Cotton sewing thread and silk sewing thread are no longer the factory default. They remain the right spec when heat, hand or fibre match matter more than tenacity. Cotton takes a hot needle and a durable press that would glaze or melt polyester. Silk is still the visible stitch on silk and fine wool when the buyer will pay. TrimGem University keeps both on the syllabus because a Guangzhou one-stop accessory programme that keeps expanding still has to thread an atelier sample the way a mill threads a shirt line — with the fibre named.
Cotton: three finishes, three behaviours
Cotton thread is even, shade-rich, soft and moisture-friendly. Elasticity and abrasion are the tax versus polyester. Three mill finishes:
Soft (unglazed, unmercerised). No singe, no mercerise, no size. Matte, hairy, higher friction in the needle, better fatigue in the hand. Hand sewing and low-speed machines, or seams nobody will see.
Mercerised. Combed cotton under tension in sodium hydroxide, then washed. Fibre swells toward round, lustre rises, dye uptake rises, strength rises. Elongation is low (around 4%); shrinkage is the lowest of the cotton family (about 1.5–2%). This is the thread for better cotton shirts, dresses and cotton knits that must look clean.
Glazed (waxed, polished). Sized, waxed and brushed. Slick, firm, twist-stable, ~10% more strength than soft cotton, built for high-speed sewing on firm cloth, leather and garments that will see a hot iron. Elongation around 5%. Because it was made under tension, shrinkage is higher — glazed cotton can pucker a seam that mercerised would not.
Counts: singles often 7.3–64.8 tex (about 9s–80s cotton count); plies 2, 3, 4, 6, sometimes 9 or 12. Old ticket 803 meant 80s/3; 602 meant 60s/2. Write tex × ply on new POs (7.3 tex × 3, 9.7 tex × 2).
Silk: still a specified yarn
Filament Bombyx silk (or spun silk) is smooth, lustrous, more elastic than cotton (elongation often 15–25%), stronger dry than cotton, weaker wet. It hates hard sun, mildew and strong alkali. Degum, dye (acid or direct), soften and glaze are the finishing path.
Classic tickets are denier constructions, not cotton counts: fine dress thread around (20–22 den) × 7 × 2 for silk garments; coarser cabled tickets for wool coating, buttonhole and bedding. Leather used to live on a heavy silk; polyester filament took that seat on price and wet strength.
Silk remains the right visible stitch on silk and some wool tailoring. It is the wrong volume thread for a cotton chino laundry.
When natural still wins the BOM
Match fibre to shell when shrinkage, heat and ageing must travel together: cotton thread on cotton that will be pressed hard; silk on silk. Baby programmes sometimes want cotton’s hand. Fire-adjacent or hot-needle lockstitch still uses cotton or cotton-wrapped corespun because polyester’s melt is the risk.
Do not specify cotton because it sounds honest and then run a polyester wash and a nylon stretch pattern. Natural thread is a heat and hand choice. Write soft / mercerised / glazed (cotton) or filament silk + denier construction, plus tex or ticket, plus shrink. “Cotton thread” is three different products.