali, and it is at these stages where most of the water soluble or emulsified compounds such as wool grease is removed. The remaining bowls contain fresh water which are used to thoroughly rinse the wool.
The scoured loose wool is then passed into a continuous drier, before it is passed through a number of machines which mechanically beat the wool to remove water insoluble compounds, such as sand and dust, which are not fully removed by the wash liquors during scouring.
Next, the scoured loose wool is pneumatically transported into a closed room known as a bin where it is blended and allowed to stand to condition until its moisture content reaches equilibrium. It is then ready for the next stage of manufacturing known as top-making.
The approach to successful scouring of greasy wool in terms of quality is to effectively remove almost all of the different types of contaminants, whilst ensuring entanglement of the fibres or even felting is minimised. If the later occurs then undoubtedly this will lead to excessive fibre breakage during the subsequent stages of manufacturing, which in turn will lead to poor processing efficiencies, and poor wear performance in the final products or articles of clothing.
*It should be noted that some scouring systems for greasy wool are based on the use of an organic solvent, but today by far the most common methods of scouring wool are aqueous based system using water as described above.
Carbonising
Carbonising is a continuous process which combines scouring to remove the wool grease and a chemical process which removes vegetable matter such as seeds, burs and grass. Carbonising occurs if the greasy wool contains a high percentage of vegetable matter (%VM), typically in excess of 2% to 3%.
Most of the VM contained in wool is cellulosic in nature, which is broken down into carbon, by firstly immersing in a strong solution (6% to 7% weight for volume) of Sulphuric acid (H2SO4) then followed by baking in a dryer set at 95oC to 120oC. After the VM has been turned into carbon the wool is passed through a series of heavy metal fluted rollers which crush the carbonised burs into dust.
The wool is then passed through a de-dusting unit (rotary shaker) where the dust is separated from the wool by mechanical action.
The wool at this stage is acidic due to the action of being immersed in Sulphuric acid. It then neutralizes (pH 7) by passing it into a solution of alkali containing Sodium Carbonate (Na2Co4). Finally, the wool passes through a final bath containing a solution of Hydrogen Peroxide bleach (H2O2) to improve the colour before it is dried.