Wool is one of the oldest and most valuable versatile natural fibers used in textiles, but before it can be turned into fabric, it often undergoes a cleaning process called carbonising. Because raw wool often contains plant material and impurities that can interfere with its quality.
Wool carbonising is a chemical process used to remove vegetable matter such as burrs, seeds, and grass from raw wool. These plant-based impurities cannot be removed by regular scouring or washing, so carbonizing is necessary—especially for finer or worsted wool products.
Purpose of Wool Carbonising
Wool straight from the sheep contains natural oils (lanolin), dirt, sweat, and plant matter. While scouring removes grease and dirt, but some stubborn plant-based impurities remain. These can cause defects in yarn and fabric if not removed.
The main purpose of carbonising is to:
To improve the cleanliness of wool by removing stubborn vegetable matter.
To prepare wool for dyeing and spinning in a cleaner, more refined form.
To improve quality and softness, especially in high-end wool fabrics.
To make wool more uniform, which helps with further processing.
Wool Carbonising Process Step-by-Step
Raw wool contains seeds, burrs and other pieces of vegetable matter. Much of this may be removed in scouring and in the worsted process during carding and combing. Combing is not used in the woollen system, and vegetable matter is only partially removed in carding. In some worsted processes, small amounts of residual vegetable material get through to the fabric; in such cases, carbonising is essential to remove the residues.
The carbonising process requires that scoured wool fabric is padded, either in rope form or in open width, with a liquor containing dilute sulphuric acid (5–7% by mass; approximately 65% wet pickup), and dried at 65–90◦C in order to concentrate the acid.
Baking at 125 ◦C for 1 minute chars the cellulosic material. The charred vegetable material is brittle and easily crushed on passing through rollers; it can then be removed as dust during subsequent mechanical working. Following carbonising, the wool should be rinsed and neutralised by a wet process. Such neutralisation should be carried out immediately after baking, or else fibre damage will occur during storage of the wool in such an acidic state. It is convenient to neutralise prior to dyeing, but uneven neutralisation leads to unlevel dyeings. Many dyers of carbonised piece goods thus choose to dye with acid levelling or 1 : 1 metal-complex dyes – the latter at pH 1.8 with 6–8% on mass fabric (o.m.f.) sulphuric acid.
If faster dyes such as milling or 1 : 2 metal-complex dyes are required, very careful neutralisation with ammonia and ammonium acetate gives the best results.
The substantivity of acid dyes for carbonised wools is generally reduced, presumably by the formation of sulphate esters at serine residues (Scheme 1). The dyer may see white resist spots, which clearly arise from localised high concentrations of sulphuric acid giving rise to large amounts of serine sulphate residues.
Scheme 1: Sulphation of wool serine by sulphuric acid in carbonising
It is generally accepted that the loss in strength during carbonising (or, more importantly, during storage of carbonised goods) is due to the N→O migration (peptidyl acyl shift) reaction at serine and threonine residues (Scheme 2). Chain cleavage occurs at the ester linkage in Scheme 2, resulting in breakage of the peptide chain and consequently reduced strength.
Scheme 2: N→O shift in wool carbonising
Given the drawbacks of acid carbonising, such as excessive fibre damage and environmental pollution from the relatively large amount of acid used, it is not surprising that researchers are looking at the potential role of enzymes in wool carbonizing process.
The conventional carbonizing process comprises five main stages : scouring, acidizing, drying and baking, burr crushing and dedusting and neutralising. And recent development process includes: Acidising, Moisture removal, Drying/Baking, Burr crushing and dedusting, Neutralisation.
The conventional technique of wool carbonizing has the following basic steps:
Scouring of the raw wool with non-ionic detergent.
Rinsing.
Immersion in a long bowl containing 5-7% (w/v) sulphuric acid, 1-2 g/L detergent at 20-30oC.
Double squeezing and/or continuous centrifuging prior to drying.
Drying at 60-80oC to a low regain.
Baking at 95-120oC to carbonize the VM.
Pass through heavy fluted rollers to crush the embrittled VM.
Convey to a rotating shaker/de-duster to remove the charred VM dust.
Pass through a neutralising bowl usually containing sodium carbonate.
Rinsing with a small addition of detergent.
Bleaching wool with hydrogen peroxide at approximately pH 5 with formic acid.
Final drying.
The production rate through the process is usually low, typically 500-600 kg/h for an 1800 mm wide line, since sufficient time must elapse for the VM to adsorb the acid from the bowl and for subsequent baking to embrittle the acidified cellulose.
The wool burrs contain hemi-cellulose and lignin apart from cellulose. The lignin is not effected by carbonising process on acid hydrolysis, but can be split oxidatively. Salt of persulphuric acid accelerates degradation of wool burrs which enable shortening the time of reaction.
Natural soiling of wool and vegetable matter and skin flakes, can also be enzymatically modified. The replacement of carbonization by the use of enzymes, such as cellulases, ligninases, hydrolases, lyases, oxidoredulases etc. are reported. The main advantages of enzymatic carbonizing are reduced wool fibre damage, effluent load and energy consumption.
Carbonized wool is more porous than scoured wool and hence the dye uptake is higher. Uneven dyeing may result due to faulty carbonizing.
Benefits of Wool Carbonising
Carbonising brings several advantages, especially when high-quality or worsted wool is needed:
Cleaner Wool Output: Removes up to 100% of vegetable matter that scouring can’t handle.
Improved Dyeing Results: Clean wool absorbs dyes more evenly and vibrantly.
Better Spinning Performance: Fewer impurities mean fewer breaks in the yarn during spinning.
Enhanced Fabric Quality: Finished fabrics feel smoother and look more uniform.
Long-Term Storage Stability: Cleaner wool is less likely to attract pests or mildew.
Reduce Production Waste: Less fiber is lost in processing, which is both cost-effective and environmentally friendly.
Versatility: Carbonised wool is ideal for fine fabrics like suits and high-end textiles.
Conclusion
Wool carbonising is a crucial step in wool processing, ensuring that the final fabric is free from plant-based impurities. By using acid treatment and heat, manufacturers can produce cleaner, stronger, and more consistent wool for high-quality textiles.
References
[1] The Coloration of Wool and other Keratin Fibres. (2013). In Wiley eBooks. https://doi.org/10.1002/9781118625118
[2] Karmakar, S. (1999). Chemical technology in the Pre-Treatment processes of textiles. Elsevier.
[3] Cook, J. G. (1984). Handbook of Textile Fibres: Natural Fibres. McGraw Hill Professional.
Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. Mr. Kiron is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.
