Blocking is the step between a finished piece of knitting and a finished garment. It opens lace, evens out stitches, squares up edges and brings a piece to the measurements on the schematic. What it actually does to the fibre, though, differs so much between wool, acrylic and cotton that treating them alike is the main reason blocking sometimes disappoints.
What is happening in the fibre
A knitted fabric leaves the needles carrying strains. Every loop was formed under tension and every stitch is holding its neighbours in a shape that is not their resting shape. A review of shrink resistance in wool published in the Journal of Advanced Research separates the two things that then happen when a wool fabric meets water. Relaxation shrinkage is the release of strains imposed during manufacturing stages such as spinning, weaving and knitting; it is reversible, and it occurs in textiles made from any fibre. Felting shrinkage is different: it is irreversible, it happens during wet processing, and it is driven by the scales on the wool fibre surface acting as a ratchet under mechanical agitation, friction and pressure in the presence of moisture and heat, so that fibres migrate and cannot return.
Blocking works with the first mechanism and against the second. You wet the fabric so the strains release, pin it to the dimensions you want while it dries, and avoid the agitation, friction and heat that would cause felting instead. This is also why wool responds better to blocking than anything else: it takes up a lot of moisture, its structure relaxes, and it holds the new configuration once dry.
Wet, steam and spray
Wet blocking is total immersion. Soak the piece in cool water with a wool wash for twenty minutes, lift it out supporting its whole weight, press out water without wringing, roll it in a towel to remove more, then pin it out and let it dry completely. It gives the largest, most reliable change in dimensions and is the right method for wool, wool blends, alpaca and most animal fibres, and for anything where lace has to open up dramatically.
Steam blocking holds a steam iron or garment steamer above the fabric, never resting on it, and lets the steam penetrate while the piece is pinned. It is faster, it suits pieces already seamed, and it is the only method that will do much for acrylic. Applied with real heat it will kill acrylic, flattening the fibres permanently into a soft drapey fabric. That is sometimes exactly what you want, and it cannot be undone.
Spray blocking pins the piece dry and mists it with water until damp. It gives the gentlest, smallest correction, and it is the choice for a quick refresh, for pieces that must not be over relaxed, and for fibres you do not want fully saturated.
Mats, wires and measuring
Interlocking foam mats give you a pinnable surface and a printed grid to work against. Rustproof T pins hold edges; anything that can rust will mark. Blocking wires threaded through the edge stitches of a shawl produce a dead straight edge that pins alone will not, and they distribute the tension across many stitches instead of concentrating it at each pin.
Work to the schematic rather than by eye. Pin the centre points first, then the corners, then fill in between, checking measurements as you go. Pull firmly but not to the limit: stretching a fabric past what the yarn can hold produces a piece that will relax back unevenly, and on lace it can leave the fabric permanently thin.
How different fibres respond
- Untreated wool and other animal fibres block best and hold their new dimensions well.
- Superwash wool has had its scales smoothed or coated to prevent felting. That also removes much of the fibre friction that holds a blocked shape, so superwash grows under its own weight when wet and often keeps growing in wear. Block it to size rather than stretching it, and expect it to need reblocking.
- Cotton, linen and other plant fibres have little elastic recovery. Blocking will even out stitches and set a length, but the fabric will drop again over time under gravity.
- Acrylic and other synthetics do not respond to water at all in the way protein fibres do. Steam is the only route, and killing acrylic with heat is permanent.
Blocking is not a one time event
Because relaxation shrinkage is reversible, a blocked garment gradually returns toward its unblocked state through wear and washing, and has to be blocked again after each wash. Formal textile testing treats this dimensional movement as something to be measured rather than assumed: ASTM’s Standard Test Method for Shrinkage of Yarns determines shrinkage of yarns in skein form when treated in boiling water, dry heat, saturated steam or solvents, and states that the results are used to predict the dimensional stability of fabrics during wet and elevated temperature processing.
The practical version of that for a hand knitter is simple. Keep the blocked measurements written down, and reblock to those same numbers every time. Better still, wash and block your gauge swatch before the project, so you know before you cast on whether the yarn grows, shrinks or stays put.
